Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 256 Vibration-Driven Self-Doping of Dangling-Bond Wires on Si(553)-Au Surfaces
    Braun, C. and Neufeld, S. and Gerstmann, U. and Sanna, S. and Plaickner, J. and Speiser, E. and Esser, N. and Schmidt, W.G.
    Physical Review Letters 124 (2020)
    Density-functional theory is used to explore the Si(553)-Au surface dynamics. Our study (i) reveals a complex two-stage order-disorder phase transition where with rising temperature first the ×3 order along the Si step edges and, subsequently, the ×2 order of the Au chains is lost, (ii) identifies the transient modification of the electron chemical potential during soft Au chain vibrations as instrumental for disorder at the step edge, and (iii) shows that the transition leads to a self-doping of the Si dangling-bond wire at the step edge. The calculations are corroborated by Raman measurements of surface phonon modes and explain previous electron diffraction, scanning tunneling microscopy, and surface transport data. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstractdoi: 10.1103/PhysRevLett.124.146802
  • 2020 • 255 Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
    Duchoň, T. and Hackl, J. and Mueller, D.N. and Kullgren, J. and Du, D. and Senanayake, S.D. and Mouls, C. and Gottlob, D.M. and Khan, M.I. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Journal of Materials Chemistry A 8 5501-5507 (2020)
    The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta11784a
  • 2020 • 254 Microscopic model for the stacking-fault potential and the exciton wave function in GaAs
    Durnev, M.V. and Glazov, M.M. and Linpeng, X. and Viitaniemi, M.L.K. and Matthews, B. and Spurgeon, S.R. and Sushko, P.V. and Wieck, A.D. and Ludwig, Ar. and Fu, K.-M.C.
    Physical Review B 101 (2020)
    Two-dimensional stacking fault defects embedded in a bulk crystal can provide a homogeneous trapping potential for carriers and excitons. Here we utilize state-of-The-Art structural imaging coupled with density-functional and effective-mass theory to build a microscopic model of the stacking-fault exciton. The diamagnetic shift and exciton dipole moment at different magnetic fields are calculated and compared with the experimental photoluminescence of excitons bound to a single stacking fault in GaAs. The model is used to further provide insight into the properties of excitons bound to the double-well potential formed by stacking fault pairs. This microscopic exciton model can be used as an input into models which include exciton-exciton interactions to determine the excitonic phases accessible in this system. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.125420
  • 2020 • 253 Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations
    Eggert, B. and Gruner, M.E. and Ollefs, K. and Schuster, E. and Rothenbach, N. and Hu, M.Y. and Zhao, J. and Toellner, T.S. and Sturhahn, W. and Pentcheva, R. and Cuenya, B.R. and Alp, E.E. and Wende, H. and Keune, W.
    Physical Review Materials 4 (2020)
    We combine Fe57 Mössbauer spectroscopy and Fe57 nuclear resonant inelastic x-ray scattering (NRIXS) on nanoscale polycrystalline [bcc-Fe57/MgO] multilayers with various Fe-layer thicknesses and layer-resolved density-functional-theory (DFT)-based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure (where ML denotes monolayer) to unravel the interface-related atomic vibrational properties of a multilayer system. Being consistent in theory and experiment, we observe enhanced hyperfine magnetic fields Bhf in the multilayers as compared to Bhf in bulk bcc Fe; this effect is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.044402
  • 2020 • 252 Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)
    Haussühl, E. and Reichmann, H.J. and Schreuer, J. and Friedrich, A. and Hirschle, C. and Bayarjargal, L. and Winkler, B. and Alencar, I. and Wiehl, L. and Ganschow, S.
    Materials Research Express 7 (2020)
    A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1591/ab6ad6
  • 2020 • 251 Electron correlation and spin transition
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology 35-43 (2020)
    Theoretical treatment of functional metalorganics is non-trivial for the metal centers with narrow bands (3d, 4d of transition metals or 4f bands of rare-earth metals), featuring a sizeable Coulomb interaction. An interplay between crystal field, spin-orbit coupling and Coulomb interaction expresses the properties of the molecule. Correlated metal centers, immersed in the electron bath of organic ring makes it ideal to treat with Anderson’s impurity model. In this chapter, we will focus on the description of electron correlation in functional metalorganics with the aid of density functional theory, combined with a many body approach. For most of the illustrative purposes, we will consider iron porphyrin (FeP) molecule. The chapter will reveal the importance of the treatment of explicit electron correlation in order to accurately identify the spin transition, magnetic anisotropy, Kondo effect etc., which are key ingredients for molecular spintronics and electronics. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstractdoi: 10.1007/978-981-15-3719-6_5
  • 2020 • 250 Thermodynamic modelling of the Ni–Zr system
    Jana, A. and Sridar, S. and Fries, S.G. and Hammerschmidt, T. and Kumar, K.C.H.
    Intermetallics 116 (2020)
    In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach based on special quasirandom structures (SQS) was used for calculating the enthalpy of mixing of the fcc solid solution. The vibrational contribution to the heat capacities of NiZr, NiZr2, Ni3Zr and Ni7Zr2 phases were calculated using the quasiharmonic approximation (QHA) and the corresponding electronic contribution was obtained using an approach based on Mermin statistics. The total heat capacities for these phases were fitted to appropriate expressions and integrated to obtain the Gibbs energy functions valid down to 0 K. The calculated thermochemical properties along with critically selected experimental constitutional and thermochemical data served as input for the thermodynamic optimisation of the system. The calculated phase equilibria and the thermodynamic properties using the optimised Gibbs energy functions are in good agreement with the input data. The calculated congruent melting points of NiZr and NiZr2 phases are close to the recent experimental data. The Ni10Z7 phase forms by a peritectic reaction, which is also in agreement with the experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2019.106640
  • 2020 • 249 Chemisorption and Physisorption at the Metal/Organic Interface: Bond Energies of Naphthalene and Azulene on Coinage Metal Surfaces
    Kachel, S.R. and Klein, B.P. and Morbec, J.M. and Schöniger, M. and Hutter, M. and Schmid, M. and Kratzer, P. and Meyer, B. and Tonner, R. and Gottfried, J.M.
    Journal of Physical Chemistry C 124 8257-8268 (2020)
    Organic/inorganic hybrid interfaces play a prominent role in organic (opto)electronics, heterogeneous catalysis, sensors, and other current fields of technology. The performance of the related devices and processes depends critically on the nature and strength of interfacial interaction. Here, we use the molecular isomers naphthalene (Nt) and azulene (Az) on the Ag(111) and Cu(111) surfaces as model systems that cover different bonding regimes from physisorption to chemisorption. Az also serves as a model for nonalternant molecular electronic materials and for topological 5-7 defects in graphene. The interaction energies are determined from the quantitative analysis of temperature-programmed desorption data. On both surfaces, Az binds more strongly than Nt, with zero-coverage desorption energies (in kJ/mol) of 120 for Az/Ag and 179 for Az/Cu, compared to 103 for Nt/Ag and 114 for Nt/Cu. The integrated experimental energies are compared with adsorption energies from density-functional theory (DFT) calculations, which include van der Waals contributions using four different correction schemes for the PBE functional: (1) the DFT-D3 scheme with Becke-Johnson damping, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion correction scheme, and (4) the D3surf scheme. Differences in the performance of these methods are discussed. Periodic energy decomposition analysis reveals details of the surface chemical bond and confirms that Az/Cu forms a chemisorptive bond, while the other systems are physisorbed. The variation of the adsorbate-substrate interaction with the topology of the Ï-electron system and the type of surface can be employed to modify the interface properties in graphene-based and organic electronic devices. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c00915
  • 2020 • 248 How photocorrosion can trick you: A detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production
    Kampmann, J. and Betzler, S. and Hajiyani, H. and Häringer, S. and Beetz, M. and Harzer, T. and Kraus, J. and Lotsch, B.V. and Scheu, C. and Pentcheva, R. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 12 7766-7775 (2020)
    The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes. © The Royal Society of Chemistry .
    view abstractdoi: 10.1039/c9nr10250g
  • 2020 • 247 Rational design of thiolated polyenes as trifunctional Raman reporter molecules in surface-enhanced Raman scattering nanotags for cytokine detection in a lateral flow assay
    Keller, T. and Brem, S. and Tran, V. and Sritharan, O. and Schäfer, D. and Schlücker, S.
    Journal of Biophotonics 13 (2020)
    The characteristic vibrational spectroscopic fingerprint of Raman reporter molecules adsorbed on noble metal nanoparticles is employed for the identification of target proteins by the corresponding surface-enhanced Raman scattering (SERS) nanotag-labeled antibodies. Here, we present the modular synthesis of thiolated polyenes with two to five C═C double bonds introduced via stepwise Wittig reactions. The experimental characterization of their electronic and vibrational properties is complemented by density functional theory calculations. Highly SERS-active nanotags are generated by using the thiolated polyenes as Raman reporter molecules in Au/Au core/satellite supraparticles with multiple hot spots. The cytokines IL-1β and IFN-γ are detected in a duplex SERS-based lateral flow assay on a nitrocellulose test strip by Raman microscopy. The thiolated polyenes are suitable for use in immuno-SERS applications such as point-of-care testing as well as cellular and tissue imaging. © 2020 The Authors. Journal of Biophotonics published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/jbio.201960126
  • 2020 • 246 An aminotetracyanocyclopentadienide system: Light-induced formation of a thermally stable cyclopentadienyl radical
    Nimax, P.R. and Zoller, F. and Blockhaus, T. and Küblböck, T. and Fattakhova-Rohlfing, D. and Sünkel, K.
    New Journal of Chemistry 44 72-78 (2020)
    Crystals of the aminotetracyanocyclopentadienyl radical were obtained from the reaction of CaCl2 with Ag[C5(CN)4(NH2)] and recrystallization in MeOH, performed in sunlight. The radical was characterized by X-ray diffraction, EPR and UV Vis spectroscopy as well as by cyclovoltammetry and DFT calculations. © 2019 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstractdoi: 10.1039/c9nj04354c
  • 2020 • 245 Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer
    Pollmann, E. and Morbec, J.M. and Madauß, L. and Bröckers, L. and Kratzer, P. and Schleberger, M.
    Journal of Physical Chemistry C 124 2689-2697 (2020)
    Two-dimensional molybdenum disulfide on graphene grown by chemical vapor deposition is a promising van der Waals system for applications in optoelectronics and catalysis. To extend the fundamental understanding of growth and intrinsic properties of molybdenum disulfide on graphene, molybdenum disulfide on highly oriented pyrolytic graphite is a suitable model system. Here, we show experimentally and by density functional theory calculations that molybdenum disulfide flakes grow in two orientations. One of the orientations is energetically preferred, the other one is rotated by 30°, but both orientations are found to be stable at room temperature. Combined Kelvin probe microscopy and Raman spectroscopy measurements show that the flakes with a typical size of a few hundred nanometers are electron doped in the order of 1012/cm2, while the doping of a molybdenum disulfide single layer exfoliated on silicon dioxide is on the order of 1013/cm2. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b10120
  • 2020 • 244 One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization-Hydrosilylation Process
    Sarma, B.B. and Kim, J. and Amsler, J. and Agostini, G. and Weidenthaler, C. and Pfänder, N. and Arenal, R. and Concepción, P. and Plessow, P. and Studt, F. and Prieto, G.
    Angewandte Chemie - International Edition 59 5806-5815 (2020)
    Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru1/CeO2 and Rh1/CeO2 SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov α-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201915255
  • 2020 • 243 Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6 H1.6
    Terwey, A. and Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Eggert, B. and Ollefs, K. and Brabänder, V. and Radulov, I. and Skokov, K. and Faske, T. and Hu, M.Y. and Zhao, J. and Alp, E.E. and Giacobbe, C. and G...
    Physical Review B 101 (2020)
    We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 in terms of the vibrational (phonon) density of states (VDOS), which we determined with Fe57 nuclear resonant inelastic X-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy ΔSlat across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change ΔSlat of (3.2±1.9)JkgK across the FM-to-PM transition contributes with ∼35% significantly and cooperatively to the total isothermal entropy change ΔSiso. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.064415
  • 2020 • 242 Structural stability of Co–V intermetallic phases and thermodynamic description of the Co–V system
    Wang, P. and Hammerschmidt, T. and Kattner, U.R. and Olson, G.B.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 68 (2020)
    The Co–V system has been reviewed. Density functional theory (DFT) calculations using the generalized gradient approximation (GGA) were used to obtain the energies for the end-members for all three intermediate phases, Co3V, σ and CoV3. Results from DFT calculations considering spin polarization were used to evaluate the CALPHAD (Calculation of phase diagrams) model parameters. The method to evaluate the contribution of the magnetism to the energies of Co-rich compounds that was introduced in our previous work is presented in more detail in the present work. For the description of the σ phase, the magnetic part of the total energy is included in the description of the pure Co end-member compound resulting in a non-linear description of the magnetic contribution over composition. The calculated phase diagram obtained from the present CALPHAD description is in good agreement with the experimental data. The metastable FCC-L12 phase diagram was calculated and compared with experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2019.101729
  • 2020 • 241 Role of magnetic ordering for the design of quinary TWIP-TRIP high entropy alloys
    Wu, X. and Li, Z. and Rao, Z. and Ikeda, Y. and Dutta, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Physical Review Materials 4 (2020)
    We reveal the impact of magnetic ordering on stacking fault energy (SFE) and its influence on the deformation mechanisms and mechanical properties in a class of nonequiatomic quinary Mn-containing compositional complex alloys or high entropy alloys (HEAs). By combining ab initio simulation and experimental validation, we demonstrate magnetic ordering as an important factor in the activation and transition of deformation modes from planar dislocation slip to TWIP (twinning-induced plasticity) and/or TRIP (transformation-induced plasticity). A wide compositional space of Cr20MnxFeyCo20Niz(x+y+z=60, at. %) was probed by density-functional theory calculations to search for potential alloys displaying the TWIP/TRIP effects. Three selected promising HEA compositions with varying Mn concentrations were metallurgically synthesized, processed, and probed for microstructure, deformation mechanism, and mechanical property evaluation. The differences in the deformation modes of the probed HEAs are interpreted in terms of the computed SFEs and their dependence on the predicted magnetic state, as revealed by ab initio calculations and validated by explicit magnetic measurements. It is found that the Mn content plays a key role in the stabilization of antiferromagnetic configurations which strongly impact the SFEs and eventually lead to the prevalent deformation behavior. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.033601
  • 2020 • 240 Mapping the mechanical properties in nitride coatings at the nanometer scale
    Zhang, Z. and Chen, Z. and Holec, D. and Liebscher, C.H. and Koutná, N. and Bartosik, M. and Zheng, Y. and Dehm, G. and Mayrhofer, P.H.
    Acta Materialia 194 343-353 (2020)
    We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. Gradient structures are also accompanied by a modulation of the chemical compositions. A combined experimental analysis based on valence electrons and inner shell electrons allowed mapping the mechanical properties of the multilayer at the nanometer scale and further unveiled the effect of oxygen impurities on the bulk modulus. We found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.04.024
  • 2020 • 239 Amorphization-governed elasto-plastic deformation under nanoindentation in cubic (3C) silicon carbide
    Zhao, L. and Alam, M. and Zhang, J. and Janisch, R. and Hartmaier, A.
    Ceramics International 46 12470-12479 (2020)
    Amorphization plays an important role in ceramic deformation under mechanical loading. In the present work, we investigate the elasto-plastic deformation mechanisms of monocrystalline cubic silicon carbide (3C–SiC) in spherical nanoindentation by means of molecular dynamics simulations. The indentation-induced amorphization and its interactions with other deformation modes are emphasized. Initially, the suitable empirical potential capable of accurately characterizing the mechanical and defect properties of monocrystalline 3C–SiC, as well as the propensity of phase transformation from 3C–SiC to amorphous SiC, is rationally selected by benchmarking of different empirical potentials with experimental data and density functional theory calculations. Subsequently, the inhomogeneous elastic-plastic transitions during nanoindentation of monocrystalline 3C–SiC, as well as their dependence on crystallographic orientation, are investigated. Phase transformations including amorphization are analyzed using combined methods based on radial distribution function and bond angle distribution. Our simulation results demonstrate that before plasticity initiation-related “pop-in” event, each indented-monocrystalline 3C–SiC experiences a pure quasi-elastic deformation governed by the formation of amorphous structures. And this process of amorphization is fully reversible for small indentation depths. Further amorphization and dislocation nucleation jointly dominate the incipient plasticity in 3C–SiC nanoindentation. It is found that the indentation-induced defect zone composed of amorphous phase and dislocations is more pronounced in 3C–SiC(010) than that in the other two orientations of (110) and (111). © 2020 Elsevier Ltd and Techna Group S.r.l.
    view abstractdoi: 10.1016/j.ceramint.2020.02.009
  • 2019 • 238 The 2019 materials by design roadmap
    Alberi, K. and Nardelli, M.B. and Zakutayev, A. and Mitas, L. and Curtarolo, S. and Jain, A. and Fornari, M. and Marzari, N. and Takeuchi, I. and Green, M.L. and Kanatzidis, M. and Toney, M.F. and Butenko, S. and Meredig, B. and L...
    Journal of Physics D: Applied Physics 52 (2019)
    Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aad926
  • 2019 • 237 Selective 2-Propanol Oxidation over Unsupported Co3O4 Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols
    Anke, S. and Bendt, G. and Sinev, I. and Hajiyani, H. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Pentcheva, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    ACS Catalysis 9 5974-5985 (2019)
    Crystalline Co3O4 nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleylamine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as a function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR), and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O2 with the exposed spinel surfaces. Co3O4 selectively catalyzes the oxidative dehydrogenation of 2-propanol, yielding acetone and H2O and only to a minor extent the total oxidation to CO2 and H2O at higher temperatures. The high catalytic activity of Co3O4 reaching nearly full conversion with 100% selectivity to acetone at 430 K is attributed to the high amount of active Co3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of the 5-fold-coordinated octahedral surface Co5c3+ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co3+ to Co2+ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction and poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01048
  • 2019 • 236 Role of hole confinement in the recombination properties of InGaN quantum structures
    Anikeeva, M. and Albrecht, M. and Mahler, F. and Tomm, J.W. and Lymperakis, L. and Chèze, C. and Calarco, R. and Neugebauer, J. and Schulz, T.
    Scientific Reports 9 (2019)
    We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41598-019-45218-8
  • 2019 • 235 Mechanical properties of VMoNO as a function of oxygen concentration: Toward development of hard and tough refractory oxynitrides
    Edström, D. and Sangiovanni, D.G. and Landälv, L. and Eklund, P. and Greene, J.E. and Petrov, I. and Hultman, L. and Chirita, V.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 37 (2019)
    Improved toughness is a central goal in the development of wear-resistant refractory ceramic coatings. Extensive theoretical and experimental research has revealed that NaCl-structure VMoN alloys exhibit surprisingly high ductility combined with high hardness and toughness. However, during operation, protective coatings inevitably oxidize, a problem that may compromise material properties and performance. Here, the authors explore the role of oxidation in altering VMoN properties. Density functional theory and theoretical intrinsic hardness models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx solid solutions as a function of the oxygen concentration x. Elastic constant and intrinsic hardness calculations show that oxidation does not degrade the mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate that the presence of oxygen reduces the covalent bond character, which slightly lowers the alloy strength and intrinsic hardness. Nevertheless, the character of metallic d-d states, which are crucial for allowing plastic deformation and enhancing toughness, remains unaffected. Overall, the authors' results suggest that VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high intrinsic hardness, while still being ductile. © 2019 Author(s).
    view abstractdoi: 10.1116/1.5125302
  • 2019 • 234 Discovery of Elusive K4O6, a Compound Stabilized by Configurational Entropy of Polarons
    Freysoldt, C. and Merz, P. and Schmidt, M. and Mohitkar, S. and Felser, C. and Neugebauer, J. and Jansen, M.
    Angewandte Chemie - International Edition 58 149-153 (2019)
    Synthesis of elusive K4O6 has disclosed implications of crucial relevance for new solid materials discovery. K4O6 forms in equilibrium from K2O2 and KO2, in an all-solid state, endothermic reaction at elevated temperature, undergoing back reaction upon cooling to ambient conditions. This tells that the compound is stabilized by entropy alone. Analyzing possible entropic contributions reveals that the configurational entropy of “localized” electrons, i.e., of polaronic quasi-particles, provides the essential contribution to the stabilization. We corroborate this assumption by measuring the relevant heats of transformation and tracking the origin of entropy of formation computationally. These findings challenge current experimental and computational approaches towards exploring chemical systems for new materials by searching the potential energy landscape: one would fail in detecting candidates that are crucially stabilized by the configurational entropy of localized polarons. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201809409
  • 2019 • 233 Oxygen-mediated deformation and grain refinement in Cu-Fe nanocrystalline alloys
    Guo, J. and Duarte, M.J. and Zhang, Y. and Bachmaier, A. and Gammer, C. and Dehm, G. and Pippan, R. and Zhang, Z.
    Acta Materialia 166 281-293 (2019)
    Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.040
  • 2019 • 232 Shape-preserving machining produces gradient nanolaminate medium entropy alloys with high strain hardening capability
    Guo, W. and Pei, Z. and Sang, X. and Poplawsky, J.D. and Bruschi, S. and Qu, J. and Raabe, D. and Bei, H.
    Acta Materialia 170 176-186 (2019)
    A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.03.024
  • 2019 • 231 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2018.2856461
  • 2019 • 230 Photooxidation of Water on Pristine, S- And N-Doped TiO2(001) Nanotube Surfaces: A DFT + U Study
    Kenmoe, S. and Spohr, E.
    Journal of Physical Chemistry C 123 22691-22698 (2019)
    Using density functional theory calculations, we study the structure, energetics, and the photoelectrochemical oxidation of water on pristine, S-, N-, and (N + S)-doped anatase TiO2(001) nanotube (NT) surfaces. We found that water adsorbs molecularly on pristine and S-doped surfaces, while N doping promotes dissociative adsorption (both in the presence and absence of the S codopant) and leads to more favorable adsorbate-substrate interactions. Under photoelectrochemical conditions, OH groups are the most stable species on each surface with decreasing stability in the sequence (N + S) ≈ N → S → pristine. Surface Ti5C are the active sites and the anion impurity sites are not structurally affected during the water oxidation reaction. Nanostructuring TiO2 by forming three monolayer-thick (3 ML) TiO2(001) NT surfaces and subsequent anion doping yield an overpotential drop from 1.31 V on the flat (2D) TiO2(001) surface to 0.90, 0.71, 0.94, and 0.96 V on pristine, S-, N-, and (N + S)-doped nanotube surfaces, respectively. This reduction is a consequence of the strain-induced weakening of hydroxyl adsorption on the NT surfaces; the presence of an N dopant atom does not change the overpotential relative to the pristine nanotube, irrespective of the presence of a codoped S atom, while single S doping produces a slight decrease of the overpotential by 0.2 V. In all cases, the overpotential-determining step is the hydroxyl group dehydrogenation. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b01166
  • 2019 • 229 A review of the intrinsic ductility and toughness of hard transition-metal nitride alloy thin films
    Kindlund, H. and Sangiovanni, D.G. and Petrov, I. and Greene, J.E. and Hultman, L.
    Thin Solid Films 688 (2019)
    Over the past decades, enormous effort has been dedicated to enhancing the hardness of refractory ceramic materials. Typically, however, an increase in hardness is accompanied by an increase in brittleness, which can result in intergranular decohesion when materials are exposed to high stresses. In order to avoid brittle failure, in addition to providing high strength, films should also be ductile, i.e., tough. However, fundamental progress in obtaining hard-yet-ductile ceramics has been slow since most toughening approaches are based on empirical trial-and-error methods focusing on increasing the strength and ductility extrinsically, with a limited focus on understanding thin-film toughness as an inherent physical property of the material. Thus, electronic structure investigations focusing on the origins of ductility vs. brittleness are essential in understanding the physics behind obtaining both high strength and high plastic strain in ceramics films. Here, we review recent progress in experimental validation of density functional theory predictions on toughness enhancement in hard ceramic films, by increasing the valence electron concentration, using examples from the V1-xWxN and V1-xMoxN alloy systems. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137479
  • 2019 • 228 Molecule-Metal Bond of Alternant versus Nonalternant Aromatic Systems on Coinage Metal Surfaces: Naphthalene versus Azulene on Ag(111) and Cu(111)
    Klein, B.P. and Morbec, J.M. and Franke, M. and Greulich, K.K. and Sachs, M. and Parhizkar, S. and Bocquet, F.C. and Schmid, M. and Hall, S.J. and Maurer, R.J. and Meyer, B. and Tonner, R. and Kumpf, C. and Kratzer, P. and Gottfried, J.M.
    Journal of Physical Chemistry C 123 29219-29230 (2019)
    Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopies (UPS and XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 »Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b08824
  • 2019 • 227 Interaction-driven spin-orbit effects and Chern insulating phases in corundum-based 4d and 5d oxide honeycomb lattices
    Köksal, O. and Pentcheva, R.
    Journal of Physics and Chemistry of Solids 128 301-309 (2019)
    Using density functional theory calculations with a Hubbard U, we explore topologically nontrivial phases in X2O3 honeycomb layers with X= 4d and 5d cation inserted in the band insulator α-Al2O3 along the [0001]-direction. Several promising candidates for quantum anomalous Hall insulators (QAHI)are identified. In particular, for X = Tc and Pt spin-orbit coupling (SOC)opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI)with C = −2 and −1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: X = Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of Uc=2.5 eV. A comparison between the isoelectronic metastable FM phases of X = Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, X = Os turns out to be a FM Mott insulator with an unpaired electron in the t2g manifold where SOC induces an unusually high orbital moment of 0.34 μB along the z-axis. Parallels to the 3d honeycomb corundum cases are discussed. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jpcs.2018.01.049
  • 2019 • 226 Relaxation of electrons in quantum-confined states in Pb/Si(111) thin films from master equation with first-principles-derived rates
    Kratzer, P. and Zahedifar, M.
    New Journal of Physics 21 (2019)
    Atomically thin films of Pb on Si(111) provide an experimentally tunable system comprising a highly structured electronic density of states. The lifetime of excited electrons in these states is limited by both electron-electron (e-e) and electron-phonon (e-ph) scattering. We employ the description by a master equation for the electronic occupation numbers to analyze the relative importance of both scattering mechanisms. The electronic and phononic band structures, as well as the matrix elements for electron-phonon coupling within deformation potential theory were obtained from density functional calculations, thus taking into account quantum confinement effects. For the relaxation dynamics, the contribution of impact ionization processes to the lifetime is estimated from the imaginary part of the electronic self-energy calculated in the GW approximation. By numerically solving rate equations for the occupations of the Pb-derived electronic states coupled to a phononic heat bath, we are able to follow the distribution of the electronic excitation energy to the various modes of Pb lattice vibrations. While e-e scattering is the dominant relaxation mechanism, we demonstrate that the e-ph scattering is highly phonon-mode-specific, with a large contribution from surface phonons. At electron energies of about 0.3 eV above the Fermi surface, a 'phonon bottleneck' characteristic of relaxation in nanostructures with well-separated electronic states is observed. The time scales extracted from the simulations are compared to data from pump-probe experiments using time-resolved two-photon photoemission. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab5c76
  • 2019 • 225 Vibrational spectroscopic characterization of 2-(2,4-dinitrobenzyl)-pyridine (α-DNBP) in solution by polarization-resolved spontaneous raman scattering and broadband CARS
    Küpper, S. and Kumar, V. and Schlücker, S.
