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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  • 2021 • 201 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 200 Ab initio study of the structural response to magnetic disorder and van der Waals interactions in FeSe
    Lochner, F. and Eremin, I.M. and Hickel, T. and Neugebauer, J.
    Physical Review B 103 (2021)
    The electronic structure in unconventional superconductors holds a key to understanding the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the kz dependence of the electronic dispersion, the gap structure, and the pairing mechanisms. Here, we use density functional theory to investigate the underlying structural properties in combination with a sophisticated real-space treatment of magnetic disorder for the prototype system FeSe. Our calculations demonstrate that interlayer and intralayer interactions need to be considered and that charge-driven van der Waals interactions between Se atoms instead of magnetic coupling effects drive the interlayer binding. The methodological advances and physical insights are important for upcoming investigations of the three-dimensional effects, including nontrivial topology, of FeSe1-xTex and FeSe1-xSx systems. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.054506
  • 2021 • 199 Angular-dependent interatomic potential for large-scale atomistic simulation of iron: Development and comprehensive comparison with existing interatomic models
    Starikov, S. and Smirnova, D. and Pradhan, T. and Lysogorskiy, Y. and Chapman, H. and Mrovec, M. and Drautz, R.
    Physical Review Materials 5 (2021)
    The development of classical interatomic potential for iron is a quite demanding task with a long history background. A new interatomic potential for simulation of iron was created with a focus on description of crystal defects properties. In contrast with previous studies, here the potential development was based on force-matching method that requires only ab initio data as reference values. To verify our model, we studied various features of body-centered-cubic iron including the properties of point defects (vacancy and self-interstitial atom), the Peierls energy barrier for dislocations (screw and mix types), and the formation energies of planar defects (surfaces, grain boundaries, and stacking fault). The verification also implies thorough comparison of a potential with 11 other interatomic potentials reported in literature. This potential correctly reproduces the largest number of iron characteristics which ensures its advantage and wider applicability range compared to the other considered classical potentials. Here application of the model is illustrated by estimation of self-diffusion coefficients and the calculation of fcc lattice properties at high temperature. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.5.063607
  • 2021 • 198 B2 ordering in body-centered-cubic AlNbTiV refractory high-entropy alloys
    Körmann, F. and Kostiuchenko, T. and Shapeev, A. and Neugebauer, J.
    Physical Review Materials 5 (2021)
    The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder. © 2021 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.5.053803
  • 2021 • 197 Chemically induced local lattice distortions versus structural phase transformations in compositionally complex alloys
    Ikeda, Y. and Gubaev, K. and Neugebauer, J. and Grabowski, B. and Körmann, F.
    npj Computational Materials 7 (2021)
    Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti–Zr–Hf–Ta and Ti–Zr–Nb–Hf–Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41524-021-00502-y
  • 2021 • 196 Design of refractory compositionally complex alloys with optimal mechanical properties
    Ferrari, A. and Lysogorskiy, Y. and Drautz, R.
    Physical Review Materials 5 (2021)
    We use an analytical model to propose candidate compositionally complex alloys of the Mo-Nb-Ta-W family with optimal yield stress. We then introduce a computationally tractable method based on first-principles calculations to model phase equilibria in complex alloys at arbitrary concentrations. We utilize this method to predict the phase diagram at the optimized compositions and observe a tendency towards ordering for some of the proposed alloys. By combining yield stress data and thermodynamic equilibria, we suggest two alloy compositions with optimal mechanical properties and a strong solid solution forming ability for further experimental validation. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.5.063606
  • 2021 • 195 First-principles computational exploration of ferromagnetism in monolayer GaS via substitutional doping
    Khan, R. and Rahman, A.U. and Zhang, Q. and Kratzer, P. and Ramay, S.M.
    Journal of Physics Condensed Matter 33 (2021)
    Using first-principles calculations, functionalization of the monolayer-GaS crystal structure through N or Cr-doping at all possible lattice sites has been investigated. Our results show that pristine monolayer-GaS is an indirect-bandgap, non-magnetic semiconductor. The bandgap can be tuned and a magnetic moment (MM) can be induced by the introduction of N or Cr atomic anion/cation doping in monolayer GaS. For instance, the intrinsic character of monolayer GaS can be changed by substitution of N for the S-site to p-type, while substitution of Cr at the S-site or Ga-site induces half-metallicity at sufficiently high concentrations. The defect states are located in the electronic bandgap region of the GaS monolayer. These findings help to extend the application of monolayer-GaS structures in nano-electronics and spintronics. Since the S-sites at the surface are more easily accessible to doping in experiment, we chose the S-site for further investigations. Finally, we perform calculations with ferromagnetic (FM) and antiferromagnetic (AFM) alignment of the MMs at the dopants. For pairs of impurities of the same species at low concentrations we find Cr atoms to prefer the FM state, while N atoms prefer the AFM state, both for impurities on opposite surfaces of the GaS monolayer and for impurities sharing a common Ga neighbor sitting at the same surface. Extending our study to higher concentrations of Cr atoms, we find that clusters of four Cr atoms prefer AFM coupling, whereas the FM coupling is retained for Cr atoms at larger distance arranged on a honeycomb lattice. For the latter arrangement, we estimate the FM Curie temperature T C to be 241 K. We conclude that the Cr-doped monolayer-GaS crystal structure offers enhanced electronic and magnetic properties and is an appealing candidate for spintronic devices operating close to room temperature. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/ac04ce
  • 2021 • 194 Free Molecule Studies by Perturbed γ-γ Angular Correlation: A New Path to Accurate Nuclear Quadrupole Moments
    Haas, H. and Röder, J. and Correia, J.G. and Schell, J. and Fenta, A.S. and Vianden, R. and Larsen, E.M.H. and Aggelund, P.A. and Fromsejer, R. and Hemmingsen, L.B.S. and Sauer, S.P.A. and Lupascu, D.C. and Amaral, V.S.
    Physical Review Letters 126 (2021)
    Accurate nuclear quadrupole moment values are essential as benchmarks for nuclear structure models and for the interpretation of experimentally determined nuclear quadrupole interactions in terms of electronic and molecular structure. Here, we present a novel route to such data by combining perturbed γ-γ angular correlation measurements on free small linear molecules, realized for the first time within this work, with state-of-the-art ab initio electronic structure calculations of the electric field gradient at the probe site. This approach, also feasible for a series of other cases, is applied to Hg and Cd halides, resulting in Q(Hg199,5/2-)=+0.674(17) b and Q(Cd111,5/2+)=+0.664(7) b. © 2021 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.103001
  • 2021 • 193 Influence of crystalline defects on magnetic nanodomains in a rare-earth-free magnetocrystalline anisotropic alloy
    Palanisamy, D. and Kovács, A. and Hegde, O. and Dunin-Borkowski, R.E. and Raabe, D. and Hickel, T. and Gault, B.
    Physical Review Materials 5 (2021)
    A complex interplay between magnetic domain structure and crystalline imperfections, here twins, is revealed in a rare-earth-free MnAl bulk magnet. The magnetic domains are observed to be in the nanometer range for a large part of the magnetic structure and to scale with the number density of twins formed during thermal processing. We explain this phenomenon by a reduction in domain-wall energy at the twinned regions as proven by ab initio calculations. In addition, our atomic-scale analysis reveals that the twin boundaries contain excess Mn atoms that reduce the local magnetization, serving as an obstacle for domain wall motion. These insights can help guide the strategic design of magnetic materials by controlling the initial phase distribution to tailor the twin density and hence, the distribution of domains. © 2021 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.5.064403
  • 2021 • 192 Surface localized phonon modes at the Si(553)-Au nanowire system
    Plaickner, J. and Speiser, E. and Braun, C. and Schmidt, W.G. and Esser, N. and Sanna, S.
    Physical Review B 103 (2021)
    The vibrational properties of the Si(553)-Au surface are studied by Raman spectroscopy and ab initio calculations. A multitude of surface localized phonon modes with wave number below 200 cm-1 is experimentally observed, along with two modes at about 400 cm-1. Atomistic models within density functional theory allow to assign the low-energy spectral features to vibrations within the Au chain, while the Raman signatures at higher energies are mostly localized at the Si step edge. The Raman activity of nominally silent modes associated with a large charge transfer between Au chain and Si step edge states is explained by scattering at charge density fluctuations. Temperature-dependent measurements reveal specific mode shifts that are discussed in terms of a recently proposed order-disorder phase transition. The presence of model-specific displacement patterns allows us to identify the structural models compatible with the measured spectra at low and at room temperature. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.115441
  • 2020 • 191 A group additivity methodology for predicting the thermochemistry of oxygen-containing organosilanes
    Janbazi, H. and Schulz, C. and Wlokas, I. and Wang, H. and Peukert, S.
    International Journal of Chemical Kinetics 52 918-932 (2020)
    A combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group-additivity values (GAVs) for this class of molecules. The reference species include 62 stable single-bonded, 19 cyclic, and nine double-bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature. © 2020 The Authors. International Journal of Chemical Kinetics published by Wiley Periodicals LLC
    view abstractdoi: 10.1002/kin.21410
  • 2020 • 190 A Rare Low-Spin CoIV Bis(β-silyldiamide) with High Thermal Stability: Steric Enforcement of a Doublet Configuration
    Zanders, D. and Bačić, G. and Leckie, D. and Odegbesan, O. and Rawson, J. and Masuda, J.D. and Devi, A. and Barry, S.T.
    Angewandte Chemie - International Edition 59 14138-14142 (2020)
    Attempted preparation of a chelated CoII β-silylamide resulted in the unprecedented disproportionation to Co0 and a spirocyclic cobalt(IV) bis(β-silyldiamide): [Co[(NtBu)2SiMe2]2] (1). Compound 1 exhibited a room-temperature magnetic moment of 1.8 B.M. and a solid-state axial EPR spectrum diagnostic of a rare S=1/2 configuration for tetrahedral CoIV. Ab initio semicanonical coupled-cluster calculations (DLPNO-CCSD(T)) revealed the doublet state was clearly preferred (−27 kcal mol−1) over higher spin configurations only for the bulky tert-butyl-substituted analogue. Unlike other CoIV complexes, 1 had remarkable thermal stability, and was demonstrated to form a stable self-limiting monolayer in preliminary atomic layer deposition (ALD) surface saturation experiments. The ease of synthesis and high stability make 1 an attractive starting point to investigate otherwise inaccessible CoIV intermediates and for synthesizing new materials. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.202001518
  • 2020 • 189 Ab initio investigation of electronic and magnetic properties of antiferromagnetic/ferroelectric LaMnO3/BaTiO3interface
    Kabanov, V.V. and Piyanzina, I.I. and Lysogorskiy, Yu.V. and Tayurskii, D.A. and Mamin, R.F.