    Journal of Physical Chemistry A 123 6291-6297 (2019)
    The photochromic molecule 2-(2,4-dinitro-benzyl)-pyridine (α-DNBP) is characterized in solution by a combination of density functional theory employing a polarizable continuum model and polarization-resolved spontaneous and nonlinear Raman spectroscopies. By the comparison of theoretically predicted wavenumber positions and depolarization ratios with the experimental spectra acquired under electronically nonresonant conditions, polarized and depolarized Raman bands are assigned. Specifically, the symmetric stretching vibrations of the two nitro groups in ortho and para positions to the pyridine ring can be experimentally differentiated, mainly because of their different Raman depolarization ratios, which supports our prediction from theory. Compared to the polarization-resolved spontaneous Raman experiments, the vibrational spectroscopic differentiation of the two nitro groups is more pronounced in time-delayed polarization-resolved coherent anti-Stokes Raman scattering experiments. Overall, this linear and nonlinear vibrational spectroscopic characterization of the CH form paves the way for the interpretation of future time-resolved pump/nonlinear Raman probe studies on the ultrafast photoinduced intramolecular proton transfer in α-DNBP involving a nitro group as an intramolecular proton acceptor. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.9b05142
  • 2019 • 224 Engineering atomic-level complexity in high-entropy and complex concentrated alloys
    Oh, H.S. and Kim, S.J. and Odbadrakh, K. and Ryu, W.H. and Yoon, K.N. and Mu, S. and Körmann, F. and Ikeda, Y. and Tasan, C.C. and Raabe, D. and Egami, T. and Park, E.S.
    Nature Communications 10 (2019)
    Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10012-7
  • 2019 • 223 Elemental site occupancy in the L12 A3B ordered intermetallic phase in Co-based superalloys and its influence on the microstructure
    Pandey, P. and Makineni, S.K. and Samanta, A. and Sharma, A. and Das, S.M. and Nithin, B. and Srivastava, C. and Singh, A.K. and Raabe, D. and Gault, B. and Chattopadhyay, K.
    Acta Materialia 163 140-153 (2019)
    We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (KMo>1) into the γ matrix (KMo<1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.049
  • 2019 • 222 Oxygen vacancies and hydrogen doping in LaAlO3/SrTiO3 heterostructures: Electronic properties and impact on surface and interface reconstruction
    Piyanzina, I.I. and Eyert, V. and Lysogorskiy, Yu.V. and Tayurskii, D.A. and Kopp, T.
    Journal of Physics Condensed Matter 31 (2019)
    We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization. © 2019 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/1361-648X/ab1831
  • 2019 • 221 Surface structural phase transition induced by the formation of metal-organic networks on the Si(111) - In surface
    Suzuki, T. and Lawrence, J. and Morbec, J.M. and Kratzer, P. and Costantini, G.
    Nanoscale 11 21790-21798 (2019)
    We studied the adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Si(111)- √7 × √3-In surface, a known surface superconductor. Scanning tunneling microscopy shows the development of a surface-confined metal-organic network (SMON) where TCNQ molecules coordinate with indium atoms from the underlying √7 × √3 reconstruction. The formation of the SMON causes a surface structural phase transition from the √7 × √3 reconstruction to a previously unknown 5 × 5 reconstruction of the Si(111)-In surface. Scanning tunneling spectroscopy measurements indicate that the 5 × 5 reconstruction has a stronger insulating character than the √7 × √3 reconstruction. Density-functional-theory calculations are used to evaluate the atomic arrangement and stability of the 5 × 5 and √7 × √3 reconstructions as a function of In coverage, and suggest that the structural phase transition is driven by a slight reduction of the In coverage, caused by the incorporation of indium atoms into the SMON. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr07074e
  • 2019 • 220 Effect of confinement and octahedral rotations on the electronic, magnetic, and thermoelectric properties of correlated SrX O3/SrTiO3(001) superlattices (X= V, Cr, or Mn)
    Verma, M. and Geisler, B. and Pentcheva, R.
    Physical Review B 100 (2019)
    By using density functional theory calculations with an on-site Coulomb repulsion term combined with Boltzmann transport theory, we explore the effect of t2g orbital occupation on the electronic, magnetic, and thermoelectric properties of (SrXO3)1/(SrTiO3)n(001) superlattices with n=1,3 and X=V, Cr, and Mn. In order to disentangle the effect of quantum confinement and octahedral rotations and to account for a wider temperature range, P4/mmm (untilted) and P21/c (tilted) phases are considered. We find that the ground-state superlattice geometries always display finite octahedral rotations, which drive an orbital reconstruction and a concomitant metal-to-insulator transition in confined SrVO3 and SrCrO3 single layers with ferro- A nd antiferromagnetic spin alignments, respectively. On the other hand, the confined SrMnO3 single layer exhibits electronic properties similar to bulk. We show that confinement enhances the thermoelectric properties, particularly for SrVO3 and SrCrO3 due to the emergent Mott phase. Large room-temperature Seebeck coefficients are obtained for the tilted superlattices, ranging from 500 to 600μV/K near the band edges. The estimated attainable power factors of 27.9(26.6)μWK-2cm-1 in plane for the (SrCrO3)1/(SrTiO3)1(001) superlattice with P4/mmm(P21/c) symmetry and 28.1μWK-2cm-1 cross plane for the (SrMnO3)1/(SrTiO3)1(001) superlattice with P21/c symmetry compare favorably with some of the best-performing oxide thermoelectrics. This demonstrates that the idea to use quantum confinement to enhance the thermoelectric response in correlated transition-metal oxide superlattices [Phys. Rev. Mater. 2, 055403 (2018)2475-995310.1103/PhysRevMaterials.2.055403] can be applied to a broader class of materials combinations. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.165126
  • 2019 • 219 Thermodynamic assessment of the Co-Ta system
    Wang, P. and Koßmann, J. and Kattner, U.R. and Palumbo, M. and Hammerschmidt, T. and Olson, G.B.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 64 205-212 (2019)
    The Co-Ta system has been reviewed and the thermodynamic description was re-assessed in the present work. DFT (density functional theory) calculations considering spin polarization were performed to obtain the energies for all end-member configurations of the C14, C15, C36 and μ phases for the evaluation of the Gibbs energies of these phases. The phase diagram calculated with the present description agrees well with the experimental and theoretical data. Considering the DFT results was essential for giving a better description of the μ phase at lower temperatures. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2018.12.002
  • 2019 • 218 Epitaxial strain adaptation in chemically disordered FeRh thin films
    Witte, R. and Kruk, R. and Wang, D. and Schlabach, S. and Brand, R.A. and Gruner, M.E. and Wende, H. and Hahn, H.
    Physical Review B 99 (2019)
    Strain and strain adaptation mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaptation mechanism in a thin film model system as a function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g., epitaxial boundary condition. It is shown by means of high-resolution x-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response is explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a≈0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaptation behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.134109
  • 2019 • 217 Deciphering Charge Transfer and Electronic Polarization Effects at Gold Nanocatalysts on Reduced Titania Support
    Yoo, S.-H. and Siemer, N. and Todorova, M. and Marx, D. and Neugebauer, J.
    Journal of Physical Chemistry C 123 5495-5506 (2019)
    Gold nanoparticles supported on reduced TiO2 (110) surfaces are widely used as catalysts for oxidation reactions. Despite extensive studies, the role of oxygen vacancies in such systems remains elusive and is controversially discussed. Combining ab initio molecular dynamics simulations with methods originally developed to describe defects in semiconductor physics we study how the electronic charge originally located at the vacancy modifies the charge on the cluster. Despite differences resulting from the employed level of density functional theory (namely semilocal/GGA, GGA + U, and hybrid functionals), we consistently find that the Au clusters remain either neutral or acquire a positive charge. The intuitively expected electron transfer from the oxygen vacancy to the gold cluster can be safely ruled out. Analyzing these findings, we discuss the role of the oxygen vacancy in the bonding between Au clusters and support and the catalytic activity of the system. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b12015
  • 2018 • 216 Local dynamics of copper active sites in zeolite catalysts for selective catalytic reduction of NOx with NH3
    Chen, P. and Khetan, A. and Jabłońska, M. and Simböck, J. and Muhler, M. and Palkovits, R. and Pitsch, H. and Simon, U.
    Applied Catalysis B: Environmental 237 263-272 (2018)
    In Cu-zeolite based selective catalytic reduction of NOx with NH3 (NH3-SCR), Cu species (in particular CuI) solvated by NH3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH3-SCR reactivity at low temperatures (<250 °C). Direct experimental observation of the Cu mobility after NH3 solvation, however, has not been achieved yet. Here we show that complex impedance-based modulus spectroscopy, performed by following the corresponding dielectric relaxation processes at high frequencies (104 to 106 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH3-SCR related reaction conditions. Simultaneous in situ impedance and infrared spectroscopy studies, assisted by periodic DFT calculations with reliable van der Waals dispersion corrections, allowed us to identify the key factors determining the local dynamics of Cu ions in two representative Cu-zeolites, i.e. Cu-ZSM-5 and Cu-SAPO-34. The co-adsorption and interaction of NO and NH3 on CuII sites led to the formation of highly mobile CuI species and NH4+ intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of CuI, which is the rate-determining step of NH3-SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH4+ intermediate and Cu site promoting the formation of CuII-NO2/NH4+. As a result, the overall local dynamics of Cu, largely determined by CuI species, is less dependent on the NH4+ intermediate in Cu-SAPO-34 than in Cu-ZSM-5. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2018.05.091
  • 2018 • 215 Surface Termination and Composition Control of Activity of the CoxNi1- xFe2O4(001) Surface for Water Oxidation: Insights from DFT+ U Calculations
    Hajiyani, H. and Pentcheva, R.
    ACS Catalysis 8 11773-11782 (2018)
    Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the CoxNi1-xFe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b00574
  • 2018 • 214 Why Tin-Doping Enhances the Efficiency of Hematite Photoanodes for Water Splitting—The Full Picture
    Hufnagel, A.G. and Hajiyani, H. and Zhang, S. and Li, T. and Kasian, O. and Gault, B. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Scheu, C. and Pentcheva, R.
    Advanced Functional Materials 28 (2018)
    The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α-Fe 2 O 3 ) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe 2 O 3 and SnO x . Combined data from spectrophotometry and intensity-modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn-doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn-doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe 2 O 3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe 2 O 3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn-doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201804472
  • 2018 • 213 Proton Mobility, Intrinsic Acid Strength, and Acid Site Location in Zeolites Revealed by Varying Temperature Infrared Spectroscopy and Density Functional Theory Studies
    Losch, P. and Joshi, H.R. and Vozniuk, O. and Grünert, A. and Ochoa-Hernández, C. and Jabraoui, H. and Badawi, M. and Schmidt, W.
    Journal of the American Chemical Society 140 17790-17799 (2018)
    The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. < 40 kJ mol -1 ) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O-(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O-D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/jacs.8b11588
  • 2018 • 212 Fracture ab initio: A force-based scaling law for atomistically informed continuum models
    Möller, J.J. and Bitzek, E. and Janisch, R. and Ul Hassan, H. and Hartmaier, A.
    Journal of Materials Research 33 3750-3761 (2018)
    In fracture mechanics, established methods exist to model the stability of a crack tip or the kinetics of crack growth on both the atomic and the macroscopic scale. However, approaches to bridge the two scales still face the challenge in terms of directly converting the atomic forces at which bonds break into meaningful continuum mechanical failure stresses. Here we use two atomistic methods to investigate cleavage fracture of brittle materials: (i) we analyze the forces in front of a sharp crack and (ii) we study the bond breaking process during rigid body separation of half crystals without elastic relaxation. The comparison demonstrates the ability of the latter scheme, which is often used in ab initio density functional theory calculations, to model the bonding situation at a crack tip. Furthermore, we confirm the applicability of linear elastic fracture mechanics in the nanometer range close to crack tips in brittle materials. Based on these observations, a fracture mechanics model is developed to scale the critical atomic forces for bond breaking into relevant continuum mechanical quantities in the form of an atomistically informed scale-sensitive traction separation law. Such failure criteria can then be applied to describe fracture processes on larger length scales, e.g., in cohesive zone models or extended finite element models. Copyright © Materials Research Society 2018 This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (.
    view abstractdoi: 10.1557/jmr.2018.384
  • 2018 • 211 Single-atom vacancy in monolayer phosphorene: A comprehensive study of stability and magnetism under applied strain
    Morbec, J.M. and Rahman, G. and Kratzer, P.
    Journal of Magnetism and Magnetic Materials 465 546-553 (2018)
    Using first-principles calculations based on density-functional theory we systematically investigate the effect of applied strain on the stability and on the electronic and magnetic properties of monolayer phosphorene with single-atom vacancy. We consider two types of single vacancies: the symmetric SV-55|66, which has a metallic and non-magnetic ground state, and the asymmetric SV-5|9, which is energetically more favorable and exhibits a semiconducting and magnetic character. Our results show that compressive strain up to 10%, both biaxial and uniaxial along the zigzag direction, reduces the formation energy of both single-atom vacancies with respect to the pristine configuration and can stabilize these defects in phosphorene. We found that the magnetic moment of the SV-5|9 system is robust under uniaxial strain in the range of −10 to +10%, and it is only destroyed under biaxial compressive strain larger than 8%, when the system also suffers a semiconductor-to-metal transition. Additionally, we found that magnetism can be induced in the SV-55|66 system under uniaxial compressive strain larger than 4% along the zigzag direction and under biaxial tensile strain larger than 6%. Our findings of small formation energies and non-zero magnetic moments for both SV-5|9 and SV-55|66 systems under zigzag uniaxial compressive strain larger than 4% strongly suggest that a magnetic configuration in monolayer phosphorene can be easily realized by single-vacancy formation under uniaxial compressive strain. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.06.016
  • 2018 • 210 Synthesis of Furan-Annelated BINOL Derivatives: Acid-Catalyzed Cyclization Induces Partial Racemization
    Octa-Smolin, F. and Van Der Vight, F. and Yadav, R. and Bhangu, J. and Soloviova, K. and Wölper, C. and Daniliuc, C.G. and Strassert, C.A. and Somnitz, H. and Jansen, G. and Niemeyer, J.
    Journal of Organic Chemistry 83 14568-14587 (2018)
    In this account, we describe the synthesis of a series of BINOL-based bis- and trisphosphoric acids 11d/e/f, which commonly feature an unusual phosphoric acid monoester motif. This motif is generated by an acid-catalyzed 5-endo-dig cyclization of the 3-alkynyl-substituted BINOL precursors to give the corresponding Furan-annelated derivatives, followed by phosphorylation of the remaining phenolic alcohols. In the cyclization reaction, we observed an unexpected partial racemization in the bis- and tris-BINOL scaffolds, leading to mixtures of diastereomers that were separated and characterized spectroscopically and by X-ray crystal structure analyses. The cyclization and racemization processes were investigated both experimentally and by DFT-calculations, showing that although the cyclization proceeds faster, the barrier for the acid-catalyzed binaphthyl-racemization is only slightly higher. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.joc.8b02353
  • 2018 • 209 Nonlocal electron correlations in an itinerant ferromagnet
    Tusche, C. and Ellguth, M. and Feyer, V. and Krasyuk, A. and Wiemann, C. and Henk, J. and Schneider, C.M. and Kirschner, J.
    Nature Communications 9 (2018)
    Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. At the same time we observe regions of anomalously large “waterfall”-like band renormalization, previously only attributed to strong electron correlations in high-TC superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-05960-5
  • 2018 • 208 Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aab8b8
  • 2018 • 207 Conformational study of melamine crosslinkers and spectroscopical comparison of HMMM molecules by practical measurements and quantum chemical calculations
    Wysoglad, J. and Ehlers, J.-E. and Lewe, T. and Dornbusch, M. and Gutmann, J.S.
    Journal of Molecular Structure 1166 456-469 (2018)
    To understand and improve network formation processes and performance properties makes structural molecular studies critically important for the coil-coating industry. Crosslinking agents such as hexamethoxymethylmelamine (HMMM) or less methylated derivatives, e.g. methoxymethylmelamine (MMM), are often added to industrial coating formulations. For molecules with considerably fewer atoms and accordingly less rotational freedom (such as MMM) it is readily accessible to identify principal conformations. Thus, an MMM conformer study is straightforward and serves as orientation concerning HMMM conformer studies. For HMMM molecules an extensive computation method was developed to investigate the conformational distribution average and probably most likely molecular structures. Using the density functional theory (DFT) B3LYP-D3BJ method with Dunning's correlation basis set, calculations were performed to investigate the three-dimensional structural geometries of HMMM (basis set cc-pVDZ) and MMM (basis set cc-pVTZ). Beginning with 1500 conformations for HMMM and using various cut-off filters we focused on final residual 22 conformers for solvent phase calculations and 16 conformers for gas phase calculations. To the best of our knowledge, this is the first time that chemical properties for melamine crosslinkers were presented under consideration of conformational population distribution. Thus, computations of fully optimized structural geometries, energies and vibrational states indicate that preferred structural alignments for the methoxymethyl (MM) group in melamine molecules exist. Nevertheless, we suppose that as a matter of principle the MM group of MMM molecules can easily perform rotations by itself and all conformational structure geometries of MMM will exist. Concerning the MM groups position compared to triazine plane HMMM molecules have two main configurations,”3up-3down” and “4up-2down”, as dominant conformer species. Computations of weighted and averaged IR and Raman spectra of final conformations for HMMM monomers, dimers and trimers at ambient temperature are novel and in good agreement with experiment. Prediction of a final UV–Vis spectrum of top ten Boltzmann-weight and averaged monomeric HMMM conformers is in perfect agreement with practical measurement of commercially available HMMM. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.molstruc.2018.04.069
  • 2018 • 206 Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid
    Xu, G.-L. and Xiao, L. and Sheng, T. and Liu, J. and Hu, Y.-X. and Ma, T. and Amine, R. and Xie, Y. and Zhang, X. and Liu, Y. and Ren, Y. and Sun, C.-J. and Heald, S.M. and Kovacevic, J. and Sehlleier, Y.H. and Schulz, C. and Matt...
    Nano Letters 18 336-346 (2018)
    Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na+ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li+, K+, Mg2+, and Al3+ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b04193
  • 2017 • 205 Synthesis and Structure of a Dimeric Iminophosphorane Stabilized Zinc Carbene: (ZnCR2)2
    Bauer, H. and Orzechowski, L. and Escalona, A. and Jansen, G. and Harder, S.
    Organometallics 36 4883-4890 (2017)
    Single deprotonation of the bis(iminophosphorano)methane ligand (PhN=PPh2)2CH2 (5-H2) with 1 equiv of p-tBu-benzylpotassium followed by reaction with zinc(II) chloride led to the formation of the homoleptic complex ((PhN=PPh2)2CH)2Zn, (5-H)2Zn. Deprotonation of 5-H2 with 2 equiv of p-tBu-benzylpotassium gave known potassium compound (5-K2)2 which reacted further with zinc(II) chloride to the dimeric carbene complex [(PhN=PPh2)2CZn]2 ((5-Zn)2). Crystal structures of (5-H)2Zn and (5-Zn)2 are compared to those of closely related Ca and Mg compounds. To analyze the charge distribution of these zinc complexes, density functional theory calculations on simple model systems were employed. Although the Zn-C bond in (5-Zn)2 is highly ionic (81%) it is significantly more covalent than that in a similar dimeric Ca carbene complex (91% ionic). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.7b00755
  • 2017 • 204 Surface-Enhanced Raman Spectroscopy and Density Functional Theory Calculations of a Rationally Designed Rhodamine with Thiol Groups at the Xanthene Ring
    Brem, S. and Schlücker, S.
    Journal of Physical Chemistry C 121 15310-15317 (2017)
    Rhodamines are widely used dyes in fluorescence and surface-enhanced Raman spectroscopy (SERS). The latter requires adsorption of the dye onto the surface of plasmonic nanostructures, a process which requires attractive molecule-surface interactions. Here, we report an experimental SERS and computational density functional theory (DFT) study investigating the role of thiol functionalization at the xanthene ring of the rhodamine in the adsorption onto gold nanoparticles. For this purpose, a new bisthiolated rhodamine derivative was rationally designed and synthesized via a PPh3/I2 reduction route. The introduction of two thiol moieties directly at the xanthene ring provides the shortest possible distance between the molecular π-system and the metal surface for maximum SERS enhancement combined with the strong Au-S interaction for chemisorption. The comparison of experimental SERS spectra obtained from gold nanostars and a film of gold nanoparticles with results from DFT calculations (molecular electrostatic potential, normal modes) suggests adsorption via the thiol groups at the xanthene moiety. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.7b01504
  • 2017 • 203 Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 2988-2995 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700376
  • 2017 • 202 Experimental and Theoretical Understanding of Nitrogen-Doping-Induced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation
    Chen, P. and Khetan, A. and Yang, F. and Migunov, V. and Weide, P. and Stürmer, S.P. and Guo, P. and Kähler, K. and Xia, W. and Mayer, J. and Pitsch, H. and Simon, U. and Muhler, M.
    ACS Catalysis 7 1197-1206 (2017)
    By doping the TiO2 support with nitrogen, strong metal-support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain high-performance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance CO FTIR (CO-DRIFTS), N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. High-angle annular dark-field scanning transmission electron microscopy study of Pd on N-doped TiO2 confirmed a predominant presence of sub-2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO-DRIFTS revealed the formation of strongly coupled Pd-N species in Pd/TiO2 with N-doped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5, or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on N-doped TiO2 outperformed those on N-free TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02963
  • 2017 • 201 The Influence of Water on the Performance of Molybdenum Carbide Catalysts in Hydrodeoxygenation Reactions: A Combined Theoretical and Experimental Study
    Engelhardt, J. and Lyu, P. and Nachtigall, P. and Schüth, F. and García, Á.M.
    ChemCatChem 9 1985-1991 (2017)
    Understanding the deactivation of transition-metal carbide catalysts during hydrodeoxygenation (HDO) reactions is of great importance for improving the production of the second generation fuels from biomass. Based on a combined experimental and theoretical study, we present a mechanistic model for the deactivation of molybdenum carbide catalysts during phenol HDO in the presence of water. At increased water pressure, water molecules preferentially bind to the surface, and active sites are no longer accessible for phenol. In line with first principle calculations, experiments reveal that this process is fully reversible because the reduction of the water partial pressure results in a threefold increase in conversion. The direct deoxygenation of phenol was calculated to be the most favorable pathway, which is governed by the structure of the phenol adsorption complex on the surface at high hydrogen coverage. This is consistent with the experimentally observed high benzene selectivity (85 %) for phenol HDO over MoCx/HCS (hollow carbon spheres) catalyst. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700181
  • 2017 • 200 Imaging techniques: Nanoparticle atoms pinpointed
    Farle, M.
    Nature 542 35-36 (2017)
    The locations of atoms in a metallic alloy nanoparticle have been determined using a combination of electron microscopy and image simulation, revealing links between the particle's structure and magnetic properties.
    view abstractdoi: 10.1038/542035a
  • 2017 • 199 Nanoparticle atoms pinpointed
    Farle, M.
    Nature 542 35-36 (2017)
    doi: 10.1038/542035a
  • 2017 • 198 Rock Salt Ni/Co Oxides with Unusual Nanoscale-Stabilized Composition as Water Splitting Electrocatalysts
    Fominykh, K. and Tok, G.C. and Zeller, P. and Hajiyani, H. and Miller, T. and Döblinger, M. and Pentcheva, R. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 27 (2017)
    The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co1− x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co1− x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co1− x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201605121
  • 2017 • 197 Synthesis, structure and dispersion interactions in bis(1,8-naphthalendiyl)distibine
    Ganesamoorthy, C. and Heimann, S. and Hölscher, S. and Haack, R. and Wölper, C. and Jansen, G. and Schulz, S.
    Dalton Transactions 46 9227-9234 (2017)
    Naph2Sb21 was synthesized by a reaction of 1,8-dilithionaphthalene NaphLi2 with SbCl3 and its solid state structure is reported on. 1 shows intermolecular interactions in the solid state, which were studied by quantum chemical calculations with dispersion corrected density functional theory, supermolecular ab initio approaches and symmetry adapted perturbation theory. The same methods were employed to compare the solid state interactions in the crystal of 1 to those in real (for E = P) and hypothetical (for E = As and Bi) crystal structures of Naph2E2. Dispersion interactions were found to provide the most important stabilising contribution in all cases, seconded by electrostatic attraction between pnictogen atoms and π-systems of neighbouring naphthyl groups. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt02165h
  • 2017 • 196 Magnetic subunits within a single molecule-surface hybrid
    Heß, V. and Friedrich, R. and Matthes, F. and Caciuc, V. and Atodiresei, N. and Bürgler, D.E. and Blügel, S. and Schneider, C.M.
    New Journal of Physics 19 (2017)
    Magnetic molecule-surface hybrids are ideal building blocks for molecular spintronic devices due to their appealing tailorable magnetic properties and nanoscale size. So far, assemblies of interacting molecular-surface hybrids needed for spintronic functionality were generated by depositing aromatic molecules onto transition-metal surfaces, resulting in a random arrangement of hybrid magnets due to the inherent and strong hybridization. Here, we demonstrate the formation of multiple intramolecular subunits within a single molecule-surface hybrid by means of spin-polarized scanning tunneling microscopy experiments and ab initio density functional theory calculations. This novel effect is realized by depositing a polycyclic aromatic molecule on a magnetic surface. A highly asymmetric chiral adsorption position induces different structural, electronic, and magnetic properties in each aromatic ring of the molecule. In particular, the induced molecular spin polarization near the Fermi energy varies among the rings due to site- and spin-dependent molecule-surface hybridization. Our results showcase a possible organic chemistry route of tailoring geometrically well-defined assemblies of magnetically distinguishable subunits in molecule-surface hybrids. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa6ece
  • 2017 • 195 Crystal Structure Induced Preferential Surface Alloying of Sb on Wurtzite/Zinc Blende GaAs Nanowires
    Hjort, M. and Kratzer, P. and Lehmann, S. and Patel, S.J. and Dick, K.A. and Palmstrøm, C.J. and Timm, R. and Mikkelsen, A.
    Nano Letters 17 3634-3640 (2017)
    We study the surface diffusion and alloying of Sb into GaAs nanowires (NWs) with controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal phases. Using atomically resolved scanning tunneling microscopy, we find that Sb preferentially incorporates into the surface layer of the {110}-terminated Zb segments rather than the {1120}-terminated Wz segments. Density functional theory calculations verify the higher surface incorporation rate into the Zb phase and find that it is related to differences in the energy barrier of the Sb-for-As exchange reaction on the two surfaces. These findings demonstrate a simple processing-free route to compositional engineering at the monolayer level along NWs. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b00806
  • 2017 • 194 Modelling of grain boundary dynamics using amplitude equations
    Hüter, C. and Neugebauer, J. and Boussinot, G. and Svendsen, B. and Prahl, U. and Spatschek, R.
    Continuum Mechanics and Thermodynamics 29 895-911 (2017)
    We discuss the modelling of grain boundary dynamics within an amplitude equations description, which is derived from classical density functional theory or the phase field crystal model. The relation between the conditions for periodicity of the system and coincidence site lattices at grain boundaries is investigated. Within the amplitude equations framework, we recover predictions of the geometrical model by Cahn and Taylor for coupled grain boundary motion, and find both (Formula presented.) and (Formula presented.) coupling. No spontaneous transition between these modes occurs due to restrictions related to the rotational invariance of the amplitude equations. Grain rotation due to coupled motion is also in agreement with theoretical predictions. Whereas linear elasticity is correctly captured by the amplitude equations model, open questions remain for the case of nonlinear deformations. © 2015 Springer-Verlag Berlin Heidelberg
    view abstractdoi: 10.1007/s00161-015-0424-7
  • 2017 • 193 The shear instability energy: A new parameter for materials design?
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Modelling and Simulation in Materials Science and Engineering 25 (2017)
    Reliable and predictive relationships between fundamental microstructural material properties and observable macroscopic mechanical behaviour are needed for the successful design of new materials. In this study we establish a link between physical properties that are defined on the atomic level and the deformation mechanisms of slip planes and interfaces that govern the mechanical behaviour of a metallic material. To accomplish this, the shear instability energy Γ is introduced, which can be determined via quantum mechanical ab initio calculations or other atomistic methods. The concept is based on a multilayer generalised stacking fault energy calculation and can be applied to distinguish the different shear deformation mechanisms occurring at TiAl interfaces during finite-temperature molecular dynamics simulations. We use the new parameter Γ to construct a deformation mechanism map for different interfaces occurring in this intermetallic. Furthermore, Γ can be used to convert the results of ab initio density functional theory calculations into those obtained with an embedded atom method type potential for TiAl. We propose to include this new physical parameter into material databases to apply it for the design of materials and microstructures, which so far mainly relies on single-crystal values for the unstable and stable stacking fault energy. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aa865a
  • 2017 • 192 Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
    Kositzki, R. and Mebs, S. and Schüth, N. and Leidel, N. and Schwartz, L. and Karnahl, M. and Wittkamp, F. and Daunke, D. and Grohmann, A. and Apfel, U.-P. and Gloaguen, F. and Ott, S. and Haumann, M.
    Dalton Transactions 46 12544-12557 (2017)
    Synthetic diiron compounds of the general formula Fe2(μ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kβ main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(i) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt02720f
  • 2017 • 191 Detection of adsorbed transition-metal porphyrins by spin-dependent conductance of graphene nanoribbon
    Kratzer, P. and Tawfik, S.A. and Cui, X.Y. and Stampfl, C.
    RSC Advances 7 29112-29121 (2017)
    Electronic transport in a zig-zag-edge graphene nanoribbon (GNR) and its modification by adsorbed transition metal porphyrins is studied by means of density functional theory calculations. The detachment reaction of the metal centre of the porphyrin is investigated both in the gas phase and for molecules adsorbed on the GNR. As most metal porphyrins are very stable against this reaction, it is found that these molecules bind only weakly to a perfect nanoribbon. However, interaction with a single-atom vacancy in the GNR results in chemical bonding by the transition metal centre being shared between nitrogen atoms in the porphyrin ring and the carbon atoms next to the vacancy in the GNR. For both the physisorbed and the chemisorbed geometry, the inclusion of van der Waals interaction results in a significant enlargement of the binding energy and reduction of the adsorption height. Electronic transport calculations using non-equilibrium Greens functions show that the conductivity of the GNR is altered by the chemisorbed porphyrin molecules. Since the metal centers of porphyrins carry an element-specific magnetic moment, not only the net conductance, but also the spin-dependent conductance of the GNR is affected. In particular, the adsorption of Ru-porphyrin on the single-atom vacancy results in a very large spin polarization of the current of 88% at small applied source-drain voltages. Based on our results, we suggest that a spin valve constructed from a GNR with ferromagnetic contacts could be used as a sensitive detector that could discriminate between various metal porphyrins. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra04594h
  • 2017 • 190 Crystal structures of Fe4C vs. Fe4N analysed by DFT calculations: Fcc-based interstitial superstructures explored
    Leineweber, A. and Hickel, T. and Azimi-Manavi, B. and Maisel, S.B.
    Acta Materialia 140 433-442 (2017)
    Knowledge of the thermodynamic and structural properties of iron carbide and nitride phases is crucial for understanding phase transformations and related microstructure formation in steels. While the existence and crystal structure of the primitive cubic fcc-based γ′-Fe4N1-z phase is experimentally well-established, there is no consensus in contemporary literature about an analogous γ′-Fe4C compound. Here, we present DFT calculations for all fcc-like Fe4C and Fe4N superstructures with up to two formula units per primitive unit cell, providing energy values and the relaxed atomic structures, which were analysed mathematically and by visual inspection of the atomic arrangement. Notably, all considered Fe4C and Fe4N superstructures are metastable with respect to α-Fe and cementite-Fe3C/ε-Fe3N. Unsurprisingly, we find the well-known γ′ compound's crystal structure to be most favourable among these metastable Fe4N superstructures. However, we find the equivalent superstructure to be quite unfavourable in Fe4C. The most favourable among these metastable Fe4C structures are stabilised by a partial Bain-like distortion into the direction of a body-centred cubic arrangement of Fe atoms. This makes the particular C-ordering interesting for comparison with the short-range order in Fe-C martensites. However, even the lowest-energy Fe4C structure releases about 0.056 eV/atom upon decomposition into α + Fe3C, much more than it is the case for Fe4N (0.019 eV/atom). That energy difference is difficult to overcome even at T > 0 K, in agreement with the lack of clear experimental evidence for existence of a Fe4C phase. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.08.059
  • 2017 • 189 Electronic structure of metastable bcc Cu-Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations
    Liebscher, C.H. and Freysoldt, C. and Dennenwaldt, T. and Harzer, T.P. and Dehm, G.
    Ultramicroscopy 178 96-104 (2017)
    Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for (110)-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2016.07.011
  • 2017 • 188 Oxygen activity and peroxide formation as charge compensation mechanisms in Li2MnO3
    Marusczyk, A. and Albina, J.-M. and Hammerschmidt, T. and Drautz, R. and Eckl, T. and Henkelman, G.
    Journal of Materials Chemistry A 5 15183-15190 (2017)
    In the search for high energy density battery materials, over-lithiated transition metal oxides have attracted the attention of many researchers worldwide. There is, however, no consensus regarding the underlying mechanisms that give rise to the large capacities and also cause the electrochemical degradation upon cycling. As a key component and prototype phase, Li2MnO3 is investigated using density functional theory. Our calculations show that hole doping into the oxygen bands is the primary charge compensation mechanism in the first stage of delithiation. Upon further delithiation, there is an energetic driving force for peroxide formation with an optimal number of peroxide dimers that is predicted as a function of lithium concentration. Unlike the defect-free phases, the peroxide structures are highly stable, which leads to two competing mechanisms for charge compensation: (i) oxygen loss and densification at the surface and (ii) peroxide formation in the bulk. Our results show that both have a detrimental effect on the electrochemical performance and therefore the stabilization of oxygen in the crystal lattice is vital for the development of high energy cathode materials. The insights into the origin and implications of peroxide formation open the door for a more profound understanding of the degradation mechanism and how to counteract it. © The Royal Society of Chemistry 2017.
    view abstractdoi: 10.1039/c7ta04164k
  • 2017 • 187 Thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition
    Mikula, M. and Sangiovanni, D.G. and Plašienka, D. and Roch, T. and Čaplovičová, M. and Truchlý, M. and Satrapinskyy, L. and Bystrický, R. and Tonhauzerová, D. and Vlčková, D. and Kúš, P.
    Journal of Applied Physics 121 (2017)
    We combine experiments and ab initio density functional theory calculations to investigate the evolution in structural and mechanical properties of TaAlN coatings as a function of the annealing temperature T. Formation of coherent cubic TaN- and AlN-rich nanometer-size domains, occurring during the initial stage of thermally induced phase separation within cubic NaCl-type (B1) TaAlN solid solutions, yields a monotonic increase in hardness from 29 GPa (as deposited coatings) up to a maximum of 35 GPa (+17%) reached after annealing at 1000 °C. Further thermal treatment at T > 1000 °C leads to the transformation of metastable cubic domains into stable hexagonal TaNx and wurtzite AlN phases, thus resulting in hardness reductions. A comparison of our results with those reported in the literature reveals that TaAlN coatings are at least as hard while considerably less stiff (lower elastic moduli) than TiAlN coatings, thus indicating a substantial increase in toughness achieved upon replacing Ti with Ta in the host lattice. Present findings suggest that cubic TaAlN solid solutions are promising candidates for applications in protective coatings possessing both high-temperature hardness and toughness. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4981534
  • 2017 • 186 Functional Mechanically Interlocked Molecules: Asymmetric Organocatalysis with a Catenated Bifunctional Brønsted Acid
    Mitra, R. and Zhu, H. and Grimme, S. and Niemeyer, J.
    Angewandte Chemie - International Edition 56 11456-11459 (2017)
    Interlocked molecules, such as catenanes, rotaxanes, and molecular knots, have become interesting candidates for the development of sophisticated chemical catalysts. Herein, we report the first application of a catenane-based catalyst in asymmetric organocatalysis, revealing that the catenated catalyst shows dramatically increased stereoselectivities (up to 98 % ee) in comparison to its non-interlocked analogues. A mechanistic rationale for the observed differences was developed by DFT studies, suggesting that the involvement of two catalytically active groups in the stereodetermining reaction step is responsible for the superior selectivity of the interlocked catalyst. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201704647
  • 2017 • 185 The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface
    Morbec, J.M. and Kratzer, P.
    Journal of Chemical Physics 146 (2017)
    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that “pure” DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973839
  • 2017 • 184 Correlative plasma-surface model for metastable Cr-Al-N: Frenkel pair formation and influence of the stress state on the elastic properties
    Music, D. and Banko, L. and Ruess, H. and Engels, M. and Hecimovic, A. and Grochla, D. and Rogalla, D. and Brögelmann, T. and Ludwig, Al. and Von Keudell, A. and Bobzin, K. and Schneider, J.M.
    Journal of Applied Physics 121 (2017)
    Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm-3 at room temperature and Young's modulus of 358-130 GPa in the temperature range of 50-700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Young's modulus. The hypothesis of a stress-induced Young's modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4985172
  • 2017 • 183 Identification of a ternary μ-phase in the Co-Ti-W system – An advanced correlative thin-film and bulk combinatorial materials investigation
    Naujoks, D. and Eggeler, Y.M. and Hallensleben, P. and Frenzel, J. and Fries, S.G. and Palumbo, M. and Koßmann, J. and Hammerschmidt, T. and Pfetzing-Micklich, J. and Eggeler, G. and Spiecker, E. and Drautz, R. and Ludwig, Al.
    Acta Materialia 138 100-110 (2017)
    The formation of a ternary μ-phase is documented for the system Co-Ti-W. The relevant compositional stability range is identified by high-throughput energy dispersive X-ray spectroscopy, electrical resistance and X-ray diffraction maps from a thin-film materials library (1 μm thickness). Bulk samples of the identified compositions were fabricated to allow for correlative film and bulk studies. Using analytical scanning and transmission electron microscopy, we demonstrate that in both, thin film and bulk samples, the D85 phase (μ-phase) coexists with the C36-phase and the A2-phase at comparable average chemical compositions. Young's moduli and hardness values of the μ-phase and the C36-phase were determined by nanoindentation. The trends of experimentally obtained elastic moduli are consistent with density functional theory (DFT) calculations. DFT analysis also supports the experimental findings, that the μ-phase can solve up to 18 at.% Ti. Based on the experimental and DFT results it is shown that CALPHAD modeling can be modified to account for the new findings. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.07.037
  • 2017 • 182 Zn-VI quasiparticle gaps and optical spectra from many-body calculations
    Riefer, A. and Weber, N. and Mund, J. and Yakovlev, D.R. and Bayer, M. and Schindlmayr, A. and Meier, C. and Schmidt, W.G.
    Journal of Physics Condensed Matter 29 (2017)
    The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0 W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn-VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe-Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa6b2a
  • 2017 • 181 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 180 Magnetic properties of nanolaminated (Mo0.5Mn0.5)2GaC MAX phase
    Salikhov, R. and Meshkian, R. and Weller, D. and Zingsem, B. and Spoddig, D. and Lu, J. and Ingason, A.S. and Zhang, H. and Rosen, J. and Wiedwald, U. and Farle, M.
    Journal of Applied Physics 121 (2017)
    The magnetic properties of hexagonal (Mo0.5Mn0.5)2GaC MAX phase synthesized as epitaxial films on MgO (111) substrates with the c-axis perpendicular to the film plane are presented. The analysis of temperature-dependent ferromagnetic resonance (FMR) and magnetometry data reveals a ferro- to paramagnetic phase transition at 220 K. The electrical transport measurements at 5 K show a negative magnetoresistance of 6% in a magnetic field of 9 T. Further analysis confirms the spin-dependent scattering of charge carriers in this layered material. A small perpendicular (c-axis) magnetocrystalline anisotropy energy density (MAE) of 4.5 kJ/m3 at 100 K was found using FMR. Accordingly, (Mo0.5Mn0.5)2GaC behaves similar to the (Cr0.5Mn0.5)2GaC MAX phase as a soft magnetic material. The density functional theory calculations reveal that the sign and the amplitude of the MAE can be very sensitive to (Mo0.5Mn0.5)2GaC lattice parameters, which may explain the measured soft magnetic properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4982197
  • 2017 • 179 Ultra-stiff metallic glasses through bond energy density design
    Schnabel, V. and Köhler, M. and Music, D. and Bednarcik, J. and Clegg, W.J. and Raabe, D. and Schneider, J.M.
    Journal of Physics Condensed Matter 29 (2017)
    The elastic properties of crystalline metals scale with their valence electron density. Similar observations have been made for metallic glasses. However, for metallic glasses where covalent bonding predominates, such as metalloid metallic glasses, this relationship appears to break down. At present, the reasons for this are not understood. Using high energy x-ray diffraction analysis of melt spun and thin film metallic glasses combined with density functional theory based molecular dynamics simulations, we show that the physical origin of the ultrahigh stiffness in both metalloid and non-metalloid metallic glasses is best understood in terms of the bond energy density. Using the bond energy density as novel materials design criterion for ultra-stiff metallic glasses, we are able to predict a Co33.0Ta3.5B63.5 short range ordered material by density functional theory based molecular dynamics simulations with a high bond energy density of 0.94 eV Å-3 and a bulk modulus of 263 GPa, which is 17% greater than the stiffest Co-B based metallic glasses reported in literature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa72cb
  • 2017 • 178 Development of a stochastic approach for fatigue life prediction of AlSi12 alloy processed by selective laser melting
    Siddique, S. and Awd, M. and Tenkamp, J. and Walther, F.
    Engineering Failure Analysis 79 34-50 (2017)
    Parts manufactured by selective laser melting (SLM) process possess unique features in terms of surface roughness, microstructure, residual stresses as well as defect distribution. These defects are responsible for failure of the parts in functional applications. When fatigue loading is applied, these defects are the dominant cause of crack initiation, resulting in scatter of fatigue properties. This scatter occurs due to interacting phenomena like defect size, location as well as the magnitude and type of load. For the purpose of investigating the effect of defects on fatigue life performance of AlSi12 manufactured by selective laser melting, a procedure was developed based on the weakest-link theory and Weibull's probability density function. Using various destructive and non-destructive techniques, defects, including remnant porosity and surface roughness, have been characterized in amount, size and location. Therefore fatigue life prediction, relying on equations constituted from crack propagation properties, was carried out. Predicted fatigue life and Weibull's statistical parameters were used to compare the effect of both defect types on fatigue reliability of AlSi12 produced by SLM. The most probable fatigue life for a sample was interpreted based on Weibull probability density function with respect to maximum probability of occurrence. The prediction of numerous possible values enabled an estimation of fatigue scatter to be made. Thus, the findings of this novel approach enabled conclusions about strength and reliability of different SLM AlSi12 configurations and gave a prelude towards application-oriented design of SLM components. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.engfailanal.2017.03.015
  • 2017 • 177 Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis
    Song, H. and Laudenschleger, D. and Carey, J.J. and Ruland, H. and Nolan, M. and Muhler, M.
    ACS Catalysis 7 7610-7622 (2017)
    A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260-300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn-Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn-Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10-10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01822
  • 2017 • 176 Bi-Axial Growth Mode of Au-TTF Nanowires Induced by Tilted Molecular Column Stacking
    Xing, Y. and Speiser, E. and Singh, D.K. and Dittrich, P.S. and Esser, N.
    Journal of Physical Chemistry C 121 23200-23206 (2017)
    In this study, to understand the molecular self-organization in metal-organic charge-transfer nanowires, single gold-tetrathiafulvalene (Au-TTF) nanowires were analyzed using polarized Raman spectroscopy, combined with density functional theory (DFT) calculations. To verify the methodology, an investigation was done for neutral tetrathiafulvalene (TTF) bulk crystals with well-known structure. On the basis of the DFT calculation of the molecular Raman tensor and simulation of the angular-dependent depolarization ratio, the molecular orientation in single TTF crystals was verified. Thereon, the combined experimental and ab initio-simulation method was applied to study single Au-TTF nanowires. Our results clearly demonstrate, in contrast to the commonly accepted parallel molecular stacking model, that at least two molecules with different orientations are located in the unit cell of the nanowire's crystal structure. The new tilted molecular column stacking wire model explains also the axial and radial growth mechanism of Au-TTF wires. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.7b05924
  • 2017 • 175 Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hicke...
    Journal of the American Chemical Society 139 14360-14363 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07902
  • 2016 • 174 The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/38/385001
  • 2016 • 173 Molecular structure of diethylaminoalane in the solid state: An X-ray powder diffraction, DFT calculation and Raman spectroscopy study
    Bernert, T. and Ley, M.B. and Ruiz-Fuertes, J. and Fischer, M. and Felderhoff, M. and Weidenthaler, C.
    Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials 72 232-240 (2016)
    The crystal structure of diethylaminoalane, [H2Al - N(C2H5)2]2, was determined by X-ray powder diffraction in conjunction with DFT calculations. Diethylaminoalane crystallizes in the monoclinic space group P21/c with a = 7.4020 (2), b = 12.9663 (3), c = 7.2878 (2) Å and β = 90.660 (2)° at 293 K. The crystal structure was confirmed by DFT calculations and Raman spectroscopy. The molecular structure of diethylaminoalane consists of dimers of [H2Al - N(CH2CH3)2] in which an Al2N2 four-membered ring is formed by a center of inversion. Such an arrangement of the aminoalane moieties in the crystal structure is well known for this class of compound, as shown by the comparison with ethylmethylaminoalane and diisopropylaminoalane.The crystal structure of diethylaminoalane, [H2Al - N(C2H5)2]2, was determined by X-ray powder diffraction, geometry optimization by density functional theory (DFT) and Raman spectroscopy. The DFT calculations were validated by calculating the ground state structures of two known aminoalanes while the Raman spectrum of diethylaminoalane was measured and compared to the simulated ones. Furthermore, the crystal structure of diethylaminoalane is compared with chemically and structurally similar compounds. © International Union of Crystallography, 2016.
    view abstractdoi: 10.1107/S2052520616000093
  • 2016 • 172 Three-Parameter Crystal-Structure Prediction for sp-d-Valent Compounds
    Bialon, A.F. and Hammerschmidt, T. and Drautz, R.
    Chemistry of Materials 28 2550-2556 (2016)
    We present a three-dimensional structure-map based on experimental data for compounds that contain sp-block elements and transition metals. The map predicts the correct crystal structure with a probability of 86% and has a confidence of better than 98% that the correct crystal structure is among three predicted crystal structures. The three descriptors of the structure map are physically intuitive functions of the number of valence electrons, atomic volume, and electronegativity of the constituent elements. We test the structure map against standard density-functional theory calculations for 1:1 sp-d-valent compounds and show that our three-parameter model has a comparable predictive power. We demonstrate the application of the structure map in conjunction with density-functional theory calculations. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b04299
  • 2016 • 171 Incomplete Bilayer Termination of the Ice (0001) Surface
    Bockstedte, M. and Michl, A. and Kolb, M. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physical Chemistry C 120 1097-1109 (2016)
    The complete bilayer is commonly considered as the termination of the (0001) surface of hexagonal ice. Experiments on thin crystalline ice structures grown on Cu(111) demonstrated a termination by admolecule structures on top of the bilayer. Modeling of complex admolecule terminations including admolecule clusters and decorated hexagon adrows within density functional theory and high-resolution STM imaging are combined for the structural analysis and to reveal possible causes for the apparent distinction. A dominant admolecule structure that appears during a short anneal at 130 K is identified as an arrangement of water dimer and trimers. By the combined approach, detailed models for decorated hexagon adrows are derived. Such structures possess low energy; however, the proton-ordered bilayer is more favorable at a small margin. Yet, energetically unfavorable bonding of water, for example, in thin ice films may drive the formation of admolecule terminations, for which kinetic effects still are an important factor. The results also shine light on the edge termination of bilayer islands. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b10836
  • 2016 • 170 Spin-hybrids: A single-molecule approach to spintronics
    Bürgler, D.E. and Heß, V. and Esat, T. and Fahrendorf, S. and Matthes, F. and Schneider, C. M. and Besson, C. and Monakhov, K.Y. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Atodiresei, N. and Caciuc, V. and Blügel, S.
    e-Journal of Surface Science and Nanotechnology 14 17-22 (2016)
    Molecular spintronics aims at exploiting and controlling spin-dependent transport processes at the molecular level. Achieving this aim requires not only appropriate molecules, molecular structures and preparation procedures. Equally important is the understanding and engineering of the electronic and spin-dependent interactions between different molecular species, molecule and substrate, as well as molecule and electrodes. These interactions may not only determine the spin-dependent functionality of the molecular structures, but also their integrity on the substrate. Likewise, there may be also a modification of the surface properties below and in the vicinity of a molecule. We have investigated several molecules on different metallic surfaces, among them magnetic Nd doubledecker phthalocyanines, a cubane-type {Ni4} complex with single-molecule magnet properties, and a nonmagnetic triazine-based molecule. For NdPc2 molecules adsorbed on a Cu(100) surface, our scanning tunneling microscopy and spectroscopy studies show specific electronic states of the molecule-substrate complex. We find that the electric field between STM tip and sample must be taken into account to properly describe the electronic states associated with the upper Pc ligand. © 2016 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2016.17
  • 2016 • 169 Interface effects on the magnetic properties of layered Ni2MnGa/Ni2MnSn alloys: A first-principles investigation
    Dutta, B. and Opahle, I. and Hickel, T.
    Functional Materials Letters 9 (2016)
    The effect of interfaces on the magnetic properties of multilayers is analyzed forNi2MnGa/Ni2MnSn system using density functional theory. The Ni spin moments at the interface change by about 30% compared to the bulk value, whereas the effect on the Mn spin moments is much less pronounced. A similar strong effect is also observed for the Ni orbital moments at the interface. The magneto-crystalline anisotropy of the multilayer systems can be understood by the additive contribution of the respective values of strained bulk materials. © 2016 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793604716420108
  • 2016 • 168 Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
    Eiteneer, D. and Pálsson, G.K. and Nemšák, S. and Gray, A.X. and Kaiser, A.M. and Son, J. and LeBeau, J. and Conti, G. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Rotenberg, E. and Gulli...
    Journal of Electron Spectroscopy and Related Phenomena 211 70-81 (2016)
    LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2016.04.008
  • 2016 • 167 Thermoelectric Properties of Half-Heusler Heterostructures from Ab Initio Calculations
    Fiedler, G. and Kratzer, P.
    Journal of Electronic Materials 45 1762-1766 (2016)
    Semiconducting half-Heusler alloys have recently emerged as a class of thermoelectric materials with outstanding performance in the medium- to high-temperature range. Heterostructures promise a further reduction of thermal conductivity as a result of phonon scattering at interfaces. Here, both the electronic and phononic spectra of half-Heusler compounds based on Ti, Zr, and Hf are calculated using density functional theory. With this input, thermoelectric properties are obtained, and the thermal conductivity of a heterostructure superlattice is estimated by extending the diffuse mismatch model of interface conductance. We find that a high power factor (Formula presented.) can be retained in a short-period superlattice, while thermal conductivity is reduced compared to that in single-phase half-Heusler crystals. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11664-015-4205-7
  • 2016 • 166 Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals
    Gehlmann, M. and Aguilera, I. and Bihlmayer, G. and Młyńczak, E. and Eschbach, M. and Döring, S. and Gospodarič, P. and Cramm, S. and Kardynał, B. and Plucinski, L. and Blügel, S. and Schneider, C.M.
    Scientific Reports 6 (2016)
    Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.
    view abstractdoi: 10.1038/srep26197
  • 2016 • 165 From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 311-319 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.059
  • 2016 • 164 The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy
    Kim, J.-K. and Ko, W.-S. and Sandlöbes, S. and Heidelmann, M. and Grabowski, B. and Raabe, D.
    Acta Materialia 112 171-183 (2016)
    We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500°C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density-functional-theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance - quantified by the number of α-Mg layers between them - at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the α-Mg matrix is: single building block → various metastable LPSO building block clusters → 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R→ various metastable LPSO building block clusters → 14H. The thermodynamically most stable structures in both the α-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and α-Mg. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.016
  • 2016 • 163 Influence of magnetic excitations on the phase stability of metals and steels
    Körmann, F. and Hickel, T. and Neugebauer, J.
    Current Opinion in Solid State and Materials Science 20 77-84 (2016)
    Within this article we highlight recent advances in the modeling of magnetic contributions to the finite temperature phase stability of structural materials. A key quantity in this context is the specific heat capacity Cp, since it provides a sensitive link to thermophysical and calorimetric experiments and to established thermodynamic databases. For iron-based materials, the Heisenberg model and its extensions are used as an elegant way for coupling ground-state ab initio calculations with concepts of many-body theory to simulate the temperature dependence. Besides analytical concepts to derive the free energy of the Heisenberg model, our work is mainly devoted to numerical approaches such as Monte-Carlo methods. In particular, we highlight the need to go beyond a classical to a fully quantum-mechanical description of magnetic excitations. In order to achieve a quantitative description of Cp, also lattice and electronic degrees of freedom as well as their dependence on magnetism are addressed. Due to the large variety of experimental data, pure iron is best suited to discuss the method developments and to perform evaluations. Nevertheless, the application to other magnetic elements (e.g. Co, Ni) and Fe-based materials (e.g. Fe3C) will also be addressed. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cossms.2015.06.001
  • 2016 • 162 Importance of inclusion of the effect of s electrons into bond-order potentials for transition bcc metals with d-band mediated bonding
    Lin, Y.-S. and Mrovec, M. and Vitek, V.