    Materials Research Express 7 (2020)
    We investigate the structural, electronic and magnetic properties of LaMnO3/BaTiO3 heterostructure by means of ab initio calculations within the GGA+U approach. We consider the heterostructure when ferroelectric polarization in the BaTiO3 film is oriented perpendicular to the LaMnO3 substrate. We present atom and spin-resolved density of states calculations for LaMnO3/BaTiO3 heterostructure with different number of BaTiO3 overlayers as well as layer-resolved spectra for the conducting heterostructure. We found that the LaMnO3/BaTiO3 heterostructure becomes conducting with a significant spin polarization indicating that the interface becomes ferromagnetically ordered. The propose concept of a ferroelectrically controlled interface ferromagnetism that offers the possibility to design novel electronic devices. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1591/ab940e
  • 2020 • 188 Are Onsager's reciprocal relations necessary to apply Thermodynamic Extremal Principles?
    Hackl, K. and Fischer, F.D. and Zickler, G.A. and Svoboda, J.
    Journal of the Mechanics and Physics of Solids 135 (2020)
    Onsager's Reciprocal Relations between thermodynamic forces and fluxes, for which Onsager was awarded the Nobel Prize, automatically follow from Thermodynamic Extremal Principles. Thus, the Principles are up to now non-applicable for the treatment of experimentally determined or theoretically modeled non-reciprocal systems as e.g. those in the magnetic field. However, we can demonstrate that adding of a certain barrier constraint as bilinear form of thermodynamic forces and fluxes accounted by the Thermodynamic Extremal Principles provides to non-reciprocal relations between the thermodynamic forces and fluxes. Such a novel idea may contribute to a better understanding of physics behind non-reciprocal systems. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2019.103780
  • 2020 • 187 Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 362-376 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.004
  • 2020 • 186 Atomistic description of self-diffusion in molybdenum: A comparative theoretical study of non-Arrhenius behavior
    Smirnova, D. and Starikov, S. and Leines, G.D. and Liang, Y. and Wang, N. and Popov, M.N. and Abrikosov, I.A. and Sangiovanni, D.G. and Drautz, R. and Mrovec, M.
    Physical Review Materials 4 (2020)
    According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavior is likely related to anharmonic vibrational effects at elevated temperatures. However, it is still debated whether anharmonicity affects more the formation or migration of monovacancies, which are known to govern the self-diffusion. In this extensive atomistic simulation study we investigated thermodynamic properties of monovacancies in bcc molybdenum, here taken as a representative model system, from zero temperature to the melting point. We combined first-principles calculations and classical simulations based on three widely used interatomic potentials for Mo. In our analysis we employ static and dynamic atomistic calculations as well as statistical sampling techniques and thermodynamic integration to achieve thorough information about temperature variations of vacancy formation and migration free energies and diffusivities. In addition, we carry out large-scale molecular dynamics simulations that enable direct observation of high-temperature self-diffusion at the atomic scale. By scrutinizing the results obtained by different models and methods, we conclude that the peculiar self-diffusion behavior is likely caused by strong temperature dependence of the vacancy formation energy. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.013605
  • 2020 • 185 Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)
    Geisler, B. and Pentcheva, R.
    Physical Review B 101 (2020)
    The origin of the 3×1 reconstruction observed in epitaxial LaCoO3 films on SrTiO3(001) is assessed by first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that (3×1)-ordered oxygen vacancies (LaCoO2.67) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO2.5 brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with 3×1 low-spin/intermediate-spin/intermediate-spin (LS/IS/IS) magnetic order and a semiconducting phase with IS/IS/IS magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing (3×1)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the t2g hole emerge at the Co sites that are also observed for unstrained bulk LaCoO3 in the IS state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the IS state with the experimentally observed low conductivity during spin-state crossover without the presence of Jahn-Teller distortions. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.165108
  • 2020 • 184 Correlation analysis of strongly fluctuating atomic volumes, charges, and stresses in body-centered cubic refractory high-entropy alloys
    Ishibashi, S. and Ikeda, Y. and Körmann, F. and Grabowski, B. and Neugebauer, J.
    Physical Review Materials 4 (2020)
    Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.023608
  • 2020 • 183 Dislocation-induced breakthrough of strength and ductility trade-off in a non-equiatomic high-entropy alloy
    Guo, W. and Su, J. and Lu, W. and Liebscher, C.H. and Kirchlechner, C. and Ikeda, Y. and Körmann, F. and Liu, X. and Xue, Y. and Dehm, G.
    Acta Materialia 185 45-54 (2020)
    In conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5 ± 216.1 µm to 18.3 ± 9.3 µm. The ultimate tensile strength increases from 543 ± 4 MPa to 621 ± 8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7%) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.11.055
  • 2020 • 182 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 • 181 Electronic structure based design of thin film metallic glasses with superior fracture toughness
    Evertz, S. and Kirchlechner, I. and Soler, R. and Kirchlechner, C. and Kontis, P. and Bednarcik, J. and Gault, B. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Materials and Design 186 (2020)
    High fracture toughness is crucial for the application of metallic glasses as structural materials to avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloying Pd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2019.108327
  • 2020 • 180 Fast diffusion mechanism in Li4P2S6: Via a concerted process of interstitial Li ions
    Stamminger, A.R. and Ziebarth, B. and Mrovec, M. and Hammerschmidt, T. and Drautz, R.
    RSC Advances 10 10715-10722 (2020)
    The synthesis of Li superionic conductor Li7P3S11 may be accompanied by the formation of a detrimental Li4P2S6 phase due to a high mixing sensitivity of precursor materials. This phase exhibits a poor ionic conductivity whose origins are not fully understood. Recently Dietrich et al. investigated the energetics of Li ion migration in Li4P2S6 with nudged elastic band (NEB) calculations. The observed large migration barrier of 0.51 eV for purely interstitial diffusion leads to an interpretation of the low ionic conductivity by kinetic limitations. Based on ab initio molecular dynamics simulations (AIMD) we propose a new and energetically much more favorable diffusion path available to interstitial Li ion charge carriers that has not been considered so far. It consists of a concerted process in which a second lithium atom is pushed out from its equilibrium lattice position by the diffusing lithium ion. A detailed analysis with NEB calculations shows that the energy barrier for this concerted diffusion is only 0.08 eV, i.e. an order of magnitude lower than the previously reported value for purely interstitial diffusion. Therefore, the observed low ionic conductivity of Li4P2S6 is likely not originating from kinetic limitations due to high diffusion barriers but rather from thermodynamic reasons associated with a low concentration of free charge carriers. We therefore expect that increasing the charge carrier concentration by doping is a viable design route to optimize the ionic conductivity of this material. © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra00932f
  • 2020 • 179 Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys
    Schweinar, K. and Beeg, S. and Hartwig, C. and Rajamathi, C.R. and Kasian, O. and Piccinin, S. and Prieto, M.J. and Tanase, L.C. and Gottlob, D.M. and Schmidt, T. and Raabe, D. and Schlögl, R. and Gault, B. and Jones, T.E. and Gr...
    ACS Applied Materials and Interfaces 12 23595-23605 (2020)
    Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c03963
  • 2020 • 178 Fundamental difference in the electronic reconstruction of infinite-layer versus perovskite neodymium nickelate films on SrTiO3 (001)
    Geisler, B. and Pentcheva, R.
    Physical Review B 102 (2020)
    Motivated by recent reports of superconductivity in Sr-doped NdNiO2 films on SrTiO3(001) [Nature (London) 572, 624 (2019)NATUAS0028-083610.1038/s41586-019-1496-5], we explore the role of the polar interface on the structural and electronic properties of NdNiOn/SrTiO3(001) (n=2,3) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films (n=2), electronic reconstruction drives the surprising emergence of a two-dimensional electron gas (2DEG) at the interface involving a strong occupation of the Ti 3d states. This effect is more pronounced than in LaAlO3/SrTiO3(001) and accompanied by a substantial reconstruction of the Fermi surface: a depletion of the self-doping Nd 5d states and an enhanced Ni eg orbital polarization reaching up to 35% at the surface, reflecting a single hole in the 3dx2-y2 states, i.e., cupratelike behavior. In contrast, no 2DEG forms for perovskite films (n=3) or if a single perovskite layer persists at the interface. We show that the topotactic reaction from the perovskite to the infinite-layer phase is confined to the nickelate film, whereas the SrTiO3 substrate remains intact. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.020502
  • 2020 • 177 Generalized dipole correction for charged surfaces in the repeated-slab approach
    Freysoldt, C. and Mishra, A. and Ashton, M. and Neugebauer, J.
    Physical Review B 102 (2020)
    First-principles calculations of surfaces or two-dimensional materials with a finite surface charge invariably include an implicit or explicit compensating countercharge. We show that an ideal constant-charge counterelectrode in the vacuum region can be introduced by means of a simple correction to the electrostatic potential in close analogy to the well-known dipole correction for charge-neutral asymmetric slabs. Our generalized dipole correction accounts simultaneously for the sheet-charge electrode and the huge voltage built up between the system of interest and the counterelectrode. We demonstrate its usefulness for two prototypical cases, namely, field evaporation in the presence of huge electric fields (20 V/nm) and the modeling of charged defects at an insulator surface. We also introduce algorithmic improvements to charge initialization and preconditioning in the density functional theory algorithm that proved crucial for ensuring rapid convergence in slab systems with high electric fields. © 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. Open access publication funded by the Max Planck Society.
    view abstractdoi: 10.1103/PhysRevB.102.045403
  • 2020 • 176 Inelastic electron tunneling spectroscopy for probing strongly correlated many-body systems by scanning tunneling microscopy
    Eickhoff, F. and Kolodzeiski, E. and Esat, T. and Fournier, N. and Wagner, C. and Deilmann, T. and Temirov, R. and Rohlfing, M. and Tautz, F.S. and Anders, F.B.
    Physical Review B 101 (2020)
    We present an extension of the tunneling theory for scanning tunneling microscopy (STM) to include different types of electron-vibrational couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronically active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the antiadiabatic strong-coupling regime as well as the Kondo effect on a free-electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a 1,4,5,8-naphthalene-Tetracarboxylic acid dianhydride molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experimental scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to the reduction of the Kondo temperature and (ii) a further electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the dI/dV curve at finite frequencies. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.125405
  • 2020 • 175 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 • 174 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 • 173 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 • 172 Phonons in magnetically disordered materials: Magnetic versus phononic time scales
    Dutta, B. and Körmann, F. and Ghosh, S. and Sanyal, B. and Neugebauer, J. and Hickel, T.