    Modelling and Simulation in Materials Science and Engineering 24 (2016)
    In bond-order potentials (BOPs) for transition metals only the bonding mediated by the d electrons is included explicitly and the covalent part of the cohesive energy is evaluated using Slater-Koster dd bond integrals. However, the effect of s electrons with orbitals centered on atoms neighboring the corresponding dd bond is not necessarily negligible. As shown in Nguyen-Manh et al (2000 Phys. Rev. Lett. 85 4136) this can be taken into account via screening of the dd bond integrals. In a recent paper (Lin et al 2014 Model. Simul. Mater. Sci. Eng. 22 034002) the dd bond integrals were determined using a projection scheme utilizing atomic orbitals that give the best representation of the electronic wave functions in the calculations based on the density functional theory (DFT) (Madsen et al 2011 Phys. Rev. B 83 4119) and it was inferred that in this case the effect of s electrons was already included. In this paper we analyze this hypothesis by comparing studies employing BOPs with both unscreened and screened dd bond integrals. In all cases results are compared with calculations based on DFT and/or experiments. Studies of structures alternate to the bcc lattice, transformation paths that connect the bcc structure with fcc, simple cubic (sc), body centered tetragonal (bct) and hcp structures via continuously distorted configurations and calculations of γ-surfaces were all found to be insensitive to the screening of bond integrals. On the other hand, when the bond integrals are screened, formation energies of vacancies are improved and calculated phonon dispersion spectra reproduce the experimentally observed ones much better. Most importantly, dislocation core structure and dislocation glide are significantly different without and with screening of dd bond integrals. The latter lead to a much better agreement with available experiments. These findings suggest that the effect of s electrons on dd bonds, emulated by the screening of corresponding bond integrals, is the least significant when the lattice is distorted away from the ideal bcc structure homogeneously even if such distortion is large. On the other hand, when the distortion is local and inhomogeneous the impact of screening of the dd bond integrals is significant. In the studies presented in this paper such local inhomogeneities occur when phonons propagate through the lattice, at point defects and in the cores of dislocations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/8/085001
  • 2016 • 161 Ab initio calculations of doped TiO2 anatase (101) nanotubes for photocatalytical water splitting applications
    Lisovski, O. and Chesnokov, A. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Materials Science in Semiconductor Processing 42 138-141 (2016)
    TiO2 (titania) is one of the promising materials for photocatalytic applications. In this paper we report on recently obtained theoretical results for N and S doped, as well as N+S co-doped 6-layer (101) anatase nanotube (NT). First principles calculations in our study have been performed using a modified B3LYP hybrid exchange-correlation functional within density functional theory (DFT). Here we discuss the energy of defect formation mechanism and electronic band structure for nanotubes under study. We also report on influence of dopant concentration on the NT's band structure and discuss the defect-defect interactions. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.mssp.2015.09.003
  • 2016 • 160 Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates
    Marocchi, S. and Candini, A. and Klar, D. and Van Den Heuvel, W. and Huang, H. and Troiani, F. and Corradini, V. and Biagi, R. and De Renzi, V. and Klyatskaya, S. and Kummer, K. and Brookes, N.B. and Ruben, M. and Wende, H. and De...
    ACS Nano 10 9353-9360 (2016)
    We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.6b04107
  • 2016 • 159 Toughness enhancement in highly NbN-alloyed Ti-Al-N hard coatings
    Mikula, M. and Plašienka, D. and Sangiovanni, D.G. and Sahul, M. and Roch, T. and Truchlý, M. and Gregor, M. and Čaplovič, L. and Plecenik, A. and Kúš, P.
    Acta Materialia 121 59-67 (2016)
    Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (>25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness, ∼32.5 ± 2 GPa, and the highest Young's modulus, ∼442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to ∼358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.08.084
  • 2016 • 158 Dioxygen binding to Fe-MOF-74: Microscopic insights from periodic QM/MM calculations
    Moeljadi, A.M.P. and Schmid, R. and Hirao, H.
    Canadian Journal of Chemistry 94 1144-1150 (2016)
    Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal-organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O2 and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O2-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O2-binding site are replaced with other metal ions (Mg2+, Ni2+, Zn2+, Co2+, or Mn2+), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O2 binding indirectly. © 2016 Published by NRC Research Press.
    view abstractdoi: 10.1139/cjc-2016-0284
  • 2016 • 157 Enhancement of the Superconducting Gap by Nesting in CaKFe4As4: A New High Temperature Superconductor
    Mou, D. and Kong, T. and Meier, W.R. and Lochner, F. and Wang, L.-L. and Lin, Q. and Wu, Y. and Bud'Ko, S.L. and Eremin, I. and Johnson, D.D. and Canfield, P.C. and Kaminski, A.
    Physical Review Letters 117 (2016)
    We use high resolution angle resolved photoemission spectroscopy and density functional theory with measured crystal structure parameters to study the electronic properties of CaKFe4As4. In contrast to the related CaFe2As2 compounds, CaKFe4As4 has a high Tc of 35 K at stochiometric composition. This presents a unique opportunity to study the properties of high temperature superconductivity in the iron arsenides in the absence of doping or substitution. The Fermi surface consists of several hole and electron pockets that have a range of diameters. We find that the values of the superconducting gap are nearly isotropic (within the explored portions of the Brillouin zone), but are significantly different for each of the Fermi surface (FS) sheets. Most importantly, we find that the momentum dependence of the gap magnitude plotted across the entire Brillouin zone displays a strong deviation from the simple cos(kx)cos(ky) functional form of the gap function, proposed by the scenario of Cooper pairing driven by a short range antiferromagnetic exchange interaction. Instead, the maximum value of the gap is observed on FS sheets that are closest to the ideal nesting condition, in contrast to previous observations in other ferropnictides. These results provide strong support for the multiband character of superconductivity in CaKFe4As4, in which Cooper pairing forms on the electron and the hole bands interacting via a dominant interband repulsive interaction, enhanced by band nesting. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.117.277001
  • 2016 • 156 Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires
    Nematollahi, Gh.A. and Grabowski, B. and Raabe, D. and Neugebauer, J.
    Acta Materialia 111 321-334 (2016)
    A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.052
  • 2016 • 155 Lattice Distortions in the FeCoNiCrMn High Entropy Alloy Studied by Theory and Experiment
    Oh, H. S. and Ma, D. and Leyson, G. P. and Grabowski, B. and Park, E. S. and Kormann, F. and Raabe, D.
    Entropy 18 321 (2016)
    Lattice distortions constitute one of the main features characterizing high entropy alloys. Local lattice distortions have, however, only rarely been investigated in these multi-component alloys. We, therefore, employ a combined theoretical electronic structure and experimental approach to study the atomistic distortions in the FeCoNiCrMn high entropy (Cantor) alloy by means of density-functional theory and extended X-ray absorption fine structure spectroscopy. Particular attention is paid to element-resolved distortions for each constituent. The individual mean distortions are small on average, <1%, but their fluctuations (i.e., standard deviations) are an order of magnitude larger, in particular for Cr and Mn. Good agreement between theory and experiment is found.
    view abstractdoi: 10.3390/e18090321
  • 2016 • 154 First principles characterisation of brittle transgranular fracture of titanium hydrides
    Olsson, P.A.T. and Mrovec, M. and Kroon, M.
    Acta Materialia 118 362-373 (2016)
    In this work we have studied transgranular cleavage and the fracture toughness of titanium hydrides by means of quantum mechanical calculations based on density functional theory. The calculations show that the surface energy decreases and the unstable stacking fault energy increases with increasing hydrogen content. This is consistent with experimental findings of brittle behaviour of titanium hydrides at low temperatures. Based on Griffith-Irwin theory we estimate the fracture toughness of the hydrides to be of the order of 1 MPa⋅m1/2, which concurs well with experimental data. To investigate the cleavage energetics, we analyse the decohesion at various crystallographic planes and determine the traction-separation laws based on the Rose's extended universal binding energy relation. The calculations predict that the peak stresses do not depend on the hydrogen content of the phases, but it is rather dependent on the crystallographic cleavage direction. However, it is found that the work of fracture decreases with increasing hydrogen content, which is an indication of hydrogen induced bond weakening in the material. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.037
  • 2016 • 153 Zero-dimensional (CH3NH3)3Bi2I9 perovskite for optoelectronic applications
    Öz, S. and Hebig, J.-C. and Jung, E. and Singh, T. and Lepcha, A. and Olthof, S. and Jan, F. and Gao, Y. and German, R. and van Loosdrecht, P.H.M. and Meerholz, K. and Kirchartz, T. and Mathur, S.
    Solar Energy Materials and Solar Cells 158 195-201 (2016)
    We present the preparation and characterization of solution processed lead free films of zero-dimensional methylammonium iodo bismuthate (CH3NH3)3Bi2I9 perovskite. Structural characterization reveals the formation of hexagonal micro-crystals with preferred growth orientation along the c-axis. The material exhibits a wide band gap of 2.9eV and upon optical excitation the photoluminescence emission is observed at 1.65eV (751nm). Photoelectron spectroscopy confirms the stoichiometry of the (CH3NH3)3Bi2I9 perovskite and yields an ionization energy of 6.24eV, while Raman spectra confirm the vibrational modes of the bioctahedral inorganic anion framework (Bi2I9)3- in the low wavenumber regime (< 200cm-1) regime. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) are employed to evaluate the experimental results. We apply this novel bismuth-based hybrid perovskite in proof of principle simple heterojunction solar cell devices, which yield power conversion efficiencies of ~0.1%. Further enhancement is expected once the film morphology and device stack are optimized. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2016.01.035
  • 2016 • 152 Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics
    Senger, M. and Mebs, S. and Duan, J. and Wittkamp, F. and Apfel, U.-P. and Heberle, J. and Haumann, M. and Stripp, S.T.
    Proceedings of the National Academy of Sciences of the United States of America 113 8454-8459 (2016)
    The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient H2-forming catalyst in nature. It comprises a diiron active site with three carbonmonoxide (CO) and two cyanide (CN-) ligands in the active oxidized state (Hox) and one additional CO ligand in the inhibited state (Hox-CO). The diatomic ligands are sensitive reporter groups for structural changes of the cofactor. Their vibrational dynamics were monitored by real- Time attenuated total reflection Fouriertransform infrared spectroscopy. Combination of 13CO gas exposure, blue or red light irradiation, and controlled hydration of three different [FeFe]-hydrogenase proteins produced 8 Hox and 16 Hox-CO species with all possible isotopic exchange patterns. Extensive density functional theory calculations revealed the vibrational mode couplings of the carbonyl ligands and uniquely assigned each infrared spectrum to a specific labeling pattern. For Hox-CO, agreement between experimental and calculated infrared frequencies improved by up to one order of magnitude for an apical CN- At the distal iron ion of the cofactor as opposed to an apical CO. For Hox, two equally probable isomers with partially rotated ligands were suggested. Interconversion between these structures implies dynamic ligand reorientation at the H-cluster. Our experimental protocol for site-selective 13CO isotope editing combined with computational species assignment opens new perspectives for characterization of functional intermediates in the catalytic cycle.
    view abstractdoi: 10.1073/pnas.1606178113
  • 2016 • 151 Atomistically informed extended Gibbs energy description for phase-field simulation of tempering of martensitic steel
    Shchyglo, O. and Hammerschmidt, T. and Čak, M. and Drautz, R. and Steinbach, I.
    Materials 9 (2016)
    In this study we propose a unified multi-scale chemo-mechanical description of the BCT (Body-Centered Tetragonal) to BCC (Body-Centered Cubic) order-disorder transition in martensitic steel by adding the mechanical degrees of freedom to the standard CALPHAD (CALculation of PHAse Diagrams) type Gibbs energy description. The model takes into account external strain, the effect of carbon composition on the lattice parameter and elastic moduli. The carbon composition effect on the lattice parameters and elastic constants is described by a sublattice model with properties obtained from DFT (Density Functional Theory) calculations; the temperature dependence of the elasticity parameters is estimated from available experimental data. This formalism is crucial for studying the kinetics of martensite tempering in realistic microstructures. The obtained extended Gibbs energy description opens the way to phase-field simulations of tempering of martensitic steel comprising microstructure evolution, carbon diffusion and lattice symmetry change due to the ordering/disordering of carbon atoms under multiaxial load. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9080669
  • 2016 • 150 Complexity analysis of simulations with analytic bond-order potentials
    Teijeiro, C. and Hammerschmidt, T. and Seiser, B. and Drautz, R. and Sutmann, G.
    Modelling and Simulation in Materials Science and Engineering 24 (2016)
    The modeling of materials at the atomistic level with interatomic potentials requires a reliable description of different bonding situations and relevant system properties. For this purpose, analytic bond-order potentials (BOPs) provide a systematic and robust approximation to density functional theory (DFT) and tight binding (TB) calculations at reasonable computational cost. This paper presents a formal analysis of the computational complexity of analytic BOP simulations, based on a detailed assessment of the most computationally intensive parts. Different implementation algorithms are presented alongside with optimizations for efficient numerical processing. The theoretical complexity study is complemented by systematic benchmarks of the scalability of the algorithms with increasing system size and accuracy level of the BOP approximation. Both approaches demonstrate that the computation of atomic forces in analytic BOPs can be performed with a similar scaling as the computation of atomic energies. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/2/025008
  • 2016 • 149 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstractdoi: 10.1038/ncomms10990
  • 2016 • 148 Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 229-238 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.007
  • 2015 • 147 Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100)
    Antczak, G. and Kamiński, W. and Sabik, A. and Zaum, C. and Morgenstern, K.
    Journal of the American Chemical Society 137 14920-14929 (2015)
    We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b08001
  • 2015 • 146 The Interaction of Formic Acid with Zinc Oxide: A Combined Experimental and Theoretical Study on Single Crystal and Powder Samples
    Buchholz, M. and Li, Q. and Noei, H. and Nefedov, A. and Wang, Y. and Muhler, M. and Fink, K. and Wöll, C.
    Topics in Catalysis 58 174-183 (2015)
    We present azimuth- and polarization-dependent infrared spectroscopy results obtained under ultra-high vacuum conditions on surface species formed by the interaction of formic acid with the mixed-terminated ZnO(101¯0) surface. Since there are no previous IRRAS data for formic-acid derived species on any ZnO single crystal surfaces, we have carried out calculations using density function theory to aid the interpretation of the results. From our combined experimental and theoretical data we conclude that two different formate species are formed. The more strongly bound species is a bidentate with the formate molecular plane oriented along the [12¯10] direction. The less strongly bound species is a quasi-bidentate with its molecular plane oriented along the [0001] direction. This second species is characterized by a strong hydrogen bond between a surface OH species and the formate. In addition, IR data were recorded for the same molecule adsorbed on commercial ZnO nanoparticles. The different bands of the powder IR-data are assigned on the basis of the experimental and theoretical results obtained for the single crystal surface. This study demonstrates the importance of the Surface Science approach to heterogeneous catalysis also for ZnO, an important catalyst for the conversion of syngas to methanol. © 2014 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11244-014-0356-7
  • 2015 • 145 Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
    Calle-Vallejo, F. and Tymoczko, J. and Colic, V. and Vu, Q.H. and Pohl, M.D. and Morgenstern, K. and Loffreda, D. and Sautet, P. and Schuhmann, W. and Bandarenka, A.S.
    Science 350 185-189 (2015)
    A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.
    view abstractdoi: 10.1126/science.aab3501
  • 2015 • 144 Martensitic transformation between competing phases in Ti-Ta alloys: A solid-state nudged elastic band study
    Chakraborty, T. and Rogal, J. and Drautz, R.
    Journal of Physics Condensed Matter 27 (2015)
    A combined density functional theory and solid-state nudged elastic band study is presented to investigate the martensitic transformation between β → (α″, ω) phases in the Ti-Ta system. The minimum energy paths along the transformation are calculated and the transformation mechanisms as well as relative stabilities of the different phases are discussed for various compositions. The analysis of the transformation paths is complemented by calculations of phonon spectra to determine the dynamical stability of the β, α ″, and ω phase. Our theoretical results confirm the experimental findings that with increasing Ta concentration there is a competition between the destabilisation of the α ″ and ω phase and the stabilisation of the high-temperature β phase. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/11/115401
  • 2015 • 143 Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications
    Chesnokov, A. and Lisovski, O. and Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Physica Scripta 90 (2015)
    In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO<inf>2</inf> nanotubes, undoped, doped, and codoped with N<inf>O</inf> and S<inf>O</inf> atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree-Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO<inf>2</inf> nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen 2p character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O<inf>2</inf>/H<inf>2</inf>O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level. © 2015 The Royal Swedish Academy of Sciences.
    view abstractdoi: 10.1088/0031-8949/90/9/094013
  • 2015 • 142 First-principles investigation of hydrogen trapping and diffusion at grain boundaries in nickel
    Di Stefano, D. and Mrovec, M. and Elsässer, C.
    Acta Materialia 98 306-312 (2015)
    Abstract In this work, the interaction of hydrogen with high-angle GBs in nickel has been investigated by means of density functional theory simulations. Two distinct types of GBs have been considered: the Σ3(111)[1¯10] with a close-packed interface structure and the Σ5(210)[001] with a less dense interface structure consisting of open structural units. Our calculations reveal that these two GBs have a markedly different interaction behavior with atomic hydrogen. The close-packed Σ3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the Σ5 GB provides numerous trapping sites for H within the open structural units as well as easy migration pathways for H diffusion along the GB plane that can enhance the H diffusivity by about two orders of magnitude compared to bulk Ni. The obtained results are analysed in detail and compared with available experimental and other theoretical data. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.031
  • 2015 • 141 Bond-order potentials: Derivation and parameterization for refractory elements
    Drautz, R. and Hammerschmidt, T. and Čák, M. and Pettifor, D.G.
    Modelling and Simulation in Materials Science and Engineering 23 (2015)
    The bond-order potentials are derived from density functional theory by a systematic coarse graining of the electronic structure. Within their functional form the bond-order potentials comprise covalent bond formation, charge transfer and magnetism. We review the derivation of the bond-order potentials from density functional theory and discuss their application to the simulation of refractory transition metals. We show that the derived functional form of the bond-order potentials ensures the transferability of the potentials to atomic environments that have not been taken into account in the parameterization. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/23/7/074004
  • 2015 • 140 The dynamics of TiNx (x = 1-3) admolecule interlayer and intralayer transport on TiN/TiN(001) islands
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Thin Solid Films 589 133-144 (2015)
    It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiN<inf>x</inf> complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiN<inf>x</inf> (x = 1-3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN<inf>2</inf> admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN<inf>2</inf> migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN<inf>2</inf> trimers descend exclusively by hopping. In contrast, TiN<inf>3</inf> admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.05.013
  • 2015 • 139 Non-collinear magnetism with analytic Bond-Order Potentials
    Ford, M.E. and Pettifor, D.G. and Drautz, R.
    Journal of Physics Condensed Matter 27 (2015)
    The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/8/086002
  • 2015 • 138 On the effect of alloy composition on martensite start temperatures and latent heats in Ni-Ti-based shape memory alloys
    Frenzel, J. and Wieczorek, A. and Opahle, I. and Maaß, B. and Drautz, R. and Eggeler, G.
    Acta Materialia 90 213-231 (2015)
    In the present work we explain the concentration dependence of the martensite start temperature (MS) in Ni-Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of MS on alloy composition in binary Ni-Ti, ternary Ni-Ti-X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni-Ti-Cu-Y (Y = Co, Pd) SMAs. For binary Ni-Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation ΔH decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite GA(T) and martensite GM(T), which in turn results in a lower MS temperature. Our DFT results suggest that the strong decrease of ΔH is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19′. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni-Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.02.029
  • 2015 • 137 Understanding anharmonicity in fcc materials: From its origin to ab initio strategies beyond the quasiharmonic approximation
    Glensk, A. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Physical Review Letters 114 (2015)
    We derive the Gibbs energy including the anharmonic contribution due to phonon-phonon interactions for an extensive set of unary fcc metals (Al, Ag, Au, Cu, Ir, Ni, Pb, Pd, Pt, Rh) by combining density-functional-theory (DFT) calculations with efficient statistical sampling approaches. We show that the anharmonicity of the macroscopic system can be traced back to the anharmonicity in local pairwise interactions. Using this insight, we derive and benchmark a highly efficient approach which allows the computation of anharmonic contributions using a few T=0K DFT calculations only. © Published by the American Physical Society 2015.
    view abstractdoi: 10.1103/PhysRevLett.114.195901
  • 2015 • 136 Element-resolved thermodynamics of magnetocaloric lafe13-xsix
    Gruner, M.E. and Keune, W. and Roldan Cuenya, B. and Weis, C. and Landers, J. and Makarov, S.I. and Klar, D. and Hu, M.Y. and Alp, E.E. and Zhao, J. and Krautz, M. and Gutfleisch, O. and Wende, H.
    Physical Review Letters 114 (2015)
    By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13-xSix. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.057202
  • 2015 • 135 Consecutive mechanism in the diffusion of D2O on a NaCl(100) bilayer
    Heidorn, S.-C. and Bertram, C. and Cabrera-Sanfelix, P. and Morgenstern, K.
    ACS Nano 9 3572-3578 (2015)
    The motion of D<inf>2</inf>O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy. The diffusion distance histogram reveals a squared diffusion lattice that agrees with the primitive unit cell of the (100) surface. From the Arrhenius dependence, we derive the diffusion energy, the pre-exponential factor, and the attempt frequency. The mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b00691
  • 2015 • 134 Manganese tetraboride, MnB4: High-temperature crystal structure, p-n transition, 55Mn NMR spectroscopy, solid solutions, and mechanical properties
    Knappschneider, A. and Litterscheid, C. and Brgoch, J. and George, N.C. and Henke, S.c and Cheetham, A.K. and Hu, J.G. and Seshadri, R. and Albert, B.
    Chemistry - A European Journal 21 8177-8181 (2015)
    The structural and electronic properties of MnB<inf>4</inf> were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P2<inf>1</inf>/c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. 55Mn NMR measurements revealed an isotropic chemical shift of -1315ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr<inf>1-x</inf>Mn<inf>x</inf>B<inf>4</inf> (two phases in space groups Pnnm and P2<inf>1</inf>/c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7)GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37GPa obtained by DFT calculations. Phase transition: Monoclinic manganese tetraboride can be transformed into an orthorhombic phase thermally or by forming solid solutions with chromium tetraboride. The structural phase transition of semiconducting MnB<inf>4</inf> is accompanied by a p-n transition. 55Mn NMR spectroscopy confirmed the oxidation state I of the metal atom, and nanoindentation experiments resulted in hardness values that are in accordance with DFT calculations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201406631
  • 2015 • 133 Solubility and ordering of Ti, Ta, Mo and W on the Al sublattice in L12-Co3Al
    Koßmann, J. and Hammerschmidt, T. and Maisel, S. and Müller, S. and Drautz, R.
    Intermetallics 64 44-50 (2015)
    Co-Al-W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L1<inf>2</inf>-Co<inf>3</inf>(Al<inf>1-x</inf>W<inf>x</inf>) precipitates, which may form even though binary Co<inf>3</inf>Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L1<inf>2</inf>-Co<inf>3</inf>Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40-80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2015.04.009
  • 2015 • 132 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 131 Structural stability and Lewis acidity of tetravalent Ti, Sn, or Zr-linked interlayer-expanded zeolite COE-4: A DFT study
    Li, H. and Wang, J. and Zhou, D. and Tian, D. and Shi, C. and Müller, U. and Feyen, M. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Zhang, W.
    Microporous and Mesoporous Materials 218 160-166 (2015)
    Density functional theory (DFT) has been performed to characterize the structural stability and Lewis acidic properties of the T-COE-4 zeolites, in which the linked site between the layers is isomorphously substituted by the tetravalent Ti-, Sn-, or Zr- heteroatom. The effects of substitution energy and equilibrium geometry parameters on the stability of T-COE-4 are investigated. The computed Fukui function values and the adsorption of ammonia, pyridine, water and trimethylphosphine oxide molecules have been employed to predict the Lewis acid strength of the T-COE-4 zeolites. It is found that the smaller the O1-T-O2 bond angle is, the more difficult is to form the regular tetrahedral unit. The substitution energies at the linker position increase in the following sequence: Ti-COE-4 < Sn-COE-4 < Zr-COE-4. The incorporation of Ti-, Sn-, or Zr-heteroatom enhances the Lewis acidity of COE-4 zeolite. It is predicted that the Lewis acid strength increases in the order of Ti-COE-4 < Zr-COE-4 & Sn-COE-4 by the adsorption of different base molecules. Six O-T-O bond angles are divided into different extent to form the analogous trigonal bipyramid structures in the optimized ligand adsorbed complexes. These findings could be beneficial for the structural design and catalytic function modification of the interlayer-expanded zeolites. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2015.07.020
  • 2015 • 130 Bonding situation in Be[N(SiMe3)2]2-an experimental and computational study
    Naglav, D. and Neumann, A. and Bläser, D. and Wölper, C. and Haack, R. and Jansen, G. and Schulz, S.
    Chemical Communications 51 3889-3891 (2015)
    The solid state structure of Be[N(SiMe3)2]2 (1) was determined by in situ crystallisation and the bonding situation investigated by quantum chemical calculations. The Be-N bond is predominantly ionic, but some evidence for the presence of a partial Be-N double bond character was found. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4cc09732g
  • 2015 • 129 Interplanar potential for tension-shear coupling at grain boundaries derived from ab initio calculations
    Pang, X.Y. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 24 (2015)
    Based on ab initio density functional theory (DFT) calculations we derive an analytical expression for the interplanar potential of grain boundaries and single crystals as a function of coupled tensile and shear displacements. This energy function captures even details of the grain boundary behaviour, such as the tension-softening of the shear instability of aluminium grain boundaries, with good accuracy. The good agreement between the analytical model and the DFT calculations is achieved by introducing two new characteristic parameters, namely the position of the generalised unstable stacking fault with respect to the stable stacking fault, and the ratio of stable and unstable generalised stacking fault energies. One of the potentials' parameters also serves as a criterion to judge if a grain boundary deforms via crack propagation or dislocation nucleation. We suggest this potential function for application in continuum models, where constitutive relationships for grain boundaries need to be derived from a sound physical model. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/1/015007
  • 2015 • 128 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 822-829 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.255
  • 2015 • 127 Detection of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases in co-evaporated Cu2ZnSnSe4 thin-films
    Schwarz, T. and Marques, M.A.L. and Botti, S. and Mousel, M. and Redinger, A. and Siebentritt, S. and Cojocaru-Mirédin, O. and Raabe, D. and Choi, P.-P.
    Applied Physics Letters 107 (2015)
    Cu2ZnSnSe4 thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu2ZnSnSe4 and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases. Possible effects of these phases on solar cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4934847
  • 2015 • 126 Multiferroic grain boundaries in oxygen-deficient ferroelectric lead titanate
    Shimada, T. and Wang, J. and Ueda, T. and Uratani, Y. and Arisue, K. and Mrovec, M. and Elsä Sser, C. and Kitamura, T.
    Nano Letters 15 27-33 (2015)
    Ultimately thin multiferroics arouse remarkable interest, motivated by the diverse utility of coexisting ferroelectric and (anti)ferromagnetic order parameters for novel functional device paradigms. However, the ferroic order is inevitably destroyed below a critical size of several nanometers. Here, we demonstrate a new path toward realization of atomically thin multiferroic monolayers while resolving a controversial origin for unexpected "-dilute ferromagnetism" emerged in nanocrystals of nonmagnetic ferroelectrics PbTiO3. The state-of-the-art hybrid functional of Hartree-Fock and density functional theories successfully identifies the origin and underlying physics; oxygen vacancies interacting with grain boundaries (GBs) bring about (anti)ferromagnetism with localized spin moments at the neighboring Ti atoms. This is due to spin-polarized defect states with broken orbital symmetries at GBs. In addition, the energetics of oxygen vacancies indicates their self-assembling nature at GBs resulting in considerably high concentration, which convert the oxygen-deficient GBs into multiferroic monolayers due to their atomically thin interfacial structure. This synthetic concept that realizes multiferroic and multifunctional oxides in a monolayered geometry through the self-assembly of atomic defects and grain boundary engineering opens a new avenue for promising paradigms of novel functional devices. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nl502471a
  • 2015 • 125 First-Principles Calculations of Magnetic Properties of Cr-Doped Ni45Co5Mn37In13 Heusler Alloys
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The magnetic and electronic properties of Co- and Cr-doped Ni50Mn37In13 Heusler alloys with a substitution of 5 at.% Co for Ni and 5 at.% Cr for Ni, Mn, or In are investigated in the framework of the density functional theory method. The chemical disorder in the off-stoichiometric Ni-Co-Mn-Cr-In systems was treated in the coherent potential approximation. Three different ferrimagnetic and one ferromagnetic (FM) spin states for austenite and martensite were considered in ab initio calculations. It is found that for both structures, the intersublattice interactions (MnY(Z)-Co, MnY(Z)-Ni, MnY(Z)-MnZ(Y), MnY(Z)-Cr, and Cr-Co) provide the largest contribution to the exchange due to the shorter distance compared with the intrasublattice interactions (MnY(Z)-MnY(Z), Co-Co, Ni-Ni, and Cr-Cr). Besides, the MnY-MnZ and MnY(Z)-Cr exchanges in the first shell become five times larger in martensite compared with austenite. The largest anti-FM interaction is observed between MnY(Z)-Cr atoms in martensite. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2439391
  • 2015 • 124 Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
    Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 179 469-474 (2015)
    Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.02.110
  • 2015 • 123 Extreme flexibility in a zeolitic imidazolate framework: Porous to dense phase transition in desolvated ZIF-4
    Wharmby, M.T. and Henke, S. and Bennett, T.D. and Bajpe, S.R. and Schwedler, I. and Thompson, S.P. and Gozzo, F. and Simoncic, P. and Mellot-Draznieks, C. and Tao, H. and Yue, Y. and Cheetham, A.K.
    Angewandte Chemie - International Edition 54 6447-6451 (2015)
    Abstract Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23% contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K. ZIF-4(Zn) undergoes a porous to non-porous transition on cooling from the high-temperature (HT) to low-temperature (LT) phase. The nature of this transition is elucidated by a combined approach of structure solution from powder diffraction, DSC measurement, and DFT calculations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201410167
  • 2015 • 122 On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys
    Wollgramm, P. and Buck, H. and Neuking, K. and Parsa, A.B. and Schuwalow, S. and Rogal, J. and Drautz, R. and Eggeler, G.
    Materials Science and Engineering A 628 382-395 (2015)
    In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni-Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.010
  • 2014 • 121 Structural and electronic properties of micellar Au nanoparticles: Size and ligand effects
    Behafarid, F. and Matos, J. and Hong, S. and Zhang, L. and Rahman, T.S. and Roldan Cuenya, B.
    ACS Nano 8 6671-6681 (2014)
    Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn406568b
  • 2014 • 120 Ferromagnetic exchange coupling between Fe phthalocyanine and Ni(111) surface mediated by the extended states of graphene
    Candini, A. and Bellini, V. and Klar, D. and Corradini, V. and Biagi, R. and De Renzi, V. and Kummer, K. and Brookes, N.B. and Del Pennino, U. and Wende, H. and Affronte, M.
    Journal of Physical Chemistry C 118 17670-17676 (2014)
    The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular dichroism, the field-dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8 K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp5041663
  • 2014 • 119 Negatively charged ions on Mg(0001) surfaces: Appearance and origin of attractive adsorbate-adsorbate interactions
    Cheng, S.-T. and Todorova, M. and Freysoldt, C. and Neugebauer, J.
    Physical Review Letters 113 (2014)
    Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.136102
  • 2014 • 118 Molecular spintronics: Topology communicates
    Cinchetti, M.
    9 965-966 (2014)
    doi: 10.1038/nnano.2014.284
  • 2014 • 117 Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
    Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 35 695-700 (2014)
    The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.
    view abstractdoi: 10.1007/s11669-014-0342-6
  • 2014 • 116 Interacting magnetic cluster-spin glasses and strain glasses in Ni-Mn based Heusler structured intermetallics
    Entel, P. and Gruner, M.E. and Comtesse, D. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Physica Status Solidi (B) Basic Research 251 2135-2148 (2014)
    Magnetic Ni-Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni-Co-Mn-Sn on the right-hand side). The cubic austenitic phase at high temperature shows long-range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non-modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long-range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non-magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape-memory and exchange-bias effects as well as magnetocaloric effects. The 'ferroic cooling' is of particular relevance since it avoids the use of ozone-depleting and greenhouse chemicals compared with conventional fluid-compression technology. Experimental phase diagram of Ni50-x Cox Mn39 Sn11 for 0≤x≤10. Here, TC is the Curie temperature of austenite; at TM the system transforms to paramagnetic martensite and at TS to superparamagnetic martensite (SPM) and then to superspin-glass martensite (SSG) at TP. The possible strain-glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field-cooling/zero-field-cooling experiments. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201451059
  • 2014 • 115 Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2330845
  • 2014 • 114 Magnetoelastic coupling and the formation of adaptive martensite in magnetic shape memory alloys
    Gruner, M.E. and Fähler, S. and Entel, P.
    Physica Status Solidi (B) Basic Research 251 2067-2079 (2014)
    Reviewing the results of recent first-principles calculations, we work out a close analogy between the two paradigmatic classes of magnetic shape memory materials, the ordered Ni2 MnGa Heusler compound and the disordered Fe70 Pd30 alloy. Despite fundamental differences between both systems, we can demonstrate that in both cases the very low formation energy for tetragonal twins on the smallest length scale opens an alternative transformation path into an adaptive hierarchical microstructure which is important for the functional behavior. The low energy of the (101) twin boundary corresponds to a shear instability which is associated to the soft transversal acoustic phonon in both systems. In turn, changes of the energy landscape upon magnetic disorder are responsible for the stability of austenite. This points out the strong influence of magnetoelastic coupling on the transformation process. Nanotwinned adaptive microstructures in Ni2 MnGa and Fe68 Pd32 magnetic shape memory alloys obtained from first-principles calculations. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201350397
  • 2014 • 113 Interplay of hydrogen treatment and nitrogen doping in ZnO nanoparticles: A first-principles study
    Gutjahr, J. and Sakong, S. and Kratzer, P.
    Nanotechnology 25 (2014)
    With the help of density functional calculations using the HSE and PBE functionals, it is shown that incorporation of nitrogen into ZnO nanoparticles is energetically less costly compared to ZnO bulk, due to charge transfer between Zn dangling bonds and the NO impurity. Neutral NO results after full passivation of the doped nanoparticles by a treatment with atomic hydrogen. A nanocomposite made from such ZnO particles could show thermally activated p-type hopping conductivity. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/14/145204
  • 2014 • 112 Electronic and structural differences between wurtzite and zinc blende inas nanowire surfaces: Experiment and theory
    Hjort, M. and Lehmann, S. and Knutsson, J. and Zakharov, A.A. and Du, Y.A. and Sakong, S. and Timm, R. and Nylund, G. and Lundgren, E. and Kratzer, P. and Dick, K.A. and Mikkelsen, A.
    ACS Nano 8 12346-12355 (2014)
    We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn504795v
  • 2014 • 111 Symmetry-adapted perturbation theory based on density functional theory for noncovalent interactions
    Jansen, G.
    Wiley Interdisciplinary Reviews: Computational Molecular Science 4 127-144 (2014)
    The combination of symmetry-adapted perturbation theory (SAPT) of intermolecular interactions with a density functional theory (DFT) description of the underlying molecular properties, known as DFT-SAPT or SAPT(DFT), is reviewed, with a focus on methodology. A theoretical formalism avoiding an overlap expansion and the single-exchange approximation for the second-order exchange contributions is presented, and ways to include higher order contributions are discussed. The influence of the exchange-correlation potential and kernel underlying any DFT-SAPT calculation will be explicated. Enhancements of the computational efficiency through density fitting are described and comparisons to coupled cluster theory and experiment benchmark the performance of the method. © 2013 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/wcms.1164
  • 2014 • 110 Interface properties in lamellar TiAl microstructures from density functional theory
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Intermetallics 54 154-163 (2014)
    The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112̄] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.06.001
  • 2014 • 109 Electron-phonon coupling in quantum-well states of the Pb/Si(1 1 1) system
    Ligges, M. and Sandhofer, M. and Sklyadneva, I. and Heid, R. and Bohnen, K.-P. and Freutel, S. and Rettig, L. and Zhou, P. and Echenique, P.M. and Chulkov, E.V. and Bovensiepen, U.
    Journal of Physics Condensed Matter 26 (2014)
    The electron-phonon coupling parameters in the vicinity of the γ̄ point, λ(γ̄), for electronic quantum well states in epitaxial lead films on a Si(1 1 1) substrate are measured using 5, 7 and 12ML films and femtosecond laser photoemission spectroscopy. The λ (γ̄) values in the range of 0.6-0.9 were obtained by temperature-dependent line width analysis of occupied quantum well states and found to be considerably smaller than the momentum averaged electron-phonon coupling at the Fermi level of bulk lead, (λ = 1.1-1.7). The results are compared to density functional theory calculations of the lead films with and without interfacial stress. It is shown that the discrepancy can not be explained by means of confinement effects or simple structural modifications of the Pb films and, thus, is attributed to the influence of the substrate on the Pb electronic and vibrational structures. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/35/352001
  • 2014 • 108 A new method for development of bond-order potentials for transition bcc metals
    Lin, Y.-S. and Mrovec, M. and Vitek, V.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A new development of numerical bond-order potentials (BOPs) for the non-magnetic transition metals V, Nb, Ta, Cr, Mo and W is presented. The principles on which the BOPs have been set up are the same as in earlier developments (Aoki et al 2007 Prog. Mater. Sci. 52 154). However, the bond integrals are based on the recently advanced method of parametrization of tight-binding from DFT calculations (Madsen et al 2011 Phys. Rev. B 83 4119, Urban et al 2011 Phys. Rev. B 84 155119) and do not require any screening. At the same time, the functional form of the environmentally dependent repulsion is identified with the functional form of the repulsion arising from the overlap of s and p electrons in argon as proposed in Aoki and Kurokawa (2007 J. Phys.: Condens. Matter 19 136228). This is justified by the same physical origin of the environment dependent repulsion, which in transition metals arises from the overlap of s electrons that are being squeezed into the ion core regions under the influence of the strong covalent d bonds. The testing of the developed BOPs involves investigation of alternative higher energy structures, transformation paths connecting the bcc structure with other structures via continuously distorted configurations, evaluation of the vacancy formation energy and calculation of phonon spectra. In all cases, the BOP calculations are in more than satisfactory agreement with either DFT calculations and/or available experimental data. The calculated γ-surfaces for {1 0 1} planes all suggest that the core of 1/21 1 1 screw dislocations is non-degenerate in the transition metals. This is also in full agreement with available calculations that account fully for the quantum-mechanical nature of the d electrons that provide the bulk of the bonding in transition metals. The testing of developed BOPs clearly demonstrates that they are transferable to structures well outside the regime of the ideal bcc lattice and are suitable for investigating the atomic structure and behaviour of extended crystal defects. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034002
  • 2014 • 107 Multiple reentrant glass transitions in confined hard-sphere glasses
    Mandal, S. and Lang, S. and Gross, M. and Oettel, M. and Raabe, D. and Franosch, T. and Varnik, F.
    Nature Communications 5 (2014)
    Glass-forming liquids exhibit a rich phenomenology upon confinement. This is often related to the effects arising from wall-fluid interactions. Here we focus on the interesting limit where the separation of the confining walls becomes of the order of a few particle diameters. For a moderately polydisperse, densely packed hard-sphere fluid confined between two smooth hard walls, we show via event-driven molecular dynamics simulations the emergence of a multiple reentrant glass transition scenario upon a variation of the wall separation. Using thermodynamic relations, this reentrant phenomenon is shown to persist also under constant chemical potential. This allows straightforward experimental investigation and opens the way to a variety of applications in micro-and nanotechnology, where channel dimensions are comparable to the size of the contained particles. The results are in line with theoretical predictions obtained by a combination of density functional theory and the mode-coupling theory of the glass transition. © 2014 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/ncomms5435
  • 2014 • 106 Molecular-Scale Imaging of Water Near Charged Surfaces
    Mehlhorn, M. and Schnur, S. and Groß, A. and Morgenstern, K.
    ChemElectroChem 1 431-435 (2014)
    The orientation of water molecules on water bilayers is investigated on Cu(111) by a combination of scanning tunneling microscopy and density functional theory. Theory predicts that the application of a field reorients the adsorbed water molecules at a distance of close to a nanometer from the surface. Experimental evidence is presented for this prediction. Furthermore, the process differs strongly between adsorption on two and on three ordered layers. We propose that these results give insight into the behavior of the diffusive layer close to electrodes. So simple? Since the basic idea of ultrahigh-vacuum (UHV) electrochemical modeling emerged, it has been claimed that UHV model experiments are too simple because they do not include the electrode potential. This combined scanning tunneling microscopy and density functional theory study gives insight into the influence of the electric field on single molecules in the diffusive layer. A field reorients adsorbed water molecules on water bilayers on Cu(111) at a distance of about 1nm from the surface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300063
  • 2014 • 105 Exceptional size-dependent activity enhancement in the electroreduction of CO2 over Au nanoparticles
    Mistry, H. and Reske, R. and Zeng, Z. and Zhao, Z.-J. and Greeley, J. and Strasser, P. and Cuenya, B.R.
    Journal of the American Chemical Society 136 16473-16476 (2014)
    The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1-8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja508879j
  • 2014 • 104 Particle size effects in the catalytic electroreduction of CO2 on Cu nanoparticles
    Reske, R. and Mistry, H. and Behafarid, F. and Roldan Cuenya, B. and Strasser, P.
    Journal of the American Chemical Society 136 6978-6986 (2014)
    A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2-15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity-selectivity-size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja500328k
  • 2014 • 103 Identifying the role of terahertz vibrations in metal-organic frameworks: From gate-opening phenomenon to shear-driven structural destabilization
    Ryder, M.R. and Civalleri, B. and Bennett, T. and Henke, S. and Rudić, S. and Cinque, G. and Fernandez-Alonso, F. and Tan, J.-C.
    Physical Review Letters 113 (2014)
    We present an unambiguous identification of low-frequency terahertz vibrations in the archetypal imidazole-based metal-organic framework (MOF) materials: ZIF-4, ZIF-7, and ZIF-8, all of which adopt a zeolite-like nanoporous structure. Using inelastic neutron scattering and synchrotron radiation far-infrared absorption spectroscopy, in conjunction with density functional theory (DFT), we have pinpointed all major sources of vibrational modes. Ab initio DFT calculations revealed the complex nature of the collective THz modes, which enable us to establish detailed correlations with experiments. We discover that low-energy conformational dynamics offers multiple pathways to elucidate novel physical phenomena observed in MOFs. New evidence demonstrates that THz modes are intrinsically linked, not only to anomalous elasticity underpinning gate-opening and pore-breathing mechanisms, but also to shear-induced phase transitions and the onset of structural instability. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.215502
  • 2014 • 102 Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)
    Sandhofer, M. and Sklyadneva, I.Yu. and Sharma, V. and Trontl, V.M. and Zhou, P. and Ligges, M. and Heid, R. and Bohnen, K.-P. and Chulkov, E.V. and Bovensiepen, U.
    Journal of Electron Spectroscopy and Related Phenomena 195 278-284 (2014)
    The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the over(Γ, -) → over(M, -) direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6pz states with the Si substrate. The experimentally obtained dispersion E(k||) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k||) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6pz states with the Si band. However, the momentum range at which mixing of 6pz and 6px,y derived subbands is found to occur in the presence of the Si substrate is closer to over(Γ, -) than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a px,y mediated interaction of the pz states with the Si conduction band. © 2014.
    view abstractdoi: 10.1016/j.elspec.2014.04.006
  • 2014 • 101 Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations
    Sangiovanni, D.G. and Edströma, D. and Hultmana, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Surface Science 627 34-41 (2014)
    Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane b100N channels. b100N and b110N single jumps, as well as b100N double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea/ (kBT)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients Ds(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures ≤600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower Ds values (Ds corr) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D s corr (T)=(4.5 × 10-4 cm2 s-1) exp[.0.55 eV/ (kBT)]. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2014.04.007
  • 2014 • 100 Ab initio and classical molecular dynamics simulations of N2 desorption from TiN(001) surfaces
    Sangiovanni, D.G. and Edström, D. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Surface Science 624 25-31 (2014)
    Ab initio molecular dynamics simulations based on density functional theory show that N adatoms are chemisorbed in threefold sites close to a N surface atom and between the two diagonally opposed neighboring Ti surface atoms on TiN(001). The most probable N adatom reaction pathway, even in the presence of nearby N adatoms, is for the N adatom and N surface atom pair to first undergo several exchange reactions and then desorb as a N2 molecule, resulting in a surface anion vacancy, with an activation barrier Edes of 1.37 eV and an attempt frequency Ades = 3.4 × 10 13 s- 1. Edes is essentially equal to the N adatom surface diffusion barrier, Es = 1.39 eV, while As is only three to four times larger than Ades, indicating that isolated N adatoms migrate for only short distances prior to N2 desorption. The probability of N2 desorption via recombination of N adatoms on TiN(001) is much lower due to repulsive adatom/adatom interactions at separations less than ~ 3 Å which rapidly increase to ~ 2 eV at a separation of 1.5 Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2014.01.007
  • 2014 • 99 Vacancy mobility and interaction with transition metal solutes in Ni
    Schuwalow, S. and Rogal, J. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Interaction of Re, Ta, W and Mo solutes with vacancies and their diffusion in fcc Ni is investigated by density-functional theory in combination with kinetic Monte Carlo simulations. Interaction energies are calculated for the first six neighbor shells around the solutes and a complete set of diffusion barriers for these shells is provided. Further, diffusion coefficients for the four elements in Ni as well as for vacancies in the presence of these elements are calculated. The calculated solute diffusion coefficients based on our ab initio data are found to compare favorably to experimental values. The mobility of the vacancies as a key factor in dislocation climb is only minimally influenced by the solute atoms within the dilute limit. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/48/485014
  • 2014 • 98 Energetics of the water-gas-shift reaction on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst
    Studt, F. and Behrens, M. and Abild-Pedersen, F.
    Catalysis Letters 144 1973-1977 (2014)
    The energy profile for the water-gas-shift reaction has been calculated on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst using the BEEF-vdW functional. Our theoretical results suggest that both active site motifs, a copper (211) step as well as a zinc decorated step, are equally active for the water-gas-shift reaction. We find that the splitting of water into surface OH∗and H∗constitutes the rate-limiting step and that the reaction proceeds through the carboxyl mechanism. Our findings also suggest that mixed copper-zinc step sites are most likely to exhibit superior activity. © Springer Science+Business Media New York 2014.
    view abstractdoi: 10.1007/s10562-014-1363-9
  • 2014 • 97 Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe-Mn alloys: A first-principles study
    Von Appen, J. and Dronskowski, R. and Chakrabarty, A. and Hickel, T. and Spatschek, R. and Neugebauer, J.
    Journal of Computational Chemistry 35 2239-2244 (2014)
    Hydrogen interstitials in austenitic Fe-Mn alloys were studied using density-functional theory to gain insights into the mechanisms of hydrogen embrittlement in high-strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest-neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.23742
  • 2014 • 96 Potential-induced degradation in solar cells: Electronic structure and diffusion mechanism of sodium in stacking faults of silicon
    Ziebarth, B. and Mrovec, M. and Elsässer, C. and Gumbsch, P.
    Journal of Applied Physics 116 (2014)
    Sodium decorated stacking faults (SFs) were recently identified as the primary cause of potential-induced degradation in silicon (Si) solar-cells due to local electrical short-circuiting of the p-n junctions. In the present study, we investigate these defects by first principles calculations based on density functional theory in order to elucidate their structural, thermodynamic, and electronic properties. Our calculations show that the presence of sodium (Na) atoms leads to a substantial elongation of the Si-Si bonds across the SF, and the coverage and continuity of the Na layer strongly affect the diffusion behavior of Na within the SF. An analysis of the electronic structure reveals that the presence of Na in the SF gives rise to partially occupied defect levels within the Si band gap that participate in electrical conduction along the SF. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4894007
  • 2013 • 95 Spectral ellipsometry study in the range of electronic excitations and band structure of [(CH3)2CHNH3]4Cd 3Cl10 crystals
    Andriyevsky, B. and Dorywalski, K. and Jaskólski, M. and Czapla, Z. and Patryn, A. and Esser, N.
    Materials Chemistry and Physics 139 770-774 (2013)
    Optical dielectric functions ε(E) of the (IPA)4Cd 3Cl10 crystal were measured in the spectral range of fundamental electronic excitations 3.5-10 eV and in the temperature range of 310-400 K containing the phase transition point between the orthorhombic phases Cmce and Pbca. Measurements were performed by spectroscopic ellipsometry with using of synchrotron radiation. Electronic band structure, density of states and dielectric functions ε(E) of (IPA)4Cd3Cl 10 were calculated and analyzed on the basis of the density functional theory. Top valence and bottom conduction bands were found to be formed mainly by the cadmium-chlorine complexes of the crystals. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.matchemphys.2013.02.030
  • 2013 • 94 Molecular understanding of reactivity and selectivity for methanol oxidation at the Au/TiO2 interface
    Farnesicamellone, M. and Zhao, J. and Jin, L. and Wang, Y. and Muhler, M. and Marx, D.
    Angewandte Chemie - International Edition 52 5780-5784 (2013)
    Gold catalysis: Experimental and theoretical data demonstrated consistently that the interfacial sites on a Au/TiO2 catalyst show both high reactivity and selectivity for low-temperature methanol oxidation with O 2 to give formaldehyde. The microscopic mechanism of this complex reaction has been unraveled in full molecular detail (see picture, gold cluster on TiO2 surface). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301868
  • 2013 • 93 Symmetrization driven spin transition in ε-FeOOH at high pressure
    Gleason, A.E. and Quiroga, C.E. and Suzuki, A. and Pentcheva, R. and Mao, W.L.
    Earth and Planetary Science Letters 379 49-55 (2013)
    Structural and electronic spin transitions in high-pressure ε-FeOOH are studied using a combination of high pressure X-ray emission spectroscopy (XES), X-ray diffraction (XRD) and density functional theory (DFT) calculations. Using XES, a high- to low-spin transition in trivalent iron is found in ε-FeOOH on compression between 40 and 60 GPa. This is accompanied by a sudden discontinuity in unit cell volume at 53( ± 2) GPa, obtained from XRD data collected over the same compression range. These results are consistent with DFT calculations using an on-site Coulomb repulsion term (GGA+U), which predict a spin transition in ε-FeOOH at 64.8 GPa. A second order phase transition from P21nm to Pnnm is predicted from DFT at ~43 GPa and evidenced in the XRD data from the anisotropic stiffening of the lattice parameters around the spin transition. In addition, the DFT results give evidence that the spin collapse is assisted by symmetrization of hydrogen bonds during the transition from P21nm to Pnnm. As the presence of hydrogen, even in small quantities, can affect phase relations, melting temperature, rheology, and other key properties of the Earth's mantle, our study unveils a connection between water (hydroxyl) content and the spin-transition pressure of Fe3+ in the Earth's mantle. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.epsl.2013.08.012
  • 2013 • 92 Compositional trends and magnetic excitations in binary and ternary Fe-Pd-X magnetic shape memory alloys
    Gruner, M.E. and Hamann, S. and Brunken, H. and Ludwig, Al. and Entel, P.
    Journal of Alloys and Compounds 577 S333-S337 (2013)
    High throughput thin film experiments and first-principles calculations are combined in order to get insight into the relation between finite temperature transformation behavior and structural ground state properties of ternary Fe-Pd-X alloys. In particular, we consider the binding surface, i.e., the energy of the disordered alloy calculated along the Bain path between bcc and fcc which we model by a 108 atom supercell. We compare stoichiometric Fe 75Pd25 with ternary systems, where 4.6% of the Fe atoms were substituted by Cu and Mn, respectively. The computational trends are related to combinatorial experiments on thin film libraries for the systems Fe-Pd-Mn and Fe-Pd-Cu which reveal a systematic evolution of the martensitic start temperature with composition within the relevant concentration range for magnetic shape memory (MSM) applications. Our calculations include atomic relaxations, which were shown to be relevant for a correct description of the structural properties. Furthermore, we find that magnetic excitations can substantially alter the binding surface. The comparison of experimental and theoretical trends indicates that, both, compositional changes and magnetic excitations contribute significantly to the structural stability which may thus be tailored by specifically adding antiferromagnetic components. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.02.033
  • 2013 • 91 Trends in spin and orbital magnetism of free and encapsulated FePt nanoparticles
    Gruner, M.E.