    Physical Review B 101 (2020)
    The lattice dynamics in magnetic materials, such as Fe depends on the degree of disorder of the atomic magnetic moments and the time scale of spin fluctuations. Using first-principles methods, we have studied this effect by determining the force constant matrix in two limits: (i) When spin fluctuations are much faster than the atom vibrations, their combined impact is captured by a spin-space averaged force constant matrix, (ii) when individual spin fluctuations are sufficiently slow to scatter the phonon modes, the itinerant coherent potential approximation with spin-pair resolved force constants (i.e., φ↑↑,φ↓↓, and φ↑↓) is employed in this paper. The physical consequences for the vibrational spectral functions are analyzed by systematically modifying the input parameters (magnetization and ratio of force constants betweens atoms with equal and opposite spin directions) and by deriving them for the prototype material system bcc Fe from first-principles calculations. In the paramagnetic regime, the two limits yield identical phonon spectra. Below the Curie temperature, however, there are regions in the parametric study that show qualitative differences, including a broadening of the phonon peaks. For bcc Fe, however, the quantitative modifications of phonon frequencies turn out to be small. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the http://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. Open access publication funded by the Max Planck Society.
    view abstractdoi: 10.1103/PhysRevB.101.094201
  • 2020 • 171 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 • 170 Short-range order in face-centered cubic VCoNi alloys
    Kostiuchenko, T. and Ruban, A.V. and Neugebauer, J. and Shapeev, A. and Körmann, F.
    Physical Review Materials 4 (2020)
    Concentrated solid solutions including the class of high entropy alloys (HEAs) have attracted enormous attention recently. Among these alloys a recently developed face-centered cubic (fcc) equiatomic VCoNi alloy revealed extraordinary high yield strength, exceeding previous high-strength fcc CrCoNi and FeCoNiCrMn alloys. Significant lattice distortions had been reported in the VCoNi solid solution. There is, however, a lack of knowledge about potential short-range order (SRO) and its implications for most of these alloys. We performed first-principles calculations and Monte Carlo simulations to compute the degree of SRO for fcc VCoNi, namely, by utilizing the coherent-potential approximation in combination with the generalized perturbation method as well as the supercell method in combination with recently developed machine-learned potentials. We analyze the chemical SRO parameters as well as the impact on other properties such as relaxation energies and lattice distortions. © 2020 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.4.113802
  • 2020 • 169 Study of grain boundary self-diffusion in iron with different atomistic models
    Starikov, S. and Mrovec, M. and Drautz, R.
    Acta Materialia 188 560-569 (2020)
    We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.027
  • 2020 • 168 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 • 167 Tracking the ultrafast nonequilibrium energy flow between electronic and lattice degrees of freedom in crystalline nickel
    Maldonado, P. and Chase, T. and Reid, A.H. and Shen, X. and Li, R.K. and Carva, K. and Payer, T. and Horn Von Hoegen, M. and Sokolowski-Tinten, K. and Wang, X.J. and Oppeneer, P.M. and Dürr, H.A.
    Physical Review B 101 (2020)
    Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations. © 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. Funded by "https://www.kb.se/samverkan-och-utveckling/oppen-Tillgang-och-bibsamkonsortiet/bibsamkonsortiet.html" Bibsam.
    view abstractdoi: 10.1103/PhysRevB.101.100302
  • 2020 • 166 Trends in elastic properties of Ti-Ta alloys from first-principles calculations
    Chakraborty, T. and Rogal, J.
    Journal of Physics Condensed Matter 33 (2020)
    The martensitic start temperature (M s) is a technologically fundamental characteristic of high-temperature shape memory alloys. We have recently shown [Chakraborty et al 2016 Phys. Rev. B 94 224104] that the two key features in describing the composition dependence of M s are the T = 0 K phase stability and the difference in vibrational entropy which, within the Debye model, is directly linked to the elastic properties. Here, we use density functional theory together with special quasi-random structures to study the elastic properties of disordered martensite and austenite Ti-Ta alloys as a function of composition. We observe a softening in the tetragonal shear elastic constant of the austenite phase at low Ta content and a non-linear behavior in the shear elastic constant of the martensite. A minimum of 12.5% Ta is required to stabilize the austenite phase at T = 0 K. Further, the shear elastic constants and Young's modulus of martensite exhibit a maximum for Ta concentrations close to 30%. Phenomenological, elastic-constant-based criteria suggest that the addition of Ta enhances the strength, but reduces the ductile character of the alloys. In addition, the directional elastic stiffness, calculated for both martensite and austenite, becomes more isotropic with increasing Ta content. The reported trends in elastic properties as a function of composition may serve as a guide in the design of alloys with optimized properties in this interesting class of materials. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/abba67
  • 2020 • 165 Unfolding the complexity of phonon quasi-particle physics in disordered materials
    Mu, S. and Olsen, R.J. and Dutta, B. and Lindsay, L. and Samolyuk, G.D. and Berlijn, T. and Specht, E.D. and Jin, K. and Bei, H. and Hickel, T. and Larson, B.C. and Stocks, G.M.
    npj Computational Materials 6 (2020)
    The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum-mechanical many-body interactions. However, the quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here, we report first-principles phonon calculations and inelastic X-ray and neutron-scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and NiFeCo) with force-constant dominant disorder—confronting a key 50-year-old assumption in the Hamiltonian of all mean-field quasi-particle solutions for off-diagonal disorder. Our results have revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force-constant disorder that can dominate phonon scattering. This discovery not only identifies a critical analysis issue that has broad implications for other elementary excitations, such as magnons and skyrmions in magnetic alloys, but also provides an important tool for the design of materials with ultralow thermal conductivities. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41524-020-0271-3
  • 2020 • 164 Unveiling the mechanism of abnormal magnetic behavior of FeNiCoMnCu high-entropy alloys through a joint experimental-theoretical study
    Rao, Z. and Dutta, B. and Körmann, F. and Ponge, D. and Li, L. and He, J. and Stephenson, L. and Schäfer, L. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Li, Z.
    Physical Review Materials 4 (2020)
    We combined experimental investigations and theoretical calculations to unveil an abnormal magnetic behavior caused by addition of the nonmagnetic element Cu in face-centered-cubic FeNiCoMn-based high-entropy alloys (HEAs). Upon Cu addition, the probed HEAs show an increase of both Curie temperature and saturation magnetization in as-cast and homogenized states. Specifically, the saturation magnetization of the as-cast HEAs at room temperature increases by 77% and 177% at a Cu content of 11 and 20 at. %, respectively, compared to the as-cast equiatomic FeNiCoMn HEA without Cu. The increase in saturation magnetization of the as-cast HEAs is associated with the formation of an Fe-Co rich phase in the dendritic regions. For the homogenized HEAs, the magnetic state at room temperature transforms from paramagnetism to ferromagnetism after 20 at. % Cu addition. The increase of the saturation magnetization and Curie temperature cannot be adequately explained by the formation of Cu enriched zones according to atom probe tomography analysis. Ab initio calculations suggest Cu plays a pivotal role in the stabilization of a ferromagnetic ordering of Fe, and reveal an increase of the Curie temperature caused by Cu addition which agrees well with the experimental results. The underlying mechanism behind this phenomenon lies in a combined change in unit-cell volume and chemical composition and the related energetic stabilization of the magnetic ordering upon Cu alloying as revealed by theoretical calculations. Thus, the work unveils the mechanisms responsible for the Cu effect on the magnetic properties of FeNiCoMn HEAs, and suggests that nonmagnetic elements are also crucial to tune and improve magnetic properties of HEAs. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.014402
  • 2020 • 163 Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures
    Speiser, E. and Esser, N. and Halbig, B. and Geurts, J. and Schmidt, W.G. and Sanna, S.
    Surface Science Reports 75 (2020)
    Low-dimensional self-organized surface structures, induced by (sub)monolayer metal adsorbates on semiconductor surfaces may give rise not only to a variety of emergent electronic properties, but also to a multitude of specific localized vibronic features. The focus of this review is on the analysis of these novel surface vibration eigenmodes. The application of in situ surface Raman spectroscopy under UHV conditions on clean semiconductor surfaces and those with self-ordered adsorbates, in close conjunction with the calculations of Raman spectra, based on the first-principles determination of the structural, electronic and vibronic properties, allows a consistent determination of the vibration eigenfrequencies, symmetry properties, and elongation patterns of the systems of interest. The localized nature of the surface eigenmodes determines the surface sensitivity, independent of the large penetration depth of light. The surface contribution can be selectively enhanced by employing resonance conditions to surface electronic transitions. Moreover, surface and bulk contributions can be separated by taking difference spectra between various stages of surface preparation. The relevant surfaces are Ge and especially Si with different orientations ((111) and vicinal (hhk)), on which the adsorption of various metals (Au, Sn, Pb, or In) gives rise to two- and quasi-one-dimensional structures (e.g. Au-(5 × 2)/Si(111)) with a variety of vibration modes. The Raman analysis of these modes not only enables the distinction between different proposed structural models (e.g. for Au-(3×3)/Si(111)), but also gives access to the role of electron-phonon coupling in structural phase transitions (e.g. for In-(8 × 2)–(4 × 1)/Si(111)). © 2020 The Authors
    view abstractdoi: 10.1016/j.surfrep.2020.100480
  • 2019 • 162 Ab initio molecular dynamics simulations of the ferroelectric-paraelectric phase transition in sodium nitrite
    Dürholt, J.P. and Schmid, R.
    Physical Review Materials 3 (2019)
    This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization Pr was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature Tc between 425 and 450K in excellent agreement with the experimental value of 437K. In addition, the anomalous lattice behavior (shrinking of the c axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a c-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by NaNO2. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.094408
  • 2019 • 161 Ab initio phase stabilities of Ce-based hard magnetic materials and comparison with experimental phase diagrams
    Sözen, H.Ä. and Ener, S. and MacCari, F. and Skokov, K.P. and Gutfleisch, O. and Körmann, F. and Neugebauer, J. and Hickel, T.
    Physical Review Materials 3 (2019)
    Recent developments in electrical transportation and renewable energies have significantly increased the demand of hard magnetic materials with a reduced critical rare-earth content, but with properties comparable to (Nd,Dy)-Fe-B permanent magnets. Though promising alternative compositions have been identified in high-throughput screenings, the thermodynamic stability of these phases against decomposition into structures with much less favorable magnetic properties is often unclear. In the case of Ce-Fe-Ti alloys, we have used finite temperature ab initio methods to provide this missing information. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, F(T,V), is calculated for the desired hard magnetic CeFe11Ti phase and all relevant competing phases. The latter have been confirmed experimentally by employing reactive crucible melting (RCM) and energy-dispersive x-ray spectroscopy (EDS). Our ab initio based free energy calculations reveal that the presence of the CeFe2 Laves phase suppresses the formation of CeFe11Ti up to 700 K. The result is in agreement with RCM, in which CeFe11Ti is only observed above 1000 K, while the CeFe2 and Ce2Fe17 phases are stable at lower temperatures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.084407
  • 2019 • 160 Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)
    Bertram, C. and Fang, W. and Pedevilla, P. and Michaelides, A. and Morgenstern, K.
    Nano Letters 19 3049-3056 (2019)
    A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b00392
  • 2019 • 159 Discovery of ω -free high-temperature Ti-Ta- X shape memory alloys from first-principles calculations
    Ferrari, A. and Paulsen, A. and Langenkämper, D. and Piorunek, D. and Somsen, C. and Frenzel, J. and Rogal, J. and Eggeler, G. and Drautz, R.