    Physica Status Solidi (A) Applications and Materials Science 210 1282-1297 (2013)
    Owed to the large magneto-crystalline anisotropy (MCA) of the bulk FePt alloys, nanostructures with a few nm in diameter are considered for ultra-high density recording applications. First principles calculations in the framework of density functional theory (DFT) permit insight into the close interrelation between particle composition, morphology, and magnetism with access to the electronic level. The present survey will systematically highlight the impact of an additional encapsulation with Cu, Au, Al, and further main group elements on spin- and orbital magnetism and MCA with special emphasis on the role of the interface. Site resolved orbital moment anisotropy (OMA) of an uncovered 147 atom FePt nanoparticle. Large-scale first principles calculations in the framework of density functional theory offer detailed insight into the close interrelation between particle composition, morphology and magnetism with electronic resolution. Exploiting the power of contemporary supercomputers, one can identify systematic trends in spin and orbital magnetism of nanometer-sized hard magnetic particles related to their structure or chemical environment. This Feature Article concentrates on Fe-Pt nanoparticles, which are considered as promising candidates for ultra-high density recording media. Special emphasis is made on the role of the surfaces and the impact of a protective encapsulation with Cu, Au, Al or further main group elements on the hard magnetic properties. The anisotropy of the orbital moments turns out to be a valuable quantity characterizing the particular contribution of surfaces and interfaces on the atomic scale. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201329048
  • 2013 • 90 High-throughput ab initio screening of binary solid solutions in olivine phosphates for Li-ion battery cathodes
    Hajiyani, H.R. and Preiss, U. and Drautz, R. and Hammerschmidt, T.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    A promising approach to improving the performance of iron-phosphate FePO4 cathode materials for Li-ion batteries is to partly or fully substitute Fe with other metals. Here, we use high-throughput density-functional theory (DFT) calculations to investigate binary mixtures of metal atoms M and M′ in (Li)MyM'1-yPO4 olivine phosphates. We determine the formation energy for various stoichiometries of different binary combinations of metals for the cases of full lithiation and delithiation. Systematic screening of all combinations of Fe and Mn with elements of the 3d transition-metal (TM) series allows us to identify trends with average band filling and atomic size. We also included compounds that verify the observed relations or that were discussed as cathode materials, particularly Ni-Co, V-Cu and V-Ni, as well as combinations with 4d TMs (Fe-Zr, Fe-Mo, Fe-Ag) and with Mg (Fe-Mg and Ni-Mg). Based on our DFT calculations for each compound, we estimate the volume change during intercalation, the intercalation voltage, the energy density and the thermal stability with respect to reaction with oxygen. Our calculations indicate that the energy density of the binary TM phosphates increases with average band filling while the thermal stability of the compounds decreases. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/074004
  • 2013 • 89 Electrochemical formation and surface characterisation of Cu 2-xTe thin films with adjustable content of Cu
    Huang, M. and Maljusch, A. and Calle-Vallejo, F. and Henry, J.B. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    RSC Advances 3 21648-21654 (2013)
    Electrochemically driven "intercalation" of Cu into Te was used to prepare Cu<inf>2-x</inf>Te (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e-. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu<inf>2-x</inf>Te thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3ra42504e
  • 2013 • 88 Odd and even Kondo effects from emergent localization in quantum point contacts
    Iqbal, M.J. and Levy, R. and Koop, E.J. and Dekker, J.B. and De Jong, J.P. and Van Der Velde, J.H.M. and Reuter, D. and Wieck, A.D. and Aguado, R. and Meir, Y. and Van Der Wal, C.H.
    Nature 501 79-83 (2013)
    A quantum point contact (QPC) is a basic nanometre-scale electronic device: a short and narrow transport channel between two electron reservoirs. In clean channels, electron transport is ballistic and the conductance is then quantized as a function of channel width with plateaux at integer multiples of 2e 2/h (where e is the electron charge and h is Planck's constant). This can be understood in a picture where the electron states are propagating waves, without the need to account for electron-electron interactions. Quantized conductance could thus be the signature of ultimate control over nanoscale electron transport. However, even studies with the cleanest QPCs generically show significant anomalies in the quantized conductance traces, and there is consensus that these result from electron many-body effects. Despite extensive experimental and theoretical studies, understanding these anomalies is an open problem. Here we report that the many-body effects have their origin in one or more spontaneously localized states that emerge from Friedel oscillations in the electron charge density within the QPC channel. These localized states will have electron spins associated with them, and the Kondo effect-related to electron transport through such localized electron spins-contributes to the formation of the many-body state. We present evidence for such localization, with Kondo effects of odd or even character, directly reflecting the parity of the number of localized states; the evidence is obtained from experiments with length-tunable QPCs that show a periodic modulation of the many-body properties with Kondo signatures that alternate between odd and even Kondo effects. Our results are of importance for assessing the role of QPCs in more complex hybrid devices and for proposals for spintronic and quantum information applications. In addition, our results show that tunable QPCs offer a versatile platform for investigating many-body effects in nanoscale systems, with the ability to probe such physics at the level of a single site. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nature12491
  • 2013 • 87 X-ray photoemission and density functional theory study of the interaction of water vapor with the Fe3O4(001) surface at near-ambient conditions
    Kendelewicz, T. and Kaya, S. and Newberg, J.T. and Bluhm, H. and Mulakaluri, N. and Moritz, W. and Scheffler, M. and Nilsson, A. and Pentcheva, R. and Brown Jr., G.E.
    Journal of Physical Chemistry C 117 2719-2733 (2013)
    The interaction of water with the Fe3O4(001) surface was investigated in a combined ambient pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. The uptake of molecular water and hydroxyl species on the (001) surface of a natural magnetite sample at near-ambient conditions was quantified using O 1s spectra taken in the p(H 2O) range from 10-9 to 2 Torr. At low p(H2O) (≤10-4-10-5 Torr) and room temperature, we found that water does not adsorb dissociatively on the surface, except on defect sites. In contrast, progressive dissociation into surface hydroxyl species was observed between 10-4 and 10-2 Torr p(H2O). The onset of hydroxylation coincides with the increasing presence of molecular water species on the surface, which demonstrates the key role played by cooperative interactions between adsorbed water molecules, leading to dissociation and surface hydroxylation. The measured O 1s chemical shifts of hydroxyl and molecular water species from both isotherm and isobar data are on average ∼1.2 eV and ∼3.3 eV, respectively, relative to lattice oxygen. The chemical shift of the hydroxyl species on magnetite(001) agrees with previously reported values for hydroxyl species on iron oxyhydroxides such as goethite (α-FeOOH). DFT calculations including an on-site Coulomb repulsion parameter (generalized gradient approximation (GGA) + U approach) predict O 1s surface core-level shifts (SCLS) at the clean (21/2×2 1/2)R45 reconstructed Fe3O4(001) surface of up to ∼-1 eV depending on the specific bonding configuration of the surface O atoms. Hydroxyl groups formed by the dissociation of isolated water molecules at O vacancies have an SCLS value of ∼1.2 eV. With increasing coverage there is a transition toward partial dissociation on the (001) surface. The calculated SCLS for hydroxyl and adsorbed water are 1.2-1.9 and 2.6-3.0 eV, respectively, and compare very well with our experimental results. Final-state effects obtained within the Slater-Janak approach thus have the dominant contribution. In addition, the modest reduction of the work function (∼0.5 eV) predicted by DFT calculations for the mixed adsorption of dissociated and intact water molecules agrees well with work function changes measured experimentally. Finally, the similarity between isotherm and isobar data and the DFT calculations for the C-free Fe3O4(001) surface indicate that surface hydroxylation is indeed substrate induced and not catalyzed by the presence of adventitious carbonaceous species. Both theory and experiment show the importance of cooperative effects between adjacent water molecules in the dissociation reaction. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp3078024
  • 2013 • 86 A Z′ = 6 crystal structure of (E)-N,N′-dicyclohexylacetamidine
    Krasnopolski, M. and Seidel, R.W. and Goddard, R. and Breidung, J. and Winter, M.V. and Devi, A. and Fischer, R.A.
    Journal of Molecular Structure 1031 239-245 (2013)
    The crystal and molecular structure of (E)-N,N′- dicyclohexylacetamidine (1) is described. Crystalline material of 1 was obtained by sublimation. Single-crystal X-ray analysis revealed a centrosymmetric triclinic structure (space group P1̄) with six molecules in the asymmetric unit (Z′ = 6). The six crystallographically distinct molecules all exhibit an E-syn structure, but differ in the orientation of the cyclohexyl groups about the central acetamidine moiety. In the crystal, the molecules form polymeric helices via NH⋯N hydrogen bonds. The crystal structure comprises two crystallographically distinct helices of opposite handedness (P and M form). The characterisation of 1 in the solid-state is augmented by powder X-ray diffraction, infrared spectroscopy and thermal analysis. Density functional theory (DFT) structure optimisation and frequency calculation were performed at the B3LYP/cc-pVTZ level. The DFT results for the isolated molecule are compared with the experimental results for the solid-state. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.molstruc.2012.10.003
  • 2013 • 85 Blocking growth by an electrically active subsurface layer: The effect of si as an antisurfactant in the growth of GaN
    Markurt, T. and Lymperakis, L. and Neugebauer, J. and Drechsel, P. and Stauss, P. and Schulz, T. and Remmele, T. and Grillo, V. and Rotunno, E. and Albrecht, M.
    Physical Review Letters 110 (2013)
    Combining aberration corrected high resolution transmission electron microscopy and density functional theory calculations we propose an explanation of the antisurfactant effect of Si in GaN growth. We identify the atomic structure of a Si delta-doped layer (commonly called SiNx mask) as a SiGaN3 monolayer that resembles a √3×√3 R30 surface reconstruction containing one Si atom, one Ga atom, and a Ga vacancy (V Ga) in its unit cell. Our density functional theory calculations show that GaN growth on top of this SiGaN3 layer is inhibited by forming an energetically unfavorable electrical dipole moment that increases with layer thickness and that is caused by charge transfer between cation dangling bonds at the surface to VGa bound at subsurface sites. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.110.036103
  • 2013 • 84 Environmental tight-binding modeling of nickel and cobalt clusters
    McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    Journal of Physics Condensed Matter 25 (2013)
    Tight-binding models derived from density functional theory potentially provide a systematic approach to the development of accurate and transferable models of multicomponent systems. We introduce a systematic methodology for environmental tight binding in which both the overlap and environmental contributions to the electronic structure are included. The parameters of the model are determined directly from ab initio considerations, thus providing a formal conceptual link to density functional approaches. In order to test the validity of the approach, the model is applied to small clusters of Ni and Co, whose electronic structure is largely determined by the interplay of tightly bound d-valent states and the disperse s-states. We numerically illustrate that it is essential to include environmental contributions in the tight-binding approach in order to reliably reproduce the electronic structure of such clusters. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/11/115502
  • 2013 • 83 Thermodynamics of carbon solubility in ferrite and vacancy formation in cementite in strained pearlite
    Nematollahi, G.A. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 61 1773-1784 (2013)
    In order to investigate the thermodynamic driving force for the experimentally observed accumulation of C in ferritic layers of severely plastically deformed pearlitic wires, the stabilities of C interstitials in ferrite and of C vacancies in cementite are investigated as a function of uniaxial stain, using density-functional theory. In the presence of an applied strain along [1 1 0] or [1 1 1], the C interstitial in ferrite is significantly stabilized, while the C vacancy in cementite is moderately destabilized by the corresponding strain states in cementite [1 0 0] and ([0 1 0]). The enhanced stabilization of the C interstitial gives rise to an increase in the C concentration within the ferritic layers by up to two orders of magnitude. Our results thus suggest that in addition to the generally assumed non-equilibrium, dislocation-based mechanism, there is also a strain-induced thermodynamic driving force for the experimentally observed accumulation of C in ferrite. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.12.001
  • 2013 • 82 CO adsorption on a mixed-valence ruthenium metal-organic framework studied by UHV-FTIR spectroscopy and DFT calculations
    Noei, H. and Kozachuk, O. and Amirjalayer, S. and Bureekaew, S. and Kauer, M. and Schmid, R. and Marler, B. and Muhler, M. and Fischer, R.A. and Wang, Y.
    Journal of Physical Chemistry C 117 5658-5666 (2013)
    The mixed-valence metal-organic framework [Ru3 II,III(btc)2Cl1.5] (Ru-MOF) was synthesized by the controlled SBU approach and characterized by combined powder XRD, XPS, and FTIR methods. The interaction of CO molecules with Ru-MOF was studied by a novel instrumentation for ultra-high-vacuum (UHV) FTIR spectroscopy. The high-quality IR data demonstrate the presence of two different CO species within the framework: a strongly bonded CO showing a low-lying band at 2137 cm-1 and a second CO species at 2171 cm-1 with a lower binding energy. It was found that these IR bands cannot be assigned in a straightforward manner to CO molecules adsorbed on the coordinatively unsaturated RuII site (CUS) and RuIII site connected to an additional Cl- ion for charge compensation. The accurate DFT calculations reveal that the structural and electronic properties of the mixed-valence Ru-MOF are much more complex than expected. One of the Cl- counterions could be transferred to a neighboring paddle-wheel, forming an anionic SBU blocked by two Cl- counterions, whereas the other positively charged paddle-wheel with a Ru2 II,III dimer exposes two "free" CUS, which can bind two CO molecules with different frequencies and binding energies. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp3056366
  • 2013 • 81 Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys
    Pei, Z. and Zhu, L.-F. and Friák, M. and Sandlöbes, S. and Von Pezold, J. and Sheng, H.W. and Race, C.P. and Zaefferer, S. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 15 (2013)
    Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. We find a similar reduction for the stable stacking fault energy of the non-basal slip system. On the other hand, other energies along this particular γ-surface profile increase with the addition of Y. In parallel to our quantum-mechanical calculations, we have also developed a new EAM Mg-Y potential and thoroughly tested its performance. The comparison of quantum-mechanical and atomistic results indicates that the new potential is suitable for future large-scale atomistic simulations. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/4/043020
  • 2013 • 80 Ab Initio Based conformational study of the crystalline α-chitin
    Petrov, M. and Lymperakis, L. and Friák, M. and Neugebauer, J.
    Biopolymers 99 22-34 (2013)
    The equilibrium structure including the network of hydrogen bonds of an α-chitin crystal is determined combining density-functional theory (DFT), self-consistent DFT-based tight-binding (SCC-DFTB), and empirical forcefield molecular dynamics (MD) simulations. Based on the equilibrium geometry several possible crystal conformations (local energy minima) have been identified and related to hydrogen bond patterns. Our results provide new insight and allow to resolve the contradicting α-chitin structural models proposed by various experiments. © 2012 Wiley Periodicals, Inc. Copyright © 2012 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bip.22131
  • 2013 • 79 Interface defects and impurities at the growth zone of Au-catalyzed GaAs nanowire from first principles
    Sakong, S. and Du, Y.A. and Kratzer, P.
    Physica Status Solidi - Rapid Research Letters 7 882-885 (2013)
    The defects and impurities at the interface of a Au-catalyzed GaAs nanowire have been studied by the first-principles method. The interface is modeled by Au layers on the ${\rm GaAs}(\bar 1\bar 1\bar 1)$ substrate with both Ga- and As-terminations. From the energetics of interface defects and impurities, we find that a highly ordered As-terminated interface is expected under As-rich growth, but mixed Ga- and As-terminations are expected under Ga-rich growth. Comparing the interface defects and impurities to their bulk species, we expect the interface to be a sink for Au impurities in the GaAs nanowire. Based on DFT results, we estimate that materials transport by impurity diffusion through a liquid nanoparticle is sufficient for sustained GaAs growth. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201307210
  • 2013 • 78 Microscopic structure of water at elevated pressures and temperatures
    Sahle, C.J. and Sternemann, C. and Schmidt, C. and Lehtola, S. and Jahn, S. and Simonelli, L. and Huotari, S. and Hakala, M. and Pylkkänen, T. and Nyrow, A. and Mende, K. and Tolan, M. and Hämäläinen, K. and Wilke, M.
    Proceedings of the National Academy of Sciences of the United States of America 110 6301-6306 (2013)
    We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley's K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6at600 °C and p = 134 MPa.
    view abstractdoi: 10.1073/pnas.1220301110
  • 2013 • 77 Ab initio prediction of the critical thickness of a precipitate
    Sampath, S. and Janisch, R.
    Journal of Physics Condensed Matter 25 (2013)
    Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/35/355005
  • 2013 • 76 Formation of carbon nanofilms on single crystal quartz
    Samsonau, S.V. and Dzedzits, E. and Shvarkov, S.D. and Meinerzhagen, F. and Wieck, A.D. and Zaitsev, A.M.
    Sensors and Actuators, B: Chemical 186 610-613 (2013)
    In this work formation of the very first layers of carbon nanofilms on single crystal quartz substrates is studied. Films where grown by molecular beam growth, and have been characterized by Raman spectroscopy and atomic force microscopy. Formation of a non-conductive carbon layer of low crystallinity on the initial stage of the growth process is reported. Ab-initio calculations with an atom-by-atom approach have been performed to explain the experimental data. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2013.06.023
  • 2013 • 75 Surface morphology of MnSi thin films grown on Si(111)
    Suzuki, T. and Lutz, T. and Geisler, B. and Kratzer, P. and Kern, K. and Costantini, G.
    Surface Science 617 106-112 (2013)
    The surface morphology of MnSi thin films grown on Si(111)-7 × 7 substrates was investigated by systematically changing the amount of deposited Mn. A new 3 × 3 surface reconstruction was found at the very initial growth stages, whose atomic configuration was analyzed both experimentally and theoretically. At a coverage of 0.1 monolayers, the formation of nanometer-sized MnSi islands was observed in coexistence with Mn nanoclusters that fit within the 7 × 7 half unit cell. With increasing Mn deposition, the MnSi islands grow, develop extended flat tops and eventually coalesce into an atomically flat film with a high corrugated 3×3 reconstruction punctuated by several holes. The successive film growth mode is characterized by the formation of MnSi quadlayers with a low corrugated 3×3 reconstruction. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.005
  • 2013 • 74 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2012 • 73 Buckling instability of viral Capsids-A continuum approach
    Aland, S. and Rätz, A. and Röger, M. and Voigt, A.
    Multiscale Modeling and Simulation 10 82-110 (2012)
    The crystallographic structure of spherical viruses is modeled using a multiscale approach combining a macroscopic Helfrich model for morphology evolution with a microscopic approximation of a classical density functional theory for the protein interactions. The derivation of the model is based on energy dissipation and conservation of protein number density. The resulting set of equations is solved within a diffuse domain approach using finite elements and shows buckling transitions of spherical shapes into faceted viral shapes. © 2012 Society for Industrial and Applied Mathematics.
    view abstractdoi: 10.1137/110834718
  • 2012 • 72 How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry
    Armbrüster, M. and Behrens, M. and Cinquini, F. and Föttinger, K. and Grin, Y. and Haghofer, A. and Klötzer, B. and Knop-Gericke, A. and Lorenz, H. and Ota, A. and Penner, S. and Prinz, J. and Rameshan, C. and Révay, Z. and Ro...
    ChemCatChem 4 1048-1063 (2012)
    Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by insitu spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H 2 partial pressure) alkyne fragmentation leads to formation of a Pd, C surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that Pd-C hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over insitu formed bulk Pd-H. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such Pd xGa y catalysts show extended stability under insitu conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd 3Ga 7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga 2O 3, upon heating of Pd/Ga 2O 3 in hydrogen, was shown to lead to formation of Pd-Ga intermetallic compounds at moderate temperatures. In this way, Pd 5Ga 2 and Pd 2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201200100
  • 2012 • 71 The active site of methanol synthesis over Cu/ZnO/Al2O 3 industrial catalysts
    Behrens, M. and Studt, F. and Kasatkin, I. and Kühl, S. and Hävecker, M. and Abild-Pedersen, F. and Zander, S. and Girgsdies, F. and Kurr, P. and Kniep, B.-L. and Tovar, M. and Fischer, R.W. and Nørskov, J.K. and Schlögl, R.
    Science 336 893-897 (2012)
    One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al2O 3methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.
    view abstractdoi: 10.1126/science.1219831
  • 2012 • 70 Orbital directing effects in copper and zinc based paddle-wheel metal organic frameworks: The origin of flexibility
    Bureekaew, S. and Amirjalayer, S. and Schmid, R.
    Journal of Materials Chemistry 22 10249-10254 (2012)
    We have used density functional theory calculations to study non-periodic model systems for the ubiquitous layer-pillar metal organic frameworks built from paddle-wheel building blocks. Experimentally, these porous materials show nearly identical structures for both copper and zinc forming the paddle-wheel, but differ depending on the type of the metal center in their properties. Our theoretical results clearly reveal orbital directing effects for the d 9 Cu(ii) center, enforcing a square planar conformation, to be the main reason for the difference in contrast to the flexible d 10 Zn(ii) system. Surprisingly, this difference is directly visible in the structure of the bare vertex model without axial ligands, whereas in the case of pyridine coordination both copper and zinc complexes are structurally nearly indistinguishable. However, in the vibrational normal modes the higher degree of flexibility for the zinc-based systems is still noticeable, explaining the higher flexibility of the corresponding periodic MOFs. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2jm15778k
  • 2012 • 69 Basic properties of magnetic shape-memory materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Comtesse, D. and Elmers, H.-J. and Kallmayer, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 2891-2900 (2012)
    The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the lowtemperature martensite phase with body-centered-tetragonal (bct) or rather face-centeredtetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0832-7
  • 2012 • 68 The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries
    Freysoldt, C. and Pfanner, G. and Neugebauer, J.
    Journal of Non-Crystalline Solids 358 2063-2066 (2012)
    Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder. However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jnoncrysol.2011.12.090
  • 2012 • 67 Theory-guided materials design of multi-phase Ti-Nb alloys with bone-matching elastic properties
    Friák, M. and Counts, W.A. and Ma, D. and Sander, B. and Holec, D. and Raabe, D. and Neugebauer, J.
    Materials 5 1853-1872 (2012)
    We present a scale-bridging approach for modeling the integral elastic response of polycrystalline composite that is based on a multi-disciplinary combination of (i) parameter-free first-principles calculations of thermodynamic phase stability and single-crystal elastic stiffness; and (ii) homogenization schemes developed for polycrystalline aggregates and composites. The modeling is used as a theory-guided bottom-up materials design strategy and applied to Ti-Nb alloys as promising candidates for biomedical implant applications. The theoretical results (i) show an excellent agreement with experimental data and (ii) reveal a decisive influence of the multi-phase character of the polycrystalline composites on their integral elastic properties. The study shows that the results based on the density functional theory calculations at the atomistic level can be directly used for predictions at the macroscopic scale, effectively scale-jumping several orders of magnitude without using any empirical parameters. © 2012 by the authors; licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma5101853
  • 2012 • 66 Domain structure in the tetragonal phase of BaTiO 3-From bulk to nanoparticles
    Grnebohm, A. and Gruner, M.E. and Entel, P.
    Ferroelectrics 426 21-30 (2012)
    We present a first-principles density functional theory study of domain wall structures in tetragonal BaTiO 3 and its nanoparticles. For the bulk material the domain wall profiles, their width and their formation energy are computed and preliminary investigations on thin BaTiO 3 films up to 4 monolayers and small nanoparticles of 15.8 have been performed. While the 180 wall is atomically sharp, we find a lower bond for the 90 wall width of 16.5 . Although, no ferroelectric state can be stabilized neither in films nor in the nanoparticles of this small size, a large local polarization exits in both cases. © Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00150193.2012.671090
  • 2012 • 65 First-principles study of the influence of (110) strain on the ferroelectric trends of TiO 2
    GrüNebohm, A. and Siewert, M. and Ederer, C. and Entel, P.
    Ferroelectrics 429 31-42 (2012)
    We investigate the impact of uniaxial strain on atomic shifts, dipolar interactions, polarization and electric permittivity in TiO 2 (rutile) by using two different implementations of density functional theory. It is shown that calculations using the Vienna ab inito simulation package (VASP) and the plane-wave self-consistent field method (PWscf) yield qualitatively the same atomic relaxations and ferroelectric trends under strain. The phonon dispersion curves of unstrained and strained TiO 2 (rutile) obtained by employing the linear response method confirm previous calculations of the giant LO-TO splitting and the appearance of soft polar modes. A second order phase transition into a ferroelectric phase with polarization along (110) appears under expansive strain in (110) direction.
    view abstractdoi: 10.1080/00150193.2012.676945
  • 2012 • 64 Electronic and magnetic trends in martensitically transforming Fe-Pd alloys
    Gruner, M.E. and Entel, P. and Minár, J. and Polesya, S. and Mankovsky, S. and Ebert, H.
    Journal of Magnetism and Magnetic Materials 324 3524-3529 (2012)
    We discuss the compositional trends in the electronic structure of Fe-Pd alloys in the composition range relevant for martensitic transformations and magnetic shape-memory applications. The results are obtained within the framework of density functional theory based on the Korringa-Kohn-Rostoker Greens function approach in combination with the coherent potential approximation for the description of disorder. For the body centered cubic structure, we observe with increasing Pd content the gradual disappearance of the pseudogap like structure in the center of the minority spin d-band, while on the face centered cubic side a small peak passes the Fermi level around 80 at. % Fe, which has been related previously to a band-Jahn-Teller instability. In addition, we investigate the structural variation of magnetic properties of the magnetic shape-memory composition Fe 70Pd 30 based on magnetic exchange parameters obtained from our ab initio calculations being mapped onto a classical Heisenberg model. This allows us to estimate the magnetic transition temperature within a mean field approach and Monte Carlo simulations taking into account the induced nature of the Pd atoms, leading to a close, semi-quantitative agreement with experiment in the latter case. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2012.02.081
  • 2012 • 63 Structure-property relations and thermodynamic properties of monoclinic petalite, LiAlSi 4O 10
    Haussühl, E. and Schreuer, J. and Winkler, B. and Haussühl, S. and Bayarjargal, L. and Milman, V.
    Journal of Physics Condensed Matter 24 (2012)
    Structure-property relations of monoclinic petalite, LiAlSi 4O 10, were determined by experiment and atomistic modeling based on density functional theory. The elastic stiffness coefficients were measured between room temperature and 570K using a combination of the plate-resonance technique and resonant ultrasound spectroscopy. The thermal expansion was studied between 100 and 740K by means of dilatometry. The heat capacity between 2 and 398K has been obtained by microcalorimetry using a quasi-adiabatic calorimeter. The experimentally determined elastic stiffness coefficients were employed to benchmark the results of density functional theory based model calculations. The values in the two data sets agreed to within a few GPa and the anisotropy was very well reproduced. The atomistic model was then employed to predict electric field gradients, the lattice dynamics and thermodynamic properties. The theoretical charge density was analyzed to investigate the bonding between atoms. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/34/345402
  • 2012 • 62 Ab initio-based prediction of phase diagrams: Application to magnetic shape memory alloys
    Hickel, T. and Uijttewaal, M. and Al-Zubi, A. and Dutta, B. and Grabowski, B. and Neugebauer, J.
    Advanced Engineering Materials 14 547-561 (2012)
    An ultimate goal of material scientists is the prediction of the thermodynamics of tailored materials solely based on first principles methods. The present work reviews recent methodological developments and advancements providing thereby an up-to-date basis for such an approach. Key ideas and the performance of these methods are discussed with respect to the Heusler alloy Ni-Mn-Ga - a prototype magnetic shape-memory alloy of great technological interest for various applications. Ni-Mn-Ga shows an interesting and complex sequence of phase transitions, rendering it a significant theoretical challenge for any first principles approach. The primary goal of this investigation is to determine the composition dependence of the martensitic transition temperature in these alloys. Quasiharmonic phonons and the magnetic exchange interactions as well as the delicate interplay of vibrational and magnetic excitations are taken into account employing density functional theory. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200092
  • 2012 • 61 Advancing density functional theory to finite temperatures: Methods and applications in steel design
    Hickel, T. and Grabowski, B. and Körmann, F. and Neugebauer, J.
    Journal of Physics Condensed Matter 24 (2012)
    The performance of materials such as steels, their high strength and formability, is based on an impressive variety of competing mechanisms on the microscopic/atomic scale (e.g. dislocation gliding, solid solution hardening, mechanical twinning or structural phase transformations). Whereas many of the currently available concepts to describe these mechanisms are based on empirical and experimental data, it becomes more and more apparent that further improvement of materials needs to be based on a more fundamental level. Recent progress for methods based on density functional theory (DFT) now makes the exploration of chemical trends, the determination of parameters for phenomenological models and the identification of new routes for the optimization of steel properties feasible. A major challenge in applying these methods to a true materials design is, however, the inclusion of temperature-driven effects on the desired properties. Therefore, a large range of computational tools has been developed in order to improve the capability and accuracy of first-principles methods in determining free energies. These combine electronic, vibrational and magnetic effects as well as structural defects in an integrated approach. Based on these simulation tools, one is now able to successfully predict mechanical and thermodynamic properties of metals with a hitherto not achievable accuracy. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/5/053202
  • 2012 • 60 A DFT study of formation energies of Fe-Zn-Al intermetallics and solutes
    Klaver, T.P.C. and Madsen, G.K.H. and Drautz, R.
    Intermetallics 31 137-144 (2012)
    We report Density Functional Theory results on FeAl and FeZn intermetallics and Fe, Zn and Al solute atoms. The formation energies of fully relaxed intermetallic geometries were determined, as well as solution energies of the three elements in host lattices of the other two elements. Since it is know that the outcome of the magnetic states of some FeAl intermetallics and Fe solutes in Al depends on subtle details of how the calculations are carried out, we have determined many of our results with two different parameterisations, PBE and PBEsol, so see how the parameterisation influences the results. The relaxed intermetallic geometries are in good agreement with experimental results, with PBEsol calculations resulting in slightly smaller geometries than PBE calculations (0.7-2.1%). Intermetallic formation energies fall within ranges of experimental results where available, and are in excellent or reasonable agreement with other DFT results, except for the FeAl 2 phase. For this phase a structure revision was recently suggested and the heat of formation of the newly suggested structure is 0.1 eV/atom lower than for the long-accepted structure. The formation energies of Fe aluminides are an order of magnitude more negative than those of FeZn intermetallics. Most of the calculated magnetic states of the intermetallics are at odds with experimental results. However, the intermetallic formation energies are often not strongly affected by this. Fe/Al solute systems have the most negative solution energies. All other solution energies are positive and smaller in absolute value than the Fe/Al solution energies. Solution energies were all some tenths of eV. Where comparisons could be made, calculated and experimental results differed by some hundredths of eV. The magnetic moment found on an Fe solute in Al is at odds with experimental results. As with FeAl, the outcome of the magnetic state subtly depends on the details of how calculations were performed and has little energetic effect. Lattice relaxation around solute atoms is mostly in agreement with simple atomic size considerations. The slight relaxation of Al neighbours away from a Zn solute is at odds with this pattern, and also with experimental results. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.06.017
  • 2012 • 59 Catalytic role of gold nanoparticle in GaAs nanowire growth: A density functional theory study
    Kratzer, P. and Sakong, S. and Pankoke, V.
    Nano Letters 12 943-948 (2012)
    The energetics of Ga, As, and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density functional calculations. Apart from formation of the compound Au 7Ga 2, Ga is found to form a surface alloy with gold with comparable ΔH ∼ -0.5 eV for both processes. Dissociative adsorption of As 2 is found to be exothermic by more than 2 eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle in vacuo in contact with a GaAs substrate. The results of the calculations are interpreted in the context of Au-catalyzed growth of GaAs nanowires. We argue that arsenic is supplied to the growth zone of the nanowire mainly by impingement of molecules on the gold particle and identify a regime of temperatures and As 2 partial pressures suitable for Au-catalyzed nanowire growth in molecular beam epitaxy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204004p
  • 2012 • 58 Ab initio studying of topological insulator Bi2Se3 under the stress
    Lysogorskiy, Y.V. and Kijamov, A.G. and Nedopekin, O.V. and Tayurskii, D.A.
    Journal of Physics: Conference Series 394 (2012)
    A topological insulator is an unusual state of quantum matter which, while being an insulator in the bulk, has topologically protected electronic states at the surface. These states could be used in different applications, such as spintronics and quantum computing. However, it is difficult to distinguish the surface and bulk contributions into transport properties, such as conductivity. In order to distinguish surface and bulk contributions an external pressure could be applied. In the present work we have performed ab initio calculations of topological insulator Bi2Se3 under the stress for bulk and surface models. Calculations have been made by means of density functional theory within generalized gradient approximation, the spin-orbit interaction was taken into account as well. It was found that topologically protected surface states remains robust under the stress. Moreover, pressure tends to increase the Fermi velocity of surface electrons, as well as increase electronic density of states at the bottom of the conduction band of the bulk of Bi2Se 3. Thus, the results of ab initio calculations could complement the experimental investigations of high pressure transport properties of topological insulators. The experimentally detected increase of carrier density could be related to the effects of the bulk.
    view abstractdoi: 10.1088/1742-6596/394/1/012022
  • 2012 • 57 Performance of fluorene and terthiophene copolymer in bilayer photovoltaic devices: The role of the polymer conformations
    Marchiori, C.F.N. and Yamamoto, N.A.D. and Grova, I.R. and MacEdo, A.G. and Paulus, M. and Sternemann, C. and Huotari, S. and Akcelrud, L. and Roman, L.S. and Koehler, M.
    Organic Electronics: physics, materials, applications 13 2716-2726 (2012)
    We report experiments using fluorene and terthiophene copolymer as the active layer in bilayer devices with C 60. The highest short circuit current, open circuit voltage and power conversion efficiency upon AM1.5 illumination were 6.8 mA/cm 2, 0.68 V and 2.33%, respectively. Density functional theory analysis was used to identify the most stable configurations of the terthiophene moieties in the polymer: the most stable form has the thiophene rings in the alternate configuration (anti) and the second conformation has the thiophene rings pointing to the same direction (syn). Comparing theoretical results with measurements of absorbance, X-ray diffraction, and X-ray reflectometry experiments, we conclude that the annealing treatment produces conformational anti to syn transition along the backbone of poly[9,9′-n-dihexyl-2,7-fluorene-alt-2,5-terthiophene] (LaPPS45). The syn segments of the chain condensed then in a lamellar ordered structure which increases the degree of crystallinity of the annealed samples and improve the light harvest at long wavelengths. From absorption measurements of films submitted to different annealing temperatures and with the help of theoretical calculations we propose a "wave-like" aggregation pattern to the syn segments in those lamellas. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.orgel.2012.08.002
  • 2012 • 56 A flexible, plane-wave based multiband k ·p model
    Marquardt, O. and Schulz, S. and Freysoldt, C. and Boeck, S. and Hickel, T. and O'Reilly, E.P. and Neugebauer, J.
    Optical and Quantum Electronics 44 183-188 (2012)
    In this work, we present a highly generalized implementation of multiband k · p models. We have achieved a high efficiency of our approach by incorporating it in a planewave framework within the Density Functional Theory package S/PHI/nX. To demonstrate the flexibility and applicability of our code, we have chosen two example studies that are directly accessible with the standard eight-band k · p model. By employing a 14-band k · p model for the description of pyramidal InAs/GaAs quantum dots (QDs), we show that this model is able to accomodate for the correct symmetry of the underlying zincblende lattice, which is not reflected in the standard eight-band model. Our second example provides a description of site-controlled (111)-oriented InGaAs/GaAs QDs. The extremely small aspect ratio of these QDs makes a description using conventional k · p Hamiltonians computationally highly expensive.We have therefore rotated the standard eight-band Hamiltonian, to suit the description of these systems. The studies of electronic properties of the above mentioned model systems demonstrate the efficiency and flexibility of our approach. © Springer Science+Business Media, LLC. 2011.
    view abstractdoi: 10.1007/s11082-011-9506-3
  • 2012 • 55 Hydrogen adsorption and site-selective reduction of the Fe 3O 4(001) surface: Insights from first principles
    Mulakaluri, N. and Pentcheva, R.
    Journal of Physical Chemistry C 116 16447-16453 (2012)
    Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe 3O 4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe 3+ to Fe 2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe 3O 4(001) surface. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302259d
  • 2012 • 54 Quantitative label-free monitoring of peptide recognition by artificial receptors: A comparative FT-IR and UV resonance Raman spectroscopic study
    Niebling, S. and Kuchelmeister, H.Y. and Schmuck, C. and Schlücker, S.
    Chemical Science 3 3371-3377 (2012)
    Vibrational spectroscopic investigations on molecular recognition processes are surprisingly rare, even at the qualitative level. In this first comparative study, we employ Fourier-transform infrared (FT-IR) and UV resonance Raman (UVRR) spectroscopy for quantitative label-free monitoring of molecular recognition processes. Specifically, the complexation of two different tetrapeptide ligands by an artificial receptor is investigated. The central advantage of UVRR is its capability to selectively probe the binding site of the receptor in the free/unbound and complexed form. In contrast, FT-IR probes the entire receptor-ligand complex without spectral selectivity, thereby providing complementary vibrational information. Multivariate analysis of the experimental IR/UVRR binding studies is required for determining association constants and the vibrational spectrum of the complex, which is not directly accessible. Both FT-IR and UVRR spectroscopy provide similar association constants for the two different tetrapeptide ligands. Complementary DFT calculations support the interpretation of the observed spectral changes upon complexation, which is a prerequisite for extracting structural information from vibrational binding studies. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sc20811c
  • 2012 • 53 Density functional theory study of water adsorption on FeOOH surfaces
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Surface Science 606 1623-1632 (2012)
    Using density functional theory (DFT) calculations with an on-site Coulomb repulsion term, we study the composition, stability, and electronic properties of the most common FeOOH surfaces goethite(101), akaganeite(100), and lepidocrocite(010), and their interaction with water. Despite the differences in surface structure, the trends in surface stability of these FeOOH polymorphs exhibit remarkable similarities. We find that the reactivity and the binding configuration of adsorbates depend strongly on the coordination of surface iron: at the fourfold coordinated Fe2 site water is chemisorbed, whereas at the fivefold coordinated Fe1 water is only loosely bound with hydrogen pointing towards the surface. Our results show that the oxidation state of surface iron can be controlled by the surface termination where ferryl (Fe 4 +) species emerge for oxygen terminated surfaces and ferrous iron (Fe 2 +) at iron and water terminations leading to a reduced band gap. In contrast, the fully hydroxylated surfaces, identified as stable surface configurations at standard conditions from the surface phase diagram, show electronic properties and band gaps closest to bulk FeOOH with ferric surface iron (Fe 3 +). Only in the case of goethite(101), a termination with mixed surface hydroxyl and aquo groups is stabilized. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.07.009
  • 2012 • 52 Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3/SrTiO 3(001)
    Pentcheva, R. and Arras, R. and Otte, K. and Ruiz, V.G. and Pickett, W.E.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370 4904-4926 (2012)
    A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system. © 2012 The Royal Society.
    view abstractdoi: 10.1098/rsta.2012.0202
  • 2012 • 51 Toughness enhancement in TiAlN-based quarternary alloys
    Sangiovanni, D.G. and Chirita, V. and Hultman, L.
    Thin Solid Films 520 4080-4088 (2012)
    Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl) 1 - xM xN thin films in the B1 structure, with 0.06 ≤ x ≤ 0.75, obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti 0.5Al 0.5 N. For (TiAl) 1 - xW xN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t 2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2012.01.030
  • 2012 • 50 Combined ab initio, experimental, and CALPHAD approach for an improved thermodynamic evaluation of the Mg-Si system
    Schick, M. and Hallstedt, B. and Glensk, A. and Grabowski, B. and Hickel, T. and Hampl, M. and Gröbner, J. and Neugebauer, J. and Schmid-Fetzer, R.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 37 77-86 (2012)
    A new thermodynamic evaluation of the well-known Mg-Si system is presented with the aim to resolve persistent uncertainties in the Gibbs energy of its only compound, Mg 2Si. For this purpose the heat capacity and enthalpy of melting of Mg 2Si were measured by differential scanning calorimetry. Using finite temperature density functional theory and the quasiharmonic approximation, thermodynamic properties of Mg 2Si were additionally calculated up to and above its melting temperature. Using these new data, in particular the heat capacity, the Mg-Si system was evaluated thermodynamically with the CALPHAD method leading to a thermodynamic description of the system within narrow bounds. In contrast to several previous evaluations there is no problem with an inverted miscibility gap in the liquid. Although present enthalpy of melting data turned out to be inconsistent with other data in this system, the new evaluation accurately describes all other available data in this system. In particular the Gibbs energy of Mg 2Si can now be considered reliably described.© 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.calphad.2012.02.001
  • 2012 • 49 DFT calculations suggest a new type of self-protection and self-inhibition mechanism in the mammalian heme enzyme myeloperoxidase: Nucleophilic addition of a functional water rather than one-electron reduction
    Sicking, W. and Somnitz, H. and Schmuck, C.
    Chemistry - A European Journal 18 10937-10948 (2012)
    The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl- to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd IIa'. However, the suggested mechanism and our proposed functional structure of Cpd IIa' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II′, we named it FeIIa'. The proposed new intermediates Cpd IIa' and FeIIa' allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN- but not Cl-, and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general. Heme mechanisms explained: Direct visual comparison with Cpd II demonstrates that Cpd II′ is a one-electron reduced intermediate with respect to the heme system. In both cases an electron is transferred: in Cpd II from an external donor, and in Cpd II′ through charge separation caused by reaction with a water molecule (see figure). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201103477
  • 2012 • 48 Atomistic picture of charge density wave formation at surfaces
    Wall, S. and Krenzer, B. and Wippermann, S. and Sanna, S. and Klasing, F. and Hanisch-Blicharski, A. and Kammler, M. and Schmidt, W.G. and Horn-Von Hoegen, M.
    Physical Review Letters 109 (2012)
    We used ultrafast electron diffraction and density-functional theory calculations to gain insight into the charge density wave (CDW) formation on In/Si(111). Weak excitation by a femtosecond-laser pulse results in the melting of the CDW. The immediate freezing is hindered by a barrier for the motion of atoms during the phase transition: The melted CDW constitutes a long-lived, supercooled phase and is strong evidence for a first-order transition. The freezing into the CDW is triggered by preexisting adsorbates. Starting at these condensation nuclei, the CDW expands one dimensionally on the In/Si(111) surface, with a constant velocity of more than 80m/s. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.109.186101
  • 2011 • 47 Electronic properties of KDP and DKDP crystals: Ab-initio calculations and spectral ellipsometry experiment
    Andriyevsky, B. and Patryn, A. and Cobet, C. and Przesławski, J. and Kosturek, B. and Esser, N. and Dorywalski, K.
    Ferroelectrics 417 20-24 (2011)
    Electronic band structure, density of states and complex dielectric function ε(E) of KDP (KH 2PO 4) crystal at Fdd2 and F4d2 space groups of symmetry corresponding to the ferroelectric and paraelectric phases, have been calculated within the density functional theory using the VASP code. The experimental dielectric function ε(E) of KDP and DKDP (KD 2PO 4) crystals have been studied by the spectroscopic ellipsometry method in the photon energy range of 5-18 eV using synchrotron radiation. Temperature dependences of the dielectric function ε(T) and the intensity of reflected light I R(T) of KDP and DKDP crystals have been measured and discussed. Copyright © Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00150193.2011.578461
  • 2011 • 46 Graphene as a reversible spin manipulator of molecular magnets
    Bhandary, S. and Ghosh, S. and Herper, H. and Wende, H. and Eriksson, O. and Sanyal, B.
    Physical Review Letters 107 (2011)
    One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S=1→S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.107.257202
  • 2011 • 45 The object-oriented DFT program library S/PHI/nX
    Boeck, S. and Freysoldt, C. and Dick, A. and Ismer, L. and Neugebauer, J.
    Computer Physics Communications 182 543-554 (2011)
    In order to simplify the development and implementation process of quantum mechanical algorithms, we developed a set of object-oriented C++ libraries which can exploit modern computer architectures. The libraries are characterized as follows: (i) State-of-the-art computer science techniques have been applied or developed in this work to provide language elements to express algebraic expressions efficiently on modern computer platforms. (ii) Quantum mechanical algorithms are crucial in the field of materials research. The new libraries support the Dirac notation to implement such algorithms in the native language of physicists. (iii) The libraries are completed by elements to express equations of motions efficiently which is required for implementing structural algorithms such as molecular dynamics. Based on these libraries we introduce the DFT program package S/PHI/nX. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cpc.2010.09.016
  • 2011 • 44 Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting,...
    115 1295-1301 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106822s
  • 2011 • 43 Composition-dependent basics of smart heusler materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Buchelnikov, V.D. and Chernenko, V.A.
    Materials Science Forum 684 1-29 (2011)
    The structural and magnetic order are the decisive elements which vastly determine the properties of smart ternary intermetallics such as X2YZ Heusler alloys. Here, X and Y are transition metal elements and Z is an element from the III-V group. In order to give a precise prescription of the possibilities to optimize the magnetic shape memory and magnetocaloric effects of these alloys, we use density functional theory calculations. In particular, we outline how one may find new intermetallics which show higher Curie and martensite transformation temperatures when compared with the prototypical magnetic shape-memory alloy Ni2MnGa. Higher operation temperatures are needed for technological applications at elevated temperatures. © (2011) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.684.1
  • 2011 • 42 Structural and magnetic properties of ternary Fe1-xmnxpt nanoalloys from first principles
    Gruner, M.E. and Entel, P.
    Beilstein Journal of Nanotechnology 2 162-172 (2011)
    Background: Structural and magnetic properties of binary Mn-Pt and ternary Fe1-xMnxPt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. Results: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. Conclusion: Addition of Mn effectively increases the stability of single crystalline L10 particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications. © 2011 Gruner and Entel.
    view abstractdoi: 10.3762/bjnano.2.20
  • 2011 • 41 From electrons to materials
    Hammerschmidt, T. and Madsen, G.K.H. and Rogal, J. and Drautz, R.
    Physica Status Solidi (B) Basic Research 248 2213-2221 (2011)
    In this article, we discuss how microstructural length and time scales may be reached in atomistic simulations. We bridge from electronic properties to properties of materials by employing a systematic coarse graining of the electronic structure to effective interatomic interactions. In combination with extended time scale simulations the elementary processes of microstructural evolution may then be described. We present our approach to the derivation of tight-binding models from density functional theory, the characterization of the interatomic interaction using bond-order potentials and extended time scale simulations based on adaptive kinetic Monte Carlo. Applications to structural stability in iron, internal interfaces in tungsten and hydrogen diffusion in iron are discussed briefly and relate our approach to Manfred Fähnle's work. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201147121
  • 2011 • 40 Co2+xFe1-xSi/MgO(001) Heusler alloys: Influence of off-stoichiometry and lattice distortion on the magnetic properties in bulk and on MgO(001)
    Herper, H.C. and Krumme, B. and Ebke, D. and Antoniak, C. and Weis, C. and Warland, A. and Htten, A. and Wende, H. and Entel, P.
    Journal of Applied Physics 109 (2011)
    We investigate the influence of lattice distortion and off-stoichiometry on the electronic and magnetic properties of Co2+xFe1-x Si Heusler alloys in their L21 bulk phase and on MgO(001) using the density functional theory. Our investigations show that Co excess does not significantly change the magnetic properties, whereas an increase of the Fe content can reduce the spin polarization. In addition, the influence of off-stoichiometry on x-ray absorption spectra is studied. For comparison, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements have been carried out for Co2FeSi/MgO(001) at the Co and Fe L2,3 edges. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3559487
  • 2011 • 39 Solubility of carbon in α-iron under volumetric strain and close to the Σ5(3 1 0)[0 0 1] grain boundary: Comparison of DFT and empirical potential methods
    Hristova, E. and Janisch, R. and Drautz, R. and Hartmaier, A.
    Computational Materials Science 50 1088-1096 (2011)
    The solubility of carbon in α-Fe as a function of lattice strain and in the vicinity of the ∑5(310)[001] symmetrical tilt grain boundary is calculated with ab initio methods based on density-functional theory (DFT). The results are compared to four different empirical potentials: the embedded-atom method (EAM) potentials of Lau et al. [1], Ruda et al. [2] and Hepburn et al. [3], and the modified embedded-atom method (MEAM) potential of Lee [4]. The results confirm that the solubility of carbon in body-centered-cubic (bcc) Fe increases under local volume expansion and provide quantitative data for the excess enthalpy to be used in thermodynamic databases. According to our study the excess enthalpy obtained from DFT is more strain-sensitive than the ones obtained from the tested empirical potentials. The comparison of the applied methods furthermore reveals that among the empirical potentials the MEAM is most appropriate to describe the solubility of C in bcc Fe under strain. The differences between the four empirical potentials stem from different parameterizations of the EAM potentials and, in the case of the MEAM, from the altogether different formalism that also includes angular dependent terms in the binding energy. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2010.11.006
  • 2011 • 38 A density functional theory based estimation of the anharmonic contributions to the free energy of a polypeptide helix
    Ismer, L. and Ireta, J. and Neugebauer, J.
    Journal of Chemical Physics 135 (2011)
    We have employed density functional theory to determine the temperature dependence of the intrinsic stability of an infinite poly-L-alanine helix. The most relevant helix types, i.e., the - and the 310 - helix, and several unfolded conformations, which serve as reference for the stability analysis, have been included. For the calculation of the free energies for the various chain conformations we have explicitly included both, harmonic and anharmonic contributions. The latter have been calculated by means of a thermodynamic integration approach employing stochastic Langevin molecular dynamics, which is shown to provide a dramatic increase in the computational efficiency as compared to commonly employed deterministic molecular dynamics schemes. Employing this approach we demonstrate that the anharmonic part of the free energy amounts to the order of 0.1-0.4 kcal/mol per peptide unit for all analysed conformations. Although small, the anharmonic contribution stabilizes the helical conformations with respect to the fully extended structure. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3629451
  • 2011 • 37 Electronic structure of 1/6〈202̄3〉 partial dislocations in wurtzite GaN
    Kioseoglou, J. and Kalesaki, E. and Lymperakis, L. and Neugebauer, J. and Komninou, Ph. and Karakostas, Th.
    Journal of Applied Physics 109 (2011)
    The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {112̄0} (a-plane) and {11̄00} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6〈202̄3〉 PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3569856
  • 2011 • 36 Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes
    Kronast, F. and Friedenberger, N. and Ollefs, K. and Gliga, S. and Tati-Bismaths, L. and Thies, R. and Ney, A. and Weber, R. and Hassel, C. and Römer, F.M. and Trunova, A.V. and Wirtz, C. and Hertel, R. and Dürr, H.A. and Farle, M.
    Nano Letters 11 1710-1715 (2011)
    Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl200242c
  • 2011 • 35 Precise chemical, electronic, and magnetic structure of binuclear complexes studied by means of X-ray spectroscopies and theoretical methods
    Kuepper, K. and Benoit, D.M. and Wiedwald, U. and Mögele, F. and Meyering, A. and Neumann, M. and Kappler, J.-P. and Joly, L. and Weidle, S. and Rieger, B. and Ziemann, P.
    Journal of Physical Chemistry C 115 25030-25039 (2011)
    We investigate two planar complexes MnNi and CoNi (see Scheme 1) by X-ray photoelec-tron spectroscopy (XPS) and ultralow-temperature X-ray magnetic circular dichroism (XMCD). In this way the valence states as well as the presence of uncompensated magnetic moments are obtained. The magnetism has been probed at a temperature of 0.6 K in order to reveal the magnetic ground state properties. We find that divalent Ni ions are in a diamagnetic low spin ground state in both complexes; however, in MnNi a small fraction of divalent nickel high-spin ions leads to a residual XMCD signal, indicating parallel spin alignment with the Mn spins. Mn and Co are found to be in a divalent high-spin configuration in both compounds. Theoretically, we address the energetic ordering of the different possible spin states of the binuclear complexes using (zeroth-order) relativistic approximation density functional calculations and a triple-ζ quality basis set. These results show that intermediate-spin states are often favored over low-spin states for most both metal combinations, in qualitative agreement with our experimental observations. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp2069804
  • 2011 • 34 First principles potential for the acetylene dimer and refinement by fitting to experiments
    Leforestier, C. and Tekin, A. and Jansen, G. and Herman, M.
    Journal of Chemical Physics 135 (2011)
    We report the definition and refinement of a new first principles potential for the acetylene dimer. The ab initio calculations were performed with the DFT-SAPT combination of symmetry-adapted intermolecular perturbation method and density functional theory, and fitted to a model site-site functional form. Comparison of the calculated microwave spectrum with experimental data revealed that the barriers to isomerization were too low. This potential was refined by fitting the model parameters in order to reproduce the observed transitions, an excellent agreement within ∼1 MHz being achieved. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3668283
  • 2011 • 33 Temperature-induced structural changes of tetrahydrofuran clathrate and of the liquid water/tetrahydrofuran mixture
    Lehmkühler, F. and Sakko, A. and Steinke, I. and Sternemann, C. and Hakala, M. and Sahle, C.J. and Buslaps, T. and Simonelli, L. and Galambosi, S. and Paulus, M. and Pylkkänen, T. and Tolan, M. and Hämäläinen, K.
    Journal of Physical Chemistry C 115 21009-21015 (2011)
    We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system's temperature different structural behavior of THF hydrate is reported. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp207027p
  • 2011 • 32 Theoretical modeling of growth processes, extended defects, and electronic properties of III-nitride semiconductor nanostructures
    Lymperakis, L. and Abu-Farsakh, H. and Marquardt, O. and Hickel, T. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 248 1837-1852 (2011)
    Ab initio based simulations have been proven in the past to be and still are a valuable and indispensable tool in the field of III-nitride semiconductors. They have been successfully used to explain, describe and guide growth and characterization experiments and to address a large variety of material problems at different length scales. In the present report we review on five selected topics which span different length scales, various method developments, and diverse material properties that have been theoretically addressed within the research group "Physics of nitride-based, nanostructured, light emitting devices." Schematic representation of theoretical modeling in synergy with experiment. Left: Ab initio calculated potential energy surface for adatom diffusion on the side facets of a GaN nanowire [theory from L. Lymperakis et al., Phys. Rev. B 79, 241308 (2009), SEM image from T. Aschenbrenner et al., Nanotechnology 20, 075604 (2009)]. Right: Electrostatic potential in a QD calculated by Poisson solver [theory from O. Marquardt et al., J. Appl. Phys. 106, 083707 (2009), HRTEM image from A. Pretorius et al., J. Cryst. Growth 310, 748 (2008)]. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201046511
  • 2011 • 31 A flexible, plane-wave-based formulation of continuum elasticity and multiband k·p models
    Marquardt, O. and Schulz, S. and O'Reilly, E.P. and Freysoldt, C. and Boeck, S. and Hickel, T. and Neugebauer, J.
    Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 111-112 (2011)
    We present a highly flexible, plane-wave based formulation of continuum elasticity and multiband k·p-formalism to study the elastic and electronic properties of semiconductor nanostructures. This approach has been implemented in the framework of the density functional theory (DFT) software library S/Phi/nX [1] and allows the investigation of arbitrary-shaped nanostructures such as quantum wells, wires and dots consisting of various materials. Moreover, our approach grants the flexibility to employ user-generated k·p Hamiltonians suited to the requirements of the study regarding accuracy and computational costs. © 2011 IEEE.
    view abstractdoi: 10.1109/NUSOD.2011.6041165
  • 2011 • 30 Absence of ferromagnetic interaction in Co-Co nearest neighbor impurity pairs in ZnO: An analysis from GGA+U studies
    Nayak, S.K. and Ney, A. and Gruner, M.E. and Tripathi, G.S. and Behera, S.N. and Entel, P.