    Physical Review Materials 3 (2019)
    The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the ω phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high (>100°C) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-X alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below 100°C. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-X alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of ω phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress ω phase formation in a large class of alloys. ©2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.103605
  • 2019 • 158 Electronic and optical properties of pristine, N- and S-doped water-covered TiO 2 nanotube surfaces
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Chemical Physics 150 (2019)
    For rational design and improvement of electronic and optical properties of water-splitting photocatalysts, the ability to control the band edge positions relative to the water redox potentials and the photoresponse as a function of environmental conditions is essential. We combine ab initio molecular dynamics simulations with ab initio many-body theoretical calculations to predict the bandgap and band edge energies, as well as the absorption spectrum of pristine and N- and S-doped TiO 2 nanotubes using the DFT+U and G 0 W 0 approaches. Both levels of theory show similar trends, and N+S-codoping appears to be the optimal system for photocatalytic water splitting both in dry and humid conditions. However, the effect is rather moderate. Compared to DFT+U, the enhanced many-body effects in the G 0 W 0 calculations push the absolute energies of the band edges to higher values and yield increased quasi-particle bandgaps in better agreement with experiment. In dry and humid conditions, the electronic bandgap for all systems is found to be in the range of 6.0-6.2 eV with a redshift from electronic gap to optical gap. The absorption spectra show an optical anisotropy and different absorption thresholds for different light polarizations. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5050090
  • 2019 • 157 Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3
    Arab, A. and Liu, X. and Köksal, O. and Yang, W. and Chandrasena, R.U. and Middey, S. and Kareev, M. and Kumar, S. and Husanu, M.-A. and Yang, Z. and Gu, L. and Strocov, V.N. and Lee, T.-L. and Minár, J. and Pentcheva, R. and Ch...
    Nano Letters 19 8311-8317 (2019)
    Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b03962
  • 2019 • 156 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 • 155 Impact of interstitial C on phase stability and stacking-fault energy of the CrMnFeCoNi high-entropy alloy
    Ikeda, Y. and Tanaka, I. and Neugebauer, J. and Körmann, F.
    Physical Review Materials 3 (2019)
    Interstitial alloying in CrMnFeCoNi-based high-entropy alloys is known to modify their mechanical properties. Specifically, strength can be increased due to interstitial solid-solution hardening, while simultaneously affecting ductility. In this paper, first-principles calculations are carried out to analyze the impact of interstitial C atoms on CrMnFeCoNi in the fcc and the hcp phases. Our results show that C solution energies are widely spread and sensitively depend on the specific local environments. Using the computed solution-energy distributions together with statistical mechanics concepts, we determine the impact of C on the phase stability. C atoms are found to stabilize the fcc phase as compared to the hcp phase, indicating that the stacking-fault energy of CrMnFeCoNi increases due to C alloying. Using our extensive set of first-principles computed solution energies, correlations between them and local environments around the C atoms are investigated. This analysis reveals, e.g., that the local valence-electron concentration around a C atom is well correlated with its solution energy. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.113603
  • 2019 • 154 Invar effects in FeNiCo medium entropy alloys: From an Invar treasure map to alloy design
    Rao, Z. and Ponge, D. and Körmann, F. and Ikeda, Y. and Schneeweiss, O. and Friák, M. and Neugebauer, J. and Raabe, D. and Li, Z.
    Intermetallics 111 (2019)
    To facilitate the understanding of Invar effects and design of FeNiCo-base Invar alloys characterized by low thermal expansion coefficient (TEC), we investigated the magnetic and thermal expansion behavior of an equiatomic prototype medium entropy alloy FeNiCo and a non-equiatomic (super Invar)Fe63Ni32Co5 (at. %)reference alloy by means of experiments and ab initio calculations. Both alloys consist of a single face-centered cubic phase with fully recrystallized microstructure in the homogenized state. Large spontaneous volume magnetostriction is observed in both alloys below their respective Curie temperatures. The Invar effect in the non-equiatomic Fe63Ni32Co5 alloy is of step-type with nearly zero TEC over a wide temperature range (from room temperature to 120 °C)below its Curie temperature. The equiatomic FeNiCo alloy shows a peak-type Invar effect in a very narrow temperature range (from ∼675 °C to ∼730 °C)with relatively low TECs. The equiatomic FeNiCo alloy shows both higher saturation magnetization and Curie temperature than the non-equiatomic Fe63Ni32Co5 alloy. The relationships among magnetic behavior, spontaneous volume magnetostriction and Invar effects for a wider array of metallic alloys are discussed mainly based on Masumoto's rule combined with Wohlfarth's itinerant electron theory. An Invar alloy search map is constructed based on the present results and available literature data to visualize the relationships among saturation magnetization, Curie temperature and thermal expansion coefficient for a wide range of Invar alloys. Based on this treasure map a design route for further developments of new Invar alloys by tuning their magnetic properties is discussed. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2019.106520
  • 2019 • 153 Ionic Conductivity and Its Dependence on Structural Disorder in Halogenated Argyrodites Li6PS5X (X = Br, Cl, I)
    Stamminger, A.R. and Ziebarth, B. and Mrovec, M. and Hammerschmidt, T. and Drautz, R.
    Chemistry of Materials (2019)
    Halogenated argyrodites Li6PS5Br, Li6PS5Cl, and Li6PS5I exhibit large differences in the measured Li ionic conductivities. Crystallographic analysis has shown that these differences may be related to occupations of specific Wyckoff sites in different argyrodite types, but detailed understanding of the relationship between the atomic structure and operating diffusion mechanisms is still lacking. In this work, we employed ab initio molecular dynamics simulations to calculate the Li diffusivity for different argyrodite structure types. Our calculations show that the Li diffusivity does not depend implicitly on the type of halogen but is rather governed by the degree of structural disorder. Assuming disordered structures to arise naturally from the ordered structure type by thermally activated antisite defects, we are able to explain the degree of disorder found for the different types of halogens from the calculated defect formation energies. Comparing the calculated formation energies to the ionic radii of the halogen atoms, we find a strong correlation between the radii and energies required for introducing the antisite defects. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.9b02047
  • 2019 • 152 Mass transport properties of quasiharmonic vs. anharmonic transition-metal nitrides
    Sangiovanni, D.G.
    Thin Solid Films 688 (2019)
    I present a development of the color-diffusion algorithm, used in non-equilibrium (accelerated) ab initio molecular dynamics simulations of point-defect migration in crystals [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)], to determine the temperature dependence of anion vacancy jump frequencies in rocksalt-structure (B1) TiN and VN characterized by quasiharmonic (TiN) vs. strongly anharmonic (VN) lattice dynamics. Over a temperature range [≈0.6·Tm < T < ≈0.9·Tm] relatively close to the materials melting points Tm, the simulations reveal that anion vacancy migration in TiN and VN exhibits an Arrhenius-like behavior, described by activation energies Ea TiN = 4.2 ± 0.3 eV and Ea VN = 3.1 ± 0.3 eV, and attempt frequencies νTiN = 8·1015±0.7 s−1 and νVN = 2·1017±0.8 s−1. A comparison of activation energies Ea extracted by Arrhenius linear regression at elevated temperatures with ab initio Ea0K values calculated at 0 Kelvin reveals that, while the nitrogen migration energy Ea TiN varies modestly with temperature {∆Ea TiN = [Ea(Tm) – Ea(0 K)]/Ea(0 K) ≈ 0.1}, the changes in Ea VN vs. T are considerable (∆Ea VN ≈ 1). The temperature-induced variations in vacancy migration energies and diffusivities are discussed in relation to the TiN and VN vibrational properties determined via ab initio molecular dynamics at different temperatures. It is argued that static 0-K calculations, which account for thermal expansion effects within the framework of quasiharmonic transition-state theory, accurately reproduce the finite-temperature mass transport properties of TiN. Conversely, the use of molecular dynamics simulations, which explicit treat lattice vibrations at any temperature of interest, is necessary to achieve reliable atomic diffusivities in B1 VN, a crystal phase dynamically stabilized by anharmonic vibrations [Mei et al., Phys. Rev. B 91, 054101 (2015)]. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.05.016
  • 2019 • 151 Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. an...
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.174301
  • 2019 • 150 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 • 149 Partitioning of interstitial segregants during decohesion: A DFT case study of the ∑3 symmetric tilt grain boundary in ferritic steel
    Huang, X. and Janisch, R.
    Materials 12 (2019)
    The effect of hydrogen atoms at grain boundaries in metals is usually detrimental to the cohesion of the interface. This effect can be quantified in terms of the strengthening energy, which is obtained following the thermodynamic model of Rice and Wang. A critical component of this model is the bonding or solution energy of the atoms to the free surfaces that are created during decohesion. At a grain boundary in a multicomponent system, it is not immediately clear how the different species would partition and distribute on the cleaved free surfaces. In this work, it is demonstrated that the choice of partitioning pattern has a significant effect on the predicted influence of H and C on grain boundary cohesion. To this end, the ∑3(112)[110] symmetric tilt grain boundary in bcc Fe with different contents of interstitial C and H was studied, taking into account all possible distributions of the elements, as well as surface diffusion effects. H as a single element has a negative influence on grain boundary cohesion, independent of the details of the H distribution. C, on the other hand, can act both ways, enhancing or reducing the cohesion of the interface. The effect of mixed H and C compositions depends on the partition pattern. However, the general trend is that the number of detrimental cases increases with increasing H content. A decomposition of the strengthening energy into chemical and mechanical contributions shows that the elastic contribution dominates at high C contents, while the chemical contribution sets the trend for high H contents. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182971
  • 2019 • 148 Phonon-induced electronic relaxation in a strongly correlated system: The Sn/Si(111) (3 × 3) adlayer revisited
    Zahedifar, M. and Kratzer, P.
    Physical Review B 100 (2019)
    The ordered adsorbate layer Sn/Si(111) (3×3) with coverage of one third of a monolayer is considered as a realization of strong electronic correlation in surface physics. Our theoretical analysis shows that electron-hole pair excitations in this system can be long lived, up to several hundred nanoseconds, since the decay into surface phonons is found to be a highly nonlinear process. We combine first-principles calculations with help of a hybrid functional (HSE06) with modeling by a Mott-Hubbard Hamiltonian coupled to phononic degrees of freedom. The calculations show that the Sn/Si(111) (3×3) surface is insulating and the two Sn-derived bands inside the substrate band gap can be described as the lower and upper Hubbard band in a Mott-Hubbard model with U=0.75 eV. Furthermore, phonon spectra are calculated with particular emphasis on the Sn-related surface phonon modes. The calculations demonstrate that the adequate treatment of electronic correlations leads to a stiffening of the wagging mode of neighboring Sn atoms; thus, we predict that the onset of electronic correlations at low temperature should be observable in the phonon spectrum, too. The deformation potential for electron-phonon coupling is calculated for selected vibrational modes and the decay rate of an electron-hole excitation into multiple phonons is estimated, substantiating the very long lifetime of these excitations. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.125427
  • 2019 • 147 pyiron: An integrated development environment for computational materials science
    Janssen, J. and Surendralal, S. and Lysogorskiy, Y. and Todorova, M. and Hickel, T. and Drautz, R. and Neugebauer, J.
    Computational Materials Science 163 24-36 (2019)
    To support and accelerate the development of simulation protocols in atomistic modelling, we introduce an integrated development environment (IDE) for computational materials science called pyiron (http://pyiron.org). The pyiron IDE combines a web based source code editor, a job management system for build automation, and a hierarchical data management solution. The core components of the pyiron IDE are pyiron objects based on an abstract class, which links application structures such as atomistic structures, projects, jobs, simulation protocols and computing resources with persistent storage and an interactive user environment. The simulation protocols within the pyiron IDE are constructed using the Python programming language. To highlight key concepts of this tool as well as to demonstrate its ability to simplify the implementation and testing of simulation protocols we discuss two applications. In these examples we show how pyiron supports the whole life cycle of a typical simulation, seamlessly combines ab initio with empirical potential calculations, and how complex feedback loops can be implemented. While originally developed with focus on ab initio thermodynamics simulations, the concepts and implementation of pyiron are general thus allowing to employ it for a wide range of simulation topics. © 2019 The Authors
    view abstractdoi: 10.1016/j.commatsci.2018.07.043
  • 2019 • 146 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 • 145 Segregation tendency of Heusler alloys
    Sokolovskiy, V.V. and Gruner, M.E. and Entel, P. and Acet, M. and Çaklr, A. and Baigutlin, D.R. and Buchelnikov, V.D.
    Physical Review Materials 3 (2019)
    Segregation in a series of Ni2Mn1+x(In,Sn,Ga,Al)1-x and Mn2Ni1+x(Ga, Al) as well as Ni2+xMn1-xGa Heusler alloys is studied by first-principles calculations. We show that Mn-rich Ni2Mn1+x(In,Sn,Al)1-x compounds are at low temperatures unstable in the whole concentration range against decomposition into a dual-phase system consisting of an L21-cubic Ni2MnX phase with ferromagnetic order and an L10-tetragonal NiMn phase ordered antiferromagnetically. In contrast, Ni2Mn1+xGa1-x and Mn2Ni1+x(Ga,Al)1-x are stable in the narrow concentration range near the 2-1-1 stoichiometry. This concentration range depends on the presence of a martensitic transformation and becomes wider with increasing energy difference between austenite and martensite phases in a parent system. We find that ferromagnetic Ni-rich Ni2+xMn1-xGa is stable in the concentration range 0<x≤0.6. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.084413
  • 2019 • 144 Spin caloric transport from density-functional theory
    Popescu, V. and Kratzer, P. and Entel, P. and Heiliger, C. and Czerner, M. and Tauber, K. and Töpler, F. and Herschbach, C. and Fedorov, D.V. and Gradhand, M. and Mertig, I. and Kováčik, R. and Mavropoulos, P. and Wortmann, D. ...
    Journal of Physics D: Applied Physics 52 (2019)
    Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer-Büttiker methods. These are showcased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin-orbit torques. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aae8c5
  • 2019 • 143 Thermal annealing effects in polycrystalline EuTiO3 and Eu2Ti2O7
    Schell, J. and Kamba, S. and Kachlik, M. and Maca, K. and Drahokoupil, J. and Rano, B.R. and Nuno Gonçalves, J. and Thanh Dang, T. and Costa, A. and Noll, C. and Vianden, R. and Lupascu, D.C.
    AIP Advances 9 (2019)
    Using time-differential perturbed angular correlation of γrays, we investigated the electric-field gradient of polycrystalline EuTiO3 and Eu2Ti2O7, with 181Hf(181Ta) as a probe, following different thermal treatments. The measurements were performed at ISOLDE-CERN following 80 keV implantation at the Bonn Radioisotope Separator. The experimental results indicated successful induction of different phases in the implantation recovery process at 1273 and 1373 K. These observations were combined with ab initio calculations and X-ray diffraction measurements. A comparison of ab initio calculated electric-field gradients with the measured values discriminates between different structures and defects and rules out many possible cases. The Ta probe at the Ti site in the Eu2Ti2O7 phase is found to be the most probable case of site occupation after annealing at 1373 K, while annealing at 1273 K keeps EuTiO3 in the vicinity of the Ta probe. A discussion of the hyperfine interactions that promote variation in the interaction strength at the 181Ta site is presented. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5115466
  • 2019 • 142 Tuning the magnetic and magnetocaloric properties of austenitic Ni-Mn-(In,Sn)Heuslers
    Cavazzini, G. and Cugini, F. and Gruner, M.E. and Bennati, C. and Righi, L. and Fabbrici, S. and Albertini, F. and Solzi, M.
    Scripta Materialia 170 48-51 (2019)
    In this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni48Mn36In16−xSnx (x = 0–16)series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.05.027
  • 2019 • 141 Ultrastrong Medium-Entropy Single-Phase Alloys Designed via Severe Lattice Distortion
    Sohn, S.S. and Kwiatkowski da Silva, A. and Ikeda, Y. and Körmann, F. and Lu, W. and Choi, W.S. and Gault, B. and Ponge, D. and Neugebauer, J. and Raabe, D.
    Advanced Materials 31 (2019)
    Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium- and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid-solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation-mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra-strong materials for structural engineering applications. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201807142
  • 2019 • 140 Unraveling the Metastability of Cn 2+ (n = 2-4) Clusters
    Peng, Z. and Zanuttini, D. and Gervais, B. and Jacquet, E. and Blum, I. and Choi, P.-P. and Raabe, D. and Vurpillot, F. and Gault, B.
    Journal of Physical Chemistry Letters 10 581-588 (2019)
    Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of Cn 2+ (n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to Cn-1 + + C+. During C2 2+ ? C+ + C+, significant kinetic energy release (5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic 12C2 2+ dications are either in weakly bound 3?u and 3Sg - states, quickly dissociating under the intense electric field, or in a deeply bound electronic 5Su - state with lifetimes &gt;180 ps. © Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.8b03449
  • 2018 • 139 Coupling Phenomena in Magnetocaloric Materials
    Waske, A. and Dutta, B. and Teichert, N. and Weise, B. and Shayanfar, N. and Becker, A. and Hütten, A. and Hickel, T.
    Energy Technology 6 1429-1447 (2018)
    Strong coupling effects in magnetocaloric materials are the key factor to achieve a large magnetic entropy change. Combining insights from experiments and ab initio calculations, we review relevant coupling phenomena, including atomic coupling, stress coupling, and magnetostatic coupling. For the investigations on atomic coupling, we have used Heusler compounds as a flexible model system. Stress coupling occurs in first-order magnetocaloric materials, which exhibit a structural transformation or volume change together with the magnetic transition. Magnetostatic coupling has been experimentally demonstrated in magnetocaloric particles and fragment ensembles. Based on the achieved insights, we have demonstrated that the materials properties can be tailored to achieve optimized magnetocaloric performance for cooling applications. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/ente.201800163
  • 2018 • 138 CVD-grown copper tungstate thin films for solar water splitting
    Peeters, D. and Mendoza Reyes, O. and Mai, L. and Sadlo, A. and Cwik, S. and Rogalla, D. and Becker, H.-W. and Schütz, H.M. and Hirst, J. and Müller, S. and Friedrich, D. and Mitoraj, D. and Nagli, M. and Toroker, M.C. and Eichb...
    Journal of Materials Chemistry A 6 10206-10216 (2018)
    In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO4) and tungsten oxide (WO3) on F:SnO2 (FTO) substrates for photocatalytic water splitting. Variation of process parameters enables us to tune the stoichiometry of the deposits to obtain stoichiometric, W-rich, and Cu-rich deposits. It is found that the presence of Cu in WO3 thin films reduces the bandgap and enhances the absorption properties of the material in the visible range. The photoelectrocatalytic performance of stoichiometric CuWO4 was found to be superior to that of WO3 oxide under frontside illumination when thin films were used. However, detailed photoelectrochemical investigations of both thin and thicker CuWO4 films reveal that the incorporation of copper also decreases the mobility of both electrons and holes, the latter being the performance-limiting factor. These results are in line with our first-principles calculations of the electronic structure of CuWO4. A charge carrier mobility and diffusion length of ∼6× 10-3 cm2 V-1 s-1 and 30 nm were determined by time-resolved microwave conductivity measurements, values comparable to those of undoped bismuth vanadate (BiVO4). Our findings establish new insights into the advantages and limits of CuWO4-based photoanodes, and suggest a possibility of using very thin CuWO4 films on top of highly absorbing semiconductors with optimal electronic properties. © The Royal Society of Chemistry 2018.
    view abstractdoi: 10.1039/c7ta10759e
  • 2018 • 137 Finite-temperature property-maps of Li-Mn-Ni-O cathode materials from: Ab initio calculations
    Albina, J.-M. and Marusczyk, A. and Hammerschmidt, T. and Eckl, T. and Drautz, R.
    Journal of Materials Chemistry A 6 5687-5694 (2018)
    We report first-principles calculations for determining the phase relationships in multi-component cathode materials. We investigate the effect of delithiation on the phase stability, chemical potential, and open circuit voltage for a selection of cathode materials based on Li-Mn-Ni oxides at various temperatures. Entropic contributions are included by calculating the phonon frequencies in the harmonic approximation. The open circuit voltage in multi-component systems is estimated by a convex hull approach. We confirm that spinel-like phases are predominant during the charging process of layered Li-Mn-O cathode materials and that the addition of Ni reduces the spinel content. The analysis of phase stability upon delithiation suggests that the Li2MnO3 component in the Li2MnO3·Li(Mn,Ni)O2 electrode material should not exceed 60% and that the amount of Ni in the LiMnO2 component should be above 40 at% for minimizing spinel-type phase formation and minimizing oxygen formation. Using the computed structural stability at room temperature, we derive property maps for the design of Li-Mn-Ni-O cathode materials. © The Royal Society of Chemistry 2018.
    view abstractdoi: 10.1039/c7ta07221j
  • 2018 • 136 First-Principles Approach to Model Electrochemical Reactions: Understanding the Fundamental Mechanisms behind Mg Corrosion
    Surendralal, S. and Todorova, M. and Finnis, M.W. and Neugebauer, J.
    Physical Review Letters 120 (2018)
    Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calculations under controlled potentiostat conditions for electrochemical systems. The proposed approach can be straightforwardly applied in standard density functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chemical reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.120.246801
  • 2018 • 135 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 • 134 Local Structure of Nanocrystalline Aluminum Nitride
    Ognjanović, S.M. and Zähres, M. and Mayer, C. and Winterer, M.
    Journal of Physical Chemistry C 122 23749-23757 (2018)
    The local structure of chemical-vapor-synthesized (CVS) crystalline AlN nanoparticles is investigated by combining magic angle spinning nuclear magnetic resonance and X-ray absorption spectroscopies. Extended X-ray absorption fine structure data are analyzed by reverse Monte Carlo method, and X-ray absorption near edge structure is interpreted by first principles FEFF calculations. The measurements show behavior characteristic of partially disordered systems. Nevertheless, combined analysis of the data, supported by Rietveld refinement of X-ray diffraction patterns, leads to the conclusion that the observed behavior is due to the small size (large surface to volume ratios) of the nanoparticles (dXRD &lt; 6 nm) and that highly crystalline wurtzite AlN is formed during the CVS process. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b06610
  • 2018 • 133 Microscopic Insight into Electron-Induced Dissociation of Aromatic Molecules on Ice
    Auburger, P. and Kemeny, I. and Bertram, C. and Ligges, M. and Bockstedte, M. and Bovensiepen, U. and Morgenstern, K.
    Physical Review Letters 121 (2018)
    We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the π∗ orbitals of the halobenzenes from where they are transferred to σ∗ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.206001
  • 2018 • 132 Origin of the low magnetic moment in Fe2AlTi: An Ab initio study
    Friák, M. and Slávik, A. and Miháliková, I. and Holec, D. and Všianská, M. and Šob, M. and Palm, M. and Neugebauer, J.
    Materials 11 (2018)
    The intermetallic compound Fe2AlTi (alternatively Fe2TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally measured ones. To unravel the source of this discrepancy, we analyze how various mechanisms present in realistic materials such as residual strain effects or deviations from stoichiometry affect magnetism. Since in spin-unconstrained calculations the system always evolves to the spin configuration which represents a local or global minimum in the total energy surface, finite temperature spin effects are not well described. We therefore turn the investigation around and use constrained spin calculations, fixing the global magnetic moment. This approach provides direct insight into local and global energy minima (reflecting metastable and stable spin phases) as well as the curvature of the energy surface, which correlates with the magnetic entropy and thus the magnetic configuration space accessible at finite temperatures. Based on this approach, we show that deviations from stoichiometry have a huge impact on the local magnetic moment and can explain the experimentally observed low magnetic moments. © 2018 by the authors.
    view abstractdoi: 10.3390/ma11091732
  • 2018 • 131 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 • 130 Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS2-superconductors
    Cobo-Lopez, S. and Bahramy, M.S. and Arita, R. and Akbari, A. and Eremin, I.
    New Journal of Physics 20 (2018)
    We develop the realistic minimal electronic model for recently discovered BiS2 superconductors including the spin-orbit (SO) coupling based on the first-principles band structure calculations. Due to strong SO coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes p x, p y, and p z orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation. For small and intermediate doping concentrations we find the dominant instabilities to be -wave, and s ±-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsions are of the same strength, which yield the strongly anisotropic behavior of the superconducting gaps on the Fermi surface. This agrees with recent angle resolved photoemission spectroscopy findings. In addition, we find that the Fermi surface topology for BiS2 layered systems at large electron doping can resemble the doped iron-based pnictide superconductors with electron and hole Fermi surfaces maintaining sufficient nesting between them. This could provide further boost to increase T c in these systems. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aaaf87
  • 2017 • 129 Ab initio assisted design of quinary dual-phase high-entropy alloys with transformation-induced plasticity
    Li, Z. and Körmann, F. and Grabowski, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 136 262-270 (2017)
    We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co20Cr20Fe40-xMn20Nix (x = 0–20 at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co20Cr20Fe34Mn20Ni6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.07.023
  • 2017 • 128 Ab initio modelling of solute segregation energies to a general grain boundary
    Huber, L. and Grabowski, B. and Militzer, M. and Neugebauer, J. and Rottler, J.
    Acta Materialia 132 138-148 (2017)
    We apply a quantum mechanical/molecular mechanical (QM/MM) multiscale approach to calculate the segregation energies of Mg and Pb to two kinds of grain boundaries in Al. The first boundary, a symmetric (310)[001] Σ5 tilt boundary, is also tractable using traditional QM calculations, and serves as a validation for the QM/MM method. The second boundary is a general, low-symmetry tilt boundary that is completely inaccessible to pure QM calculations. QM/MM results for both of these boundaries are used to evaluate the accuracy of empirical (EAM) potentials for the Al-Mg and Al-Pb alloy systems. Based on these results we develop a physical model for the segregation energy based on elastic interaction and bond breaking terms. Both MM calculations with the EAM potentials and the model work quantitatively well for describing Mg-GB interaction across a wide range of local environments. For Pb, MM performance is weaker and the model provides only qualitative insight, demonstrating the utility of a QM/MM approach. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.024
  • 2017 • 127 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 • 126 Direct Observation of the Band Gap Transition in Atomically Thin ReS2
    Gehlmann, M. and Aguilera, I. and Bihlmayer, G. and Nemšák, S. and Nagler, P. and Gospodarič, P. and Zamborlini, G. and Eschbach, M. and Feyer, V. and Kronast, F. and Młyńczak, E. and Korn, T. and Plucinski, L. and Schüller,...
    Nano Letters 17 5187-5192 (2017)
    ReS2 is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of this van der Waals compound leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS2 using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS2 are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing the transition from an indirect band gap in bulk ReS2 to a direct gap in the bilayer and the monolayer. We also find a significantly increased effective hole mass in single-layer crystals. Our results establish bilayer ReS2 as an advantageous building block for two-dimensional devices and van der Waals heterostructures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b00627
  • 2017 • 125 Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8e7c
  • 2017 • 124 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 • 123 Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7498
  • 2017 • 122 First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production
    Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E. and D'yachkov, P.N.
    Vacuum 146 562-569 (2017)
    We have estimated theoretically the photocatalytic suitability of thinnest single-wall fluorite-structured titania (4,4) nanotube (NT) possessing three layers each (O-Ti-O) and doped by Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms substituted for host Ti atoms. For this goal, we have performed large-scale ab initio calculations on TiO2 NTs with three-layer morphology doped by 3d transition metals, using (i) the method of linear combination of atom-centered Gaussian-type orbitals (LCAO) based on the hybrid density functional theory (DFT) incorporating the Hartree-Fock (HF) exchange contribution (DFT+HF) and (ii) the method of linearized augmented cylindrical waves (LACW) with the muffin-tin approximation based on the local density functional approach (LDA). We have compared the ground state electronic structure, particularly the one-electron densities of states (DOSs) from the LCAO and LACW calculations for periodic arrangements of the 3d-metal dopant atoms. The results show clear evidence for a potential photocatalytic application for water splitting in the case of the Sc-doped titania nanotubes only. These NTs show both a reduced band gap of 2.0 eV relative to the pristine NT and an absence of defect-induced levels between the redox potentials of hydrogen and oxygen, so that electron-hole recombination becomes unlikely. Other 3d dopants with higher atomic number, although their band gap also covers the favorable green to orange region of the solar spectrum, are unsuitable because their defect-induced levels are positioned between the redox potential of oxygen and hydrogen, which can be expected to lead to rapid electron-hole recombination. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2017.05.002
  • 2017 • 121 Interatomic Coulombic Decay: The Mechanism for Rapid Deexcitation of Hollow Atoms
    Wilhelm, R.A. and Gruber, E. and Schwestka, J. and Kozubek, R. and Madeira, T.I. and Marques, J.P. and Kobus, J. and Krasheninnikov, A.V. and Schleberger, M. and Aumayr, F.
    Physical Review Letters 119 (2017)
    The impact of a highly charged ion onto a solid gives rise to charge exchange between the ion and target atoms, so that a slow ion gets neutralized in the vicinity of the surface. Using highly charged Ar and Xe ions and the surface-only material graphene as a target, we show that the neutralization and deexcitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple Interatomic Coulombic Decay (ICD) model can describe the observed ultrafast deexcitation. Other deexcitation mechanisms involving nonradiative decay and quasimolecular orbital formation during the impact are not important, as follows from the comparison of our experimental data with the results of first-principles calculations. Our method also enables the estimation of ICD rates directly. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.119.103401
  • 2017 • 120 Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As 75 NMR
    Cui, J. and Ding, Q.-P. and Meier, W.R. and Böhmer, A.E. and Kong, T. and Borisov, V. and Lee, Y. and Bud'Ko, S.L. and Valentí, R. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 96 (2017)
    We report As75 nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. As75 NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. As75 spin-lattice relaxation rates 1/T1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q=(π,0) or (0,π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the As75 quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s± nodeless two-gap superconductor. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.96.104512
  • 2017 • 119 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 • 118 Mössbauer spectroscopy evidence of intrinsic non-stoichiometry in iron telluride single crystals
    Kiiamov, A.G. and Lysogorskiy, Y.V. and Vagizov, F.G. and Tagirov, L.R. and Tayurskii, D.A. and Croitori, D. and Tsurkan, V. and Loidl, A.
    Annalen der Physik 529 (2017)
    The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of over-stoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe1+ySe1-xTex compounds, which contain excess iron atoms. (Figure presented.). © 2016 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/andp.201600241
  • 2017 • 117 Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography
    Gleich, S. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Soler, R. and Pradeep, K.G. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 122 (2017)
    In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4999304
  • 2017 • 116 Origin of Structural Modulations in Ultrathin Fe Films on Cu(001)
    Zhang, X. and Hickel, T. and Rogal, J. and Neugebauer, J.
    Physical Review Letters 118 (2017)
    Employing ab initio calculations we demonstrate that the complex structural modulations experimentally observed in ultrathin Fe films on Cu(001) originate from Fe bulk phases that arise under extreme deformations. Specifically, we show that the structural modulations correspond to the motifs observed when transforming fcc Fe to bcc Fe in the Pitsch orientation relationship [(001)fcc||(110)bcc]. The observed structural equivalence between surface and unstable bulk structures naturally explains the experimentally reported magnetic and structural transitions when going from low (two to four MLs) to intermediate (four to ten MLs) film coverages. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.118.236101
  • 2017 • 115 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 • 114 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 • 113 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 • 112 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 &gt; 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
  • 2016 • 111 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 • 110 Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory
    Lischner, J. and Nemšák, S. and Conti, G. and Gloskovskii, A. and Pálsson, G.K. and Schneider, C.M. and Drube, W. and Louie, S.G. and Fadley, C.
    Journal of Applied Physics 119 (2016)
    We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4947594
  • 2016 • 109 Atomic displacement in the CrMnFeCoNi high-entropy alloy - A scaling factor to predict solid solution strengthening
    Okamoto, N.L. and Yuge, K. and Tanaka, K. and Inui, H. and George, E.P.
    AIP Advances 6 (2016)
    Although metals strengthened by alloying have been used for millennia, models to quantify solid solution strengthening (SSS) were first proposed scarcely seventy years ago. Early models could predict the strengths of only simple alloys such as dilute binaries and not those of compositionally complex alloys because of the difficulty of calculating dislocation-solute interaction energies. Recently, models and theories of SSS have been proposed to tackle complex high-entropy alloys (HEAs). Here we show that the strength at 0 K of a prototypical HEA, CrMnFeCoNi, can be scaled and predicted using the root-mean-square atomic displacement, which can be deduced from X-ray diffraction and first-principles calculations as the isotropic atomic displacement parameter, that is, the average displacements of the constituent atoms from regular lattice positions. We show that our approach can be applied successfully to rationalize SSS in FeCoNi, MnFeCoNi, MnCoNi, MnFeNi, CrCoNi, CrFeCoNi, and CrMnCoNi, which are all medium-entropy subsets of the CrMnFeCoNi HEA. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4971371
  • 2016 • 108 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 107 Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet
    Mlynczak, E. and Eschbach, M. and Borek, S. and Minár, J. and Braun, J. and Aguilera, I. and Bihlmayer, G. and Döring, S. and Gehlmann, M. and Gospodaric, P. and Suga, S. and Plucinski, L. and Blügel, S. and Ebert, H. and Schne...
    Physical Review X 6 (2016)
    We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of ΔE = 100 meV and Δk = 0.1 Å-1. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spinorbit interaction even for elements as light as 3d ferromagnets. These results set a new paradigm for the investigations of spin-orbit effects in the spintronic materials. The same methodology could be used in the bottom-up design of the devices based on the switching of spin-orbit gaps such as electric-field control of magnetic anisotropy or tunneling anisotropic magnetoresistance.
    view abstractdoi: 10.1103/PhysRevX.6.041048
  • 2016 • 106 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 • 105 First-principles study of carbon segregation in bcc iron symmetrical tilt grain boundaries
    Wang, J. and Janisch, R. and Madsen, G.K.H. and Drautz, R.
    Acta Materialia 115 259-268 (2016)
    Segregation of light elements can profoundly affect the energies and cohesive properties of grain boundaries. First-principles calculations have been performed to determine the carbon solution energies and cohesive properties of three different grain boundaries in presence of carbon. It is demonstrated that the most stable segregation sites possess the greatest coordination number and maximum Fe-C nearest neighbor distance. Thereby a geometric criterion for predicting the segregation sites is suggested. Open grain boundary structures are shown to be more attractive to C atoms than the compact grain boundary structure, vacancies and dislocations, and C segregation at open grain boundaries decreases the grain boundary energy. The theoretical fracture strength of grain boundaries increases with C concentration and tend to similar values for certain areal concentrations irrespective of the grain boundary structures. This implies that the maximum fracture strength of a grain boundary depends on the maximum C areal concentration it can accommodate. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.058
  • 2016 • 104 High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 3367-3373 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b00447
  • 2016 • 103 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 • 102 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 • 101 Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate
    Middey, S. and Meyers, D. and Doennig, D. and Kareev, M. and Liu, X. and Cao, Y. and Yang, Z. and Shi, J. and Gu, L. and Ryan, P.J. and Pentcheva, R. and Freeland, J.W. and Chakhalian, J.
    Physical Review Letters 116 (2016)
    Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, which, along with the interactions, define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO3, combined with a dielectric spacer, LaAlO3, has been layered along the pseudocubic [111] direction. The resulting artificial graphenelike Mott crystal with magnetic 3d electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and ab initio calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.116.056801
  • 2016 • 100 Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics
    Sawada, H. and Kawakami, K. and Körmann, F. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Acta Materialia 102 241-250 (2016)
    Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.09.010
  • 2016 • 99 Quaternary Al-Cu-Mg-Si Q Phase: Sample Preparation, Heat Capacity Measurement and First-Principles Calculations
    Löffler, A. and Zendegani, A. and Gröbner, J. and Hampl, M. and Schmid-Fetzer, R. and Engelhardt, H. and Rettenmayr, M. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 37 119-126 (2016)
    The quaternary Q phase is an important precipitate phase in the Al-Cu-Mg-Si alloy system and its accurate thermodynamic description is crucial for further tailoring this material class for light-weight structural applications. In order to achieve an improved thermochemical parameter set of this phase, we used a combination of experimental measurements and first-principles calculations, which was focussed on the heat capacity. Its accurate experimental determination required the preparation of pure samples of Q phase and sophisticated calorimetric measurements. On the theoretical side, a simultaneous treatment of lattice vibrations within the quasiharmonic approximation, electronic excitations, and configuration entropy within the compound energy formalism were required to achieve a complete description of the heat capacity. The evaluation demonstrates the high predictive power of the first-principles as well as the Calphad modeling. © 2015, ASM International.
    view abstractdoi: 10.1007/s11669-015-0426-y
  • 2016 • 98 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 • 97 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 • 96 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 (&gt;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
  • 2015 • 95 Ab initio study of compositional trends in solid solution strengthening in metals with low Peierls stresses
    Ma, D. and Friák, M. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 98 367-376 (2015)
    Abstract We identify and analyze general trends governing solid solution strengthening in binary alloys containing solutes across the Periodic table using quantum-mechanical calculations. Here we present calculations for the model system of Al binary solid solutions. The identified trends originate from an approximately parabolic dependence of two strengthening parameters to quantitatively predict the solid solution strengthening effect, i.e. the volume and slip misfit parameters. The volume misfit parameter shows a minimum (concave-up behavior) as a function of the solute element group number in the periodic table, whereas the slip misfit parameter shows a maximum (concave-down behavior). By analyzing reported data, a similar trend is also found in Ni and Mg (basal slip) binary systems. Hence, these two strengthening parameters are strongly anti-correlated, which can be understood in terms of the Fermi level shift in the framework of free electron model. The chemical trends identified in this study enable a rapid and efficient identification of the solutes that provide optimum solid-solution strengthening. The approach described here may thus serve as basis for ab initio guided metallurgical materials design. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.054
  • 2015 • 94 C-, N-, S-, and Fe-Doped TiO2 and SrTiO3 Nanotubes for Visible-Light-Driven Photocatalytic Water Splitting: Prediction from First Principles
    Piskunov, S. and Lisovski, O. and Begens, J. and Bocharov, D. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Journal of Physical Chemistry C 119 18686-18696 (2015)
    The ground state electronic structure and the formation energies of both TiO<inf>2</inf> and SrTiO<inf>3</inf> nanotubes (NTs) containing C<inf>O</inf>, N<inf>O</inf>, S<inf>O</inf>, and Fe<inf>Ti</inf> substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO<inf>2</inf> NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H<inf>2</inf>O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N<inf>O</inf>+S<inf>O</inf> codoped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that ε<inf>HOIL</inf> < ε<inf>O<inf>2</inf>/H<inf>2</inf>O</inf> < ε<inf>H+/H<inf>2</inf></inf> < ε<inf>LUIL</inf>), thus decreasing the photon energy required for splitting of H<inf>2</inf>O molecule. For C- and Fe-doped TiO<inf>2</inf> NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO<inf>3</inf> NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b03691
  • 2015 • 93 Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal
    Stoffers, A. and Ziebarth, B. and Barthel, J. and Cojocaru-Mirédin, O. and Elsässer, C. and Raabe, D.
    Physical Review Letters 115 (2015)
    Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.235502
  • 2015 • 92 Computationally efficient and quantitatively accurate multiscale simulation of solid-solution strengthening by ab initio calculation
    Ma, D. and Friák, M. and Von Pezold, J. and Raabe, D. and Neugebauer, J.
    Acta Materialia 85 53-66 (2015)
    We propose an approach for the computationally efficient and quantitatively accurate prediction of solid-solution strengthening. It combines the 2-D Peierls-Nabarro model and a recently developed solid-solution strengthening model. Solid-solution strengthening is examined with Al-Mg and Al-Li as representative alloy systems, demonstrating a good agreement between theory and experiments within the temperature range in which the dislocation motion is overdamped. Through a parametric study, two guideline maps of the misfit parameters against (i) the critical resolved shear stress, τ0, at 0 K and (ii) the energy barrier, ΔEb, against dislocation motion in a solid solution with randomly distributed solute atoms are created. With these two guideline maps, τ0 at finite temperatures is predicted for other Al binary systems, and compared with available experiments, achieving good agreement. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.10.044
  • 2015 • 91 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 • 90 Differences between thermal and laser-induced diffusion
    Zaum, C. and Meyer-Auf-Der-Heide, K.M. and Mehlhorn, M. and McDonough, S. and Schneider, W.F. and Morgenstern, K.
    Physical Review Letters 114 (2015)
    A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.146104
  • 2015 • 89 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 • 88 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 • 87 Existence Theorem for Geometrically Nonlinear Cosserat Micropolar Model Under Uniform Convexity Requirements
    Neff, P. and Bîrsan, M. and Osterbrink, F.
    Journal of Elasticity 121 119-141 (2015)
    We reconsider the geometrically nonlinear Cosserat model for a uniformly convex elastic energy and write the equilibrium system as a minimization problem. Applying the direct methods of the calculus of variations we show the existence of minimizers. We present a clear proof based on the coercivity of the elastically stored energy density and on the weak lower semi-continuity of the total energy functional. Use is made of the dislocation density tensor $\overline{\boldsymbol{K}}= \overline{\boldsymbol{R}}^{T}\operatorname{Curl}\overline{\boldsymbol{R}}$ as a suitable Cosserat curvature measure. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10659-015-9517-6
  • 2015 • 86 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 • 85 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 • 84 Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 6360-6368 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201500851
  • 2015 • 83 Interference effects in T-MOKE spectra of Fe thin films at the 3p edges - Theory and experiment
    Legut, D. and Tesch, M.F. and Oppeneer, P.M. and Mertins, H.-Ch. and Jansing, Ch. and Gilbert, M. and Gaupp, A. and Bürgler, D.E. and Schneider, C. M.
    Acta Physica Polonica A 127 466-468 (2015)
    We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.
    view abstractdoi: 10.12693/APhysPolA.127.466
  • 2015 • 82 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 • 81 Interplay of strain and interdiffusion in Heusler alloy bilayers
    Dutta, B. and Hickel, T. and Neugebauer, J. and Behler, C. and Fähler, S. and Behler, A. and Waske, A. and Teichert, N. and Schmalhorst, J.-M. and Hütten, A.
    Physica Status Solidi - Rapid Research Letters 9 321-325 (2015)
    Combining conventional and inverse magnetocaloric materials promises to enhance solid state refrigeration. As a first step here we present epitaxial Ni-Mn-Ga/Ni-Mn-Sn bilayer films. We examine the dependence of the lateral and normal lattice constants on the deposition sequence by combining experimental and ab initio techniques. Structural properties are determined with X-ray diffraction as well as highresolution transmission electron microscopy, while ab initio calculations explain the interplay of strain, local relaxations and the interdiffusion of atoms. The latter is confirmed by Auger electron spectroscopy and is expected to have a noticeable impact on the functional properties of the Heusler materials. ( © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510070
  • 2015 • 80 Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstractdoi: 10.1051/matecconf/20153302001
  • 2015 • 79 Large morphological sensitivity of the magneto-thermopower in Co/Cu multilayered systems
    Popescu, V. and Kratzer, P.
    New Journal of Physics 17 1-17 (2015)
    We present results of first-principles calculations on the transport properties, both under an electric field or a temperature gradient, in Co/Cu multilayered systems. The various effects brought about by the changes in the morphological parameters, such as the number of repeats and the layer thickness, are discussed in a systematic way. Our calculations show that the Seebeck coefficient and the magneto-thermopower (MTP) converge rather rapidly with the number of Co repeats. In the range of thin Co layers, we find strong variations in the amplitude and sign of both the Seebeck coefficient and the MTP. These large variations, which have no correspondent in the (magneto)conductance, are shown to be the result of quantum well states present in the minority spin channel of thin Co layers. ©2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
    view abstractdoi: 10.1088/1367-2630/17/3/033036
  • 2015 • 78 Magnetic States of the Ni1.75Co0.25Mn1.25Cr0.25In0.5 Heusler Alloy
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The equilibrium magnetic and structural reference states of Co- and Cr-doped Ni2Mn1.5In0.5 Heusler alloy are investigated by means of the first-principles method using a supercell approach. Three different ferrimagnetic and one ferromagnetic (FM) spin configurations, as well as two supercells with different distributions of excess Mn and In atoms, were considered. It is found that for supercell #1, the FM spin state in austenite is stable, where the martensite with different spin configurations is unstable, while in the case of supercell #2, a ferrimagnetic configuration for both austenite and martensite is favorable. The different trends for martensitic transformation were studied by c/a calculations for the tetragonal and orthorombic distortions of supercells along the z -axis and the y -axis, showing martensitic variants for supercell #2 at a ratio c/a &gt; 1 and c/a < 1. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2438953
  • 2015 • 77 Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 3615-3620 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01530
  • 2015 • 76 On the mechanism of Zn4O-acetate precursors ripening to ZnO: How dimerization is promoted by hydroxide incorporation
    Milek, T. and Kirschbaum, R.W. and Gernler, M.S.V. and Lübbert, C. and Segets, D. and Drewello, T. and Peukert, W. and Zahn, D.
    Journal of Chemical Physics 143 (2015)
    We report on a combined experimental and molecular modelling study on Zn<inf>4</inf>O ion clusters stabilized by acetate molecules (OAc). In particular, ab initio calculations of acetate substitution by hydroxide ions are compared with mass spectrometry data. Though quantum calculations in the gas phase indicate strong energetic preference, no experimental evidence of stable Zn<inf>4</inf>O(OAc)<inf>6-x</inf>(OH)<inf>x</inf> clusters in ethanolic solutions could be observed. This apparent contradiction is rationalized by identifying the supportive role of hydroxide ions for the association of (OAc- → OH- substituted) Zn<inf>4</inf>O(OAc)<inf>6</inf> and Zn<inf>4</inf>O(OAc)<inf>5</inf>+ clusters. Mass spectrometry and quantum calculations hint at the stability of (Zn<inf>4</inf>O)<inf>2</inf>(OAc)<inf>12-x</inf>(OH)<inf>x</inf> dimers with x = 1, 2. Therein, the hydroxide ions establish salt-bridges that allow for the formation of additional Zn<inf>3</inf> motifs with the OH above the triangle center - a structural motif close to that of the ZnO-crystal. The association of Zn<inf>4</inf>O(OAc)<inf>6</inf> clusters is thus suggested to involve OAc- → OH- substitution as an activation step, quickly followed by dimerization and the subsequent agglomeration of oligomers. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4928190
  • 2015 • 75 Origin of shear induced β to ω transition in Ti-Nb-based alloys
    Lai, M.J. and Tasan, C.C. and Zhang, J. and Grabowski, B. and Huang, L.F. and Raabe, D.
    Acta Materialia 92 55-63 (2015)
    Ti-Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti-Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> shear, explaining why {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.040
  • 2015 • 74 Predictions of a Large Magnetocaloric Effect in Co- and Cr-Substituted Heusler Alloys Using First-Principles and Monte Carlo Approaches
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Zagrebin, M.A. and Grünebohm, A. and Entel, P.
    Physics Procedia 75 1381-1388 (2015)
    The effect of Co- and Cr-doping on magnetic and magnetocaloric poperties of Ni-Mn-(In, Ga, Sn, and Al) Heusler alloys has been theoretically studied by combining first principles with Monte Carlo approaches. The magnetic and magnetocaloric properties are obtained as a function of temperature and magnetic field using a mixed type of Potts and Blume-Emery-Griffiths model where the model parameters are obtained from ab initio calculations. The Monte Carlo calculations allowed to make predictions of a giant inverse magnetocaloric effect in partially new hypothetical magnetic Heusler alloys across the martensitic transformation. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.155
  • 2015 • 73 Rapid theory-guided prototyping of ductile Mg alloys: From binary to multi-component materials
    Pei, Z. and Friák, M. and Sandlöbes, S. and Nazarov, R. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    In order to identify a method allowing for a fast solute assessment without lengthy ab initio calculations, we analyze correlations and anti-correlation between the stacking fault energies (SFEs), which were shown to be related to the macroscopic ductility in Mg alloys, and five material parameters of 18 different elemental solutes. Our analysis reveals that the atomic volume V of pure solutes, their electronegativity ν and bulk modulus B are either linearly or logarithmically related to the SFE. Comparing the impact of solutes with that of yttrium (that increases the ductility in Mg) we propose a single numerical quantity (called yttrium similarity index, YSI) that is based on these inter-relations. Subsequently, we evaluate this new figure of merit for 76 elements from the periodic table of elements in search for solutes reducing the SFE. Limiting ourselves first to binary Mg alloys, we hardly find any alternative solutes providing similar reduction as that due to rare-earth (RE) additions. Therefore, we extended our search to ternary Mg alloys. Assuming that the physical properties of solute combinations can be represented by their average values, 2850 solute combinations were checked and 133 solute pairs (not including any RE elements) have been found to have a YSI larger than 0.85. Quantum-mechanical calculations have been subsequently performed for 11 solute pairs with YSIs higher than 0.95 and they were all found to reduce the in excellent agreement with the predictions based on the YSI. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/17/9/093009
  • 2015 • 72 Shear-Induced Detachment of Polystyrene Beads from SAM-Coated Surfaces
    Cho, K.L. and Rosenhahn, A. and Thelen, R. and Grunze, M. and Lobban, M. and Karahka, M.L. and Kreuzer, H.J.
    Langmuir 31 11105-11112 (2015)
    In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b02321
  • 2015 • 71 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 • 70 Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
    Fetzer, R. and Ouardi, S. and Honda, Y. and Liu, H.-X. and Chadov, S. and Balke, B. and Ueda, S. and Suzuki, M. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M. and Fecher, G.H. and Felser, C.
    48 (2015)
    Half-metallic Co<inf>2</inf>MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co<inf>2</inf>MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co<inf>2</inf>Mn<inf>1.30</inf>Si<inf>0.84</inf>. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced from the corresponding 2p core level spectra. The valence states are investigated by linear dichroism using both hard x-ray and very-low-photon-energy excitation. When a very low photon energy is used for excitation, the valence bands exhibit a spin polarisation of about 30% at the Fermi energy. First principles calculations reveal that the low spin polarisation might be caused by a spin-flip process in the photoelectron final states. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164002
  • 2015 • 69 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 • 68 Synergy of atom-probe structural data and quantum-mechanical calculations in a theory-guided design of extreme-stiffness superlattices containing metastable phases
    Friák, M. and Tytko, D. and Holec, D. and Choi, P.-P. and Eisenlohr, P. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    A theory-guided materials design of nano-scaled superlattices containing metastable phases is critically important for future development of advanced lamellar composites with application-dictated stiffness and hardness. Our study combining theoretical and experimental methods exemplifies the strength of this approach for the case of the elastic properties of AlN/CrN superlattices that were deposited by reactive radio-frequency magnetron sputtering with a bilayer period of 4 nm. Importantly, CrN stabilizes AlN in a metastable B1 (rock salt) cubic phase only in the form of a layer that is very thin, up to a few nanometers. Due to the fact that B1-AlN crystals do not exist as bulk materials, experimental data for this phase are not available. Therefore, quantum-mechanical calculations have been applied to simulate an AlN/CrN superlattice with a similar bilayer period. The ab initio predicted Young's modulus (428 GPa) along the [001] direction is in excellent agreement with measured nano-indentation values (408 32 GPa). Aiming at a future rapid high-throughput materials design of superlattices, we have also tested predictions obtained within linear-elasticity continuum modeling using elastic properties of B1-CrN and B1-AlN phases as input. Using single-crystal elastic constants from ab initio calculations for both phases, we demonstrate the reliability of this approach to design nano-patterned coherent superlattices with unprecedented and potentially superior properties. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/17/9/093004
  • 2015 • 67 The exponentiated Hencky-logarithmic strain energy. Part II: Coercivity, planar polyconvexity and existence of minimizers
    Neff, P. and Lankeit, J. and Ghiba, I.-D. and Martin, R. and Steigmann, D.
    Zeitschrift fur Angewandte Mathematik und Physik 66 1671-1693 (2015)
    We consider a family of isotropic volumetric–isochoric decoupled strain energies (Formula Presented.) based on the Hencky-logarithmic (true, natural) strain tensor log U, where μ > 0 is the infinitesimal shear modulus, κ=2μ+3λ3>0 is the infinitesimal bulk modulus with λ the first Lamé constant, (Formula Presented.) are dimensionless parameters, F=∇φ is the gradient of deformation, (Formula Presented.) is the right stretch tensor and (Formula Presented.) is the deviatoric part (the projection onto the traceless tensors) of the strain tensor log U. For small elastic strains, the energies reduce to first order to the classical quadratic Hencky energy (Formula Presented.) which is known to be not rank-one convex. The main result in this paper is that in plane elastostatics the energies of the family (Formula Presented.) are polyconvex for (Formula Presented.) extending a previous finding on its rank-one convexity. Our method uses a judicious application of Steigmann’s polyconvexity criteria based on the representation of the energy in terms of the principal invariants of the stretch tensor U. These energies also satisfy suitable growth and coercivity conditions. We formulate the equilibrium equations, and we prove the existence of minimizers by the direct methods of the calculus of variations. © 2015, Springer Basel.
    view abstractdoi: 10.1007/s00033-015-0495-0
  • 2015 • 66 The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 193-197 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2015.03.003
  • 2015 • 65 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
  • 2014 • 64 Ab initio based study of finite