    AIP Conference Proceedings 1461 261-266 (2011)
    We study the magnetic interactions of Co doped in ZnO, with the Co atoms occupying the nearest neighbor cation sites. We perform electronic structure calculations using the local density approximation (LDA), generalized gradient approximation (GGA), and GGA+U. The Hubbard U is treated separately on d-orbitals of Zn and Co, and simultaneously on the d-orbitals of both Zn and Co. Results of GGA+U studies confirm that the nearest neighbor Co-Co pair favor antiferromagnetic interaction, where the Co spins align oppositely. This is different from the LDA and GGA predictions. A general comparison of our results with experiments shows fairly good agreement. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4736900
  • 2011 • 29 SrTiO3 nanotubes with negative strain energy predicted from first principles
    Piskunov, S. and Spohr, E.
    Journal of Physical Chemistry Letters 2 2566-2570 (2011)
    On the basis of hybrid density functional theory calculations, we predict that the most energetically favorable single-walled SrTiO3 nanotubes with negative strain energy can be folded from SrTiO3 (110) nanosheets of rectangular morphology. Further formation of multiwalled tubular nanostructure with interwall distance of ∼0.46 nm yields an additional gain in energy of 0.013 eV per formula unit. (The formation energy of the most stable nanotube is 1.36 eV/SrTiO3.) Because of increase in the Ti-O bond covalency in the outer shells, SrTiO3 nanotubes can demonstrate an enhancement of their adsorption properties. Quantum confinement leads to a widening of the energy band gap of single-walled SrTiO3 nanotubes (∼6.1 eV) relative to the bulk (∼3.6 eV), which makes them attractive for further band gap engineering. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jz201050e
  • 2011 • 28 Ab initio study of the modification of elastic properties of α-iron by hydrostatic strain and by hydrogen interstitials
    Psiachos, D. and Hammerschmidt, T. and Drautz, R.
    Acta Materialia 59 4255-4263 (2011)
    The effect of hydrostatic strain and of interstitial hydrogen on the elastic properties of α-iron is investigated using ab initio density-functional theory calculations. We find that the cubic elastic constants and the polycrystalline elastic moduli to a good approximation decrease linearly with increasing hydrogen concentration. This net strength reduction can be partitioned into a strengthening electronic effect which is overcome by a softening volumetric effect. The calculated hydrogen-dependent elastic constants are used to determine the polycrystalline elastic moduli and anisotropic shear moduli. For the key slip planes in α-iron, [11̄0] and [112̄], we find a shear modulus reduction of approximately 1.6% per at.% H. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.041
  • 2011 • 27 Density functional study of carbon doping in ZnO
    Sakong, S. and Kratzer, P.
    Semiconductor Science and Technology 26 (2011)
    The formation energy and charge states of substitutional and interstitial C impurities and their complexes in ZnO have been studied using density functional theory calculations. While single CZn defects have the highest absolute stability, interstitial C in n-type ZnO prefers to form interstitial C2 pairs or CZn-Ci complexes, thereby lowering the defect formation energy. Moreover, those atomic C impurities that have low formation energy are found to be nonmagnetic in their stable charge states. However, both in p-type and n-type ZnO, certain charge states of C2 complexes possessing a spin magnetic moment are identified. This might give a clue why both p-type and n-type magnetism have been reported for C-doped ZnO samples. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/26/1/014038
  • 2011 • 26 Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration
    Sangiovanni, D.G. and Hultman, L. and Chirita, V.
    Acta Materialia 59 2121-2134 (2011)
    We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo or W at 50% concentrations. The ternary systems obtained are predicted to be supertough, as they are shown to be harder and significantly more ductile compared with reference binary systems. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternary systems. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.013
  • 2011 • 25 Syntheses and structures of triazides of heavy group 15 elements
    Schulz, S. and Lyhs, B. and Jansen, G. and Bläser, D. and Wölper, C.
    Chemical Communications 47 3401-3403 (2011)
    Synthesis of group 15-triazides E(N3)3 (E = Sb 1, Bi 2) and Py2-Bi(N3)33 (Py = pyridine). Single crystals of 1 were in situ grown by an IR-laser-assisted technique on the diffractometer. The structure of 3, which represents the first structurally characterized neutral Bi-triazide, is influenced by crystal packing effects according to DFT calculations. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc10237k
  • 2011 • 24 Designing shape-memory Heusler alloys from first-principles
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Chakrabarti, A. and Herper, H.C. and Wuttig, M. and Barman, S.R. and Singh, S. and Al-Zubi, A. and Hickel, T. and Neugebauer, J. and Gillessen, M. and Dronskowski, R. and Entel, P.
    Applied Physics Letters 99 (2011)
    The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3655905
  • 2011 • 23 Closed-shell ring coupled cluster doubles theory with range separation applied on weak intermolecular interactions
    Toulouse, J. and Zhu, W. and Savin, A. and Jansen, G. and Ángyán, J.G.
    Journal of Chemical Physics 135 (2011)
    We explore different variants of the random phase approximation to the correlation energy derived from closed-shell ring-diagram approximations to coupled cluster doubles theory. We implement these variants in range-separated density-functional theory, i.e., by combining the long-range random phase approximations with short-range density-functional approximations. We perform tests on the rare-gas dimers He2, Ne2, and Ar2, and on the weakly interacting molecular complexes of the S22 set of Jurečka [P. Jurečka, J. Šponer, J. Černý, and P. Hobza, Phys. Chem. Chem. Phys. 8, 1985 (2006)10.1039/b600027d]. The two best variants correspond to the ones originally proposed by Szabo and Ostlund [A. Szabo and N. S. Ostlund, J. Chem. Phys. 67, 4351 (1977)10.1063/1.434580]. With range separation, they reach mean absolute errors on the equilibrium interaction energies of the S22 set of about 0.4 kcal/mol, corresponding to mean absolute percentage errors of about 4, with the aug-cc-pVDZ basis set. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3626551
  • 2011 • 22 Modeling proton transfer to charged silver electrodes
    Wilhelm, F. and Schmickler, W. and Nazmutdinov, R. and Spohr, E.
    Electrochimica Acta 56 10632-10644 (2011)
    Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study proton transfer from an aqueous solution to an Ag(1 1 1) surface. DFT is applied to study Ag-water and Ag-hydronium interactions as well as proton transfer for small systems based on the cluster model. The data gained are then used to adjust an empirical Ag-water interaction potential and to reparametrize an empirical valence-bond (EVB) model, which has been successfully applied for the study of proton transfer to a Pt(1 1 1) surface before. Employing these force fields in MD simulations enables dynamic modeling of the electrolyte-metal interface on a scale large enough to give realistic results. Results from a MD trajectory study on Ag(1 1 1) are reported and compared to the analogous study for platinum. Low discharge rates on Ag(1 1 1) are observed, and the potential range for hydrogen evolution can be estimated. The different behavior relative to Pt(1 1 1) can be traced to features of the respective potential energy surfaces and to the different structural properties of the aqueous/metallic interfaces. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.04.036
  • 2011 • 21 Anomalous scaling in heteroepitaxial island dynamics on Ag(100)
    Zaum, C. and Rieger, M. and Reuter, K. and Morgenstern, K.
    Physical Review Letters 107 (2011)
    Diffusion and decay of alloyed Cu/Ag islands are investigated in the size range from 1 to 40nm2 on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above 7nm2 show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than 4nm2 diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the Cu/Ag islands. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.107.046101
  • 2010 • 20 Electronic structure of Co-phthalocyanine calculated by GGA+U and hybrid functional methods
    Bhattacharjee, S. and Brena, B. and Banerjee, R. and Wende, H. and Eriksson, O. and Sanyal, B.
    Chemical Physics 377 96-99 (2010)
    Electronic structure calculations have been performed for the Co-phthalocyanine molecule using density functional theory (DFT) within the framework of Generalized Gradient Approximation (GGA). The electronic correlation in Co 3d orbitals is treated in terms of the GGA+U method in the framework of the Hubbard model. We find that for U = 6 eV, the calculated structural parameters as well as the spectral features are in good agreement with the experimental findings. From our calculation both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are dominated by the pyrrole carbon, with a HOMO-LUMO gap of about 1.4 eV. The GGA+U results obtained with U = 6 eV compare reasonably well with the calculations performed using Gaussian basis set and hybrid functionals in terms of ground state geometry, spin state and spectral features. The calculated valence band photoemission spectrum is in quite good agreement with the recently published experimental results. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.chemphys.2010.08.020
  • 2010 • 19 Theoretical investigation of the Pt3Al ground state
    Chauke, H.R. and Minisini, B. and Drautz, R. and Nguyen-Manh, D. and Ngoepe, P.E. and Pettifor, D.G.
    Intermetallics 18 417-421 (2010)
    The deleterious low-temperature tetragonal phases in prototypical Pt-based superalloys have variously been reported as taking the tI16-U3Si (DOc), tI16-Ir3Si (DOc′) and tP16-Pt3Ga structure-types in contrast to the high-temperature cubic cP4-Cu3Au (L12) phase. We have investigated the relative stability of these four structure-types at absolute zero by using density functional theory. We find that the ground state of stoichiometric Pt3Al is tP16-Pt3Ga and that the other three lattices are mechanically unstable at absolute zero. Experiments are needed to measure the internal displacement parameters of these three competing tetragonal phases. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.08.016
  • 2010 • 18 Using Ab initio calculations in designing bcc MgLi-X alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 1198-1205 (2010)
    Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000225
  • 2010 • 17 Ab Initio guided design of bcc ternary Mg-Li-X (X=Ca, Al, Si, Zn, Cu) alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 572-576 (2010)
    Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fundamental physical properties (like formation energies and elastic constants) of a set of bcc Mg-Li and Mg-Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/p) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg-Li system, an Ashby map containing Y/r versus G/B shows that it is not possible to increase Y/r without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg-Li-X ternaries (X=Ca, Al, Si, Cu, Zn) based on stoichiometric Mg-Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900308
  • 2010 • 16 Structural ordering tendencies in the new ferromagnetic Ni-Co-Fe-Ga-Zn Heusler alloys
    Dannenberg, A. and Siewert, M. and Gruner, M.E. and Wuttig, M. and Entela, P.
    Physics Procedia 10 144-148 (2010)
    In search for new ferromagnetic shape memory alloys (FSMA) we have calculated structural energy differences, magnetic exchange interaction constants and mixing energies of quaternary (X 1X 2)YZ Heusler alloys with X 1, X 2, Y = Ni, Co, Fe and Z = Ga, Zn using density functional theory. The comparison of the energy profiles of (NiCo)FeZ, (FeNi)CoZ, and (FeCo)NiZ with Z = Ga and Zn as a function of the tetragonal distortion c/a reveals that the energetically preferred ordering type is (NiCo)FeGa and (NiCo)FeZn which shows that Fe prefers to occupy the same cubic sublattice as Ga or Zn what implies that Fe favors Co and Ni as nearest neighbors, respectively. The Curie temperatures of (NiCo)FeGa and (NiCo)FeZn are high of the order of 600 K. NiCo)FeGa, which has the same valence electron concentration (e/a = 7.5) as Ni 2MnGa and also possesses a high martensitic ransformation temperature (&gt; 500 K), is of interest for future magnetic shape memory devices.
    view abstractdoi: 10.1016/j.phpro.2010.11.090
  • 2010 • 15 First-principles study of the structural stability of L11 order in Pt-based alloys
    Dannenberg, A. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    We investigate in the framework of density functional theory the structural and electronic properties of stoichiometric L11 ordered transition metal alloys of Pt and the 3d transition metals Mn, Fe, Co, Ni and Cu. A marked dependence of the energy difference between L11 and L10 structure on the valence electron concentration is encountered, with the L1 1 order being the preferred structure for CuPt, whereas the other alloys favor the L10 arrangement. The changes of the electronic density of states on composition are well represented within a rigid-band-picture, while the transition from L10 to L11 order is accompanied by a characteristic redistribution of the minority spin states around the Fermi level. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/7/072021
  • 2010 • 14 Influence of the substrate lattice structure on the formation of quantum well states in thin in and Pb films on silicon
    Dil, J.H. and Hülsen, B. and Kampen, T.U. and Kratzer, P. and Horn, K.
    Journal of Physics Condensed Matter 22 (2010)
    The substrate lattice structure may have a considerable influence on the formation of quantum well states in a metal overlayer material. Here we study three model systems using angle resolved photoemission and low energy electron diffraction: indium films on Si(111) and indium and lead on Si(100). Data are compared with theoretical predictions based on density functional theory. We find that the interaction between the substrate and the overlayer strongly influences the formation of quantum well states; indium layers only exhibit well defined quantum well states when the layer relaxes from an initial face-centred cubic to the bulk body-centred tetragonal lattice structure. For Pb layers on Si(100) a change in growth orientation inhibits the formation of quantum well states in films thicker than 2ML. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/13/135008
  • 2010 • 13 Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite
    Elstnerová, P. and Friák, M. and Fabritius, H.O. and Lymperakis, L. and Hickel, T. and Petrov, M. and Nikolov, S. and Raabe, D. and Ziegler, A. and Hild, S. and Neugebauer, J.
    Acta Biomaterialia 6 4506-4512 (2010)
    Arthropoda, which represent nearly 80% of all known animal species, are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional biocomposite based on chitin and proteins. Some groups, such as Crustacea, reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle (Becker et al., Dalton Trans. (2005) 1814). Motivated by these facts, we present a theoretical parameter-free quantum-mechanical study of the phase stability and structural and elastic properties of Mg-substituted calcite crystals. The Mg-substitutions were chosen as examples of states that occur in complex chemical environments typical for biological systems in which calcite crystals contain impurities, the role of which is still the topic of debate. Density functional theory calculations of bulk (Ca,Mg)CO3 were performed employing 30-atom supercells within the generalized gradient approximation as implemented in the Vienna Ab-initio Simulation Package. Based on the calculated thermodynamic results, low concentrations of Mg atoms are predicted to be stable in calcite crystals in agreement with experimental findings. Examining the structural characteristics, Mg additions nearly linearly reduce the volume of substituted crystals. The predicted elastic bulk modulus results reveal that the Mg substitution nearly linearly stiffens the calcite crystals. Due to the quite large size-mismatch of Mg and Ca atoms, Mg substitution results in local distortions such as off-planar tilting of the CO32- group. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2010.07.015
  • 2010 • 12 Combined ab initio and experimental study of structural and elastic properties of Fe3Al-based ternaries
    Friák, M. and Deges, J. and Krein, R. and Frommeyer, G. and Neugebauer, J.
    Intermetallics 18 1310-1315 (2010)
    A combined theoretical and experimental study of thermodynamical, structural, and elastic properties of Fe3Al-based ternary alloys is presented. The theoretical part is based on a scale-bridging, multi-disciplinary combination of (i) thermodynamic aspects of the site preference and (ii) elastic stiffness data for substitutional ternary elements in Fe3Al single crystals, as determined by parameter-free first-principles calculations, and (iii) Hershey's homogenization model for the polycrystalline aggregates within the frame of linear elasticity theory. The approach was employed in order to explore the relation between chemical composition and both structural and elastic properties of Fe3Al ternary alloys containing the selected substituents (Ti, V, W, Cr and Si). The ab initio calculations employ density-functional theory (DFT) and the generalized gradient approximation (GGA). The determined elastic constants are used to calculate the elastic moduli, such as the Young's and bulk modulus. The theoretical results are compared to both literature data and novel impulse excitation measurements. Specifically, for Fe3Al-Ti alloys with low to medium Ti concentrations, an unexpected non-linear compositional dependence of the polycrystalline Young's modulus was found experimentally. The origin of this behavior is analyzed and discussed based on our theoretical results. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.02.025
  • 2010 • 11 Ab initio study of the anomalous volume-composition dependence in Fe-Al alloys
    Friák, M. and Neugebauer, J.
    Intermetallics 18 1316-1321 (2010)
    The experimentally observed anomalous compositional dependence of the lattice constant of Fe-Al crystals has been theoretically investigated employing density functional theory (DFT) within the generalized gradient approximation (GGA). The formation energies, equilibrium volumes and magnetic states have been determined for a dense set of different aluminium concentrations and a large variety of atomic configurations. The spin-polarized calculations for Fe-rich compounds reproduce very well the anomalous lattice-constant behavior in contrast to both the nonmagnetic and fixed-spin-moment calculations that result in nearly linear trends without any anomaly. We thus identify the change in magnetism of iron atoms as caused by an increasing number of Al atoms in the first coordination spheres to be the decisive driving force of the anomalous behavior. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.03.014
  • 2010 • 10 Chemical trends in structure and magnetism of bimetallic nanoparticles from atomistic calculations
    Gruner, M.E.
    Journal of Physics D: Applied Physics 43 (2010)
    By means of large scale first-principles calculations in the framework of density functional theory, structure and magnetism of 561 atom nanoparticles are compared in order to obtain a systematic picture of the evolution with respect to a change in the constitutional elements. The investigation comprises ordered and disordered, cuboctahedral, icosahedral and decahedral morphologies of composition A265B296, where A is one of Mn, Fe and Co and B is Pt and, additionally, with A = Fe and B = Ni, Pd, Pt, Ir and Au. Fe-Ir and Fe-Pd and Co-Pt exhibit in comparison with Fe-Pt an increased tendency to form multiply-twinned structures and prefer segregation of the heavier element to the surface. The latter trend also applies to Fe-Au, where, on the other hand, icosahedral and crystalline motifs are very close in energy. Only in Mn-Pt the formation of multiply-twinned structures is effectively suppressed. The combinations with reduced valence electron concentration, Mn-Pt and Fe-Ir, exhibit a strong preference for antiferromagnetic spin order. The structural and magnetic trends are tentatively related to the change in features in the element and site-resolved electronic density of states. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/43/47/474008
  • 2010 • 9 Thermodynamic properties of cementite (Fe3 C)
    Hallstedt, B. and Djurovic, D. and von Appen, J. and Dronskowski, R. and Dick, A. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 34 129-133 (2010)
    Cementite (Fe3 C) is one of the most common phases in steel. In spite of its importance, thermodynamic investigations, either experimental or theoretical, of cementite are infrequent. In the present work, the thermodynamic properties of cementite are reevaluated and Gibbs energy functions valid from 0 K upwards presented. At high temperature (1000 K and above), the Gibbs energy is practically unchanged compared to previous evaluations. The energy of formation at 0 K was also calculated using density functional theory. This energy of formation (+8 kJ/mol at 0 K) is in reasonable agreement with the present thermodynamic evaluation (+23.5 kJ/mol at 0 K and +27.0 kJ/mol at 298.15 K) and with a solution calorimetric measurement of the enthalpy of formation (+18.8 kJ/mol at 298.15 K). In addition, the heat capacity was calculated theoretically using ab initio data combined with statistical concepts such as the quasiharmonic approximation. The theoretical calculation agrees equally well as the present evaluation with experimental data, but suggests a different weighting of the experimental data. In order to use it directly in the thermodynamic evaluation further modifications in the Fe-C system, primarily of the fcc phase, would be required in order to reproduce phase equilibrium data with sufficient accuracy. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.calphad.2010.01.004
  • 2010 • 8 Theoretical investigation of {110} generalized stacking faults and their relation to dislocation behavior in perovskite oxides
    Hirel, P. and Marton, P. and Mrovec, M. and Elsässer, C.
    Acta Materialia 58 6072-6079 (2010)
    Studies of generalized stacking fault energy surfaces, or γ-surfaces, provide a convenient and efficient source of information on possible dislocation dissociation mechanisms and favorable glide systems. We carried out an extensive theoretical investigation of the {110}c-surface for three technologically important perovskite oxides SrTiO3, BaTiO 3, and PbTiO3. The calculations were performed using both a highly accurate first-principles density functional theory approach and simple empirical interatomic potentials. The main characteristic features common to all {110} γ-surfaces are the low energy path along the 〈110〉 direction and the existence of a single local energy minimum along this path. This minimum corresponds to an antiphase boundary that has been observed experimentally in dissociated dislocation cores in various perovskites. The energy profiles obtained using the empirical potentials agree qualitatively well with the first-principles results but there are significant quantitative discrepancies. This comparison provides a valuable insight into the quality and limitations of empirical potentials for atomistic simulations of dislocations and other extended defects in these materials. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.025
  • 2010 • 7 Extrinsic screening of ferroelectric domains in Pb (Zr0.48 Ti0.52) O3
    Krug, I. and Barrett, N. and Petraru, A. and Locatelli, A. and Mentes, T.O. and Niño, M.A. and Rahmanizadeh, K. and Bihlmayer, G. and Schneider, C.M.
    Applied Physics Letters 97 (2010)
    The variation in the surface potential as a function of the ferroelectric polarization of micron scale domains in a thin epitaxial film of Pb (Zr 0.48 Ti0.52) O3 is measured using mirror electron microscopy. Domains were written using piezoforce microscopy. The surface potential for each polarization was deduced from the mirror to low energy electron microscopy transition in the local reflectivity curve. The effect of extrinsic screening of the fixed polarization charge at the ferroelectric surface is demonstrated. The results are compared with density functional theory calculations. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3523359
  • 2010 • 6 Anomalous energetics in tetrahydrofuran clathrate hydrate revealed by X-ray compton scattering
    Lehmkühler, F. and Sakko, A. and Sternemann, C. and Hakala, M. and Nygård, K. and Sahle, C.J. and Galambosi, S. and Steinke, I. and Tiemeyer, S. and Nyrow, A. and Buslaps, T. and Pontoni, D. and Tolan, M. and Hämääinen, K.
    Journal of Physical Chemistry Letters 1 2832-2836 (2010)
    Changes in the ground-state electron momentum density of tetrahydrofuran clathrate hydrate are studied in a temperature range between 93 and 275 K by means of X-ray Compton scattering. At temperatures above 253 K, large rather unexpected differences from the Compton profiles of ice are observed. Configurational enthalpies are extracted and exhibit a rapid rise above 253 K, whereas a constant configurational heat capacity of 0.23 ± 0.07 J g -1 K-1 is found below 253 K. Density functional theory calculations suggest that this anomalous behavior originates from the structural change of the hydrate, however, no indication was found for the formation of hydrogen bonds between tetrahydrofuran and water molecules. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jz1010362
  • 2010 • 5 Plane-wave implementation of the real-space k ṡ p formalism and continuum elasticity theory
    Marquardt, O. and Boeck, S. and Freysoldt, C. and Hickel, T. and Neugebauer, J.
    Computer Physics Communications 181 765-771 (2010)
    In this work we demonstrate how second-order continuum elasticity theory and an eight-band k ṡ p model can be implemented in an existing density functional theory (DFT) plane-wave code. The plane-wave formulation of these two formalisms allows for an accurate and efficient description of elastic and electronic properties of semiconductor nanostructures such as quantum dots, wires, and films. Gradient operators that are computationally expensive in a real-space formulation can be calculated much more efficiently in reciprocal space. The accuracy can be directly controlled by the plane-wave cutoff. Furthermore, minimization schemes typically available in plane-wave DFT codes can be applied straightforwardly with only a few modifications to a plane-wave formulation of these continuum models. As an example, the elastic and electronic properties of a III-nitride quantum dot system are calculated. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cpc.2009.12.009
  • 2010 • 4 Coverage-dependent adsorption mode of water on Fe3O 4(001): Insights from first principles calculations
    Mulakaluri, N. and Pentcheva, R. and Scheffler, M.
    Journal of Physical Chemistry C 114 11148-11156 (2010)
    Using density functional theory calculations together with an on-site Coulomb repulsion term (GGA+U), we investigate the adsorption of water on Fe3O4(001). Starting from a single water molecule per (√2 × √2)R45° unit cell, we vary the concentration and configuration of water and hydroxyl groups. Isolated water molecules on the clean surface tend to dissociate heterolytically with an OH group adsorbed on top of an octahedral iron and a proton donated to a surface oxygen. Furthermore, oxygen defects are found to promote strongly water dissociation. The released protons bind to distant surface oxygen to minimize the repulsive interaction between the surface OH groups. At higher coverages, the interplay between adsorbate-adsorbate and adsorbate-substrate interactions and the formation of hydrogen bonds between the surface species result in a crossover to a mixed adsorption mode where every second molecule is dissociated. The energetic trends are related to the underlying electronic mechanisms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp100344n
  • 2010 • 3 Hydrogen vibrational modes on graphene and relaxation of the C-H stretch excitation from first-principles calculations
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 133 (2010)
    Density functional theory (DFT) calculations are used to determine the vibrational modes of hydrogen adsorbed on graphene in the low-coverage limit. Both the calculated adsorption energy of a H atom of 0.8 eV and calculated C-H stretch vibrational frequency of 2552 cm-1 are unusually low for hydrocarbons, but in agreement with data from electron energy loss spectroscopy on hydrogenated graphite. The clustering of two adsorbed H atoms observed in scanning tunneling microscopy images shows its fingerprint also in our calculated spectra. The energetically preferred adsorption on different sublattices correlates with a blueshift of the C-H stretch vibrational modes in H adatom clusters. The C-H bending modes are calculated to be in the 1100 cm-1 range, resonant with the graphene phonons. Moreover, we use our previously developed methods to calculate the relaxation of the C-H stretch mode via vibration-phonon interaction, using the Born-Oppenheimer surface for all local modes as obtained from the DFT calculations. The total decay rate of the H stretch into other H vibrations, thereby creating or annihilating one graphene phonon, is determined from Fermi's golden rule. Our calculations using the matrix elements derived from DFT calculations show that the lifetime of the H stretch mode on graphene is only several picoseconds, much shorter than on other semiconductor surfaces such as Ge(001) and Si(001). © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3474806
  • 2010 • 2 Reactions of a β-diketiminate zinc hydride complex with heterocumulenes
    Schulz, S. and Eisenmann, T. and Schmidt, S. and Bläser, D. and Westphal, U. and Boese, R.
    Chemical Communications 46 7226-7228 (2010)
    The β-diketiminate zinc hydride MesnacnacZnH (1) reacts with CO 2, C(Ni-Pr) 2 and t-BuNCO at ambient temperature with insertion into the Zn-H bond and subsequent formation of the corresponding formato (2), formamido (3) and formamidinato (4) complexes. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cc01329c
  • 2010 • 1 Structure, lattice dynamics and Fermi surface of the magnetic shape memory system Co-Ni-Ga from first principles calculations
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Entel, P.
    Physics Procedia 10 138-143 (2010)
    Advanced magnetic shape memory materials like the prototypical Ni-Mn-Ga alloy system are limited to operating temperatures that are too low for many practical applications. To overcome this problem, an intensive search for new magnetic shape memory compounds has been started. One interesting system, showing magnetic as well as conventional shape memory behavior, is Co- Ni-Ga. In this work we report systematic studies of stoichiometric Co-Ni-Ga based alloys in the full and inverse Heusler structure by means of density functional theory. A prediction of the martensitic transition temperatures can be obtained by the structural energy differences calculated for different crystal structures. In prototype, near-stoichiometric Ni-Mn-Ga, the (pre-)martensitic transformation is accompanied by an anomalous softening of one transversal acoustic phonon branch along the [110] direction which has been frequently linked to nesting features of the Fermi surface in the past. In order to clarify this aspect for the Co-Ni- Ga system, we will discuss the influence of structure on the phonon dispersions determined from first principles and investigate whether the Fermi surface of the Co-Ni-Ga compound reveals nesting features as well.
    view abstractdoi: 10.1016/j.phpro.2010.11.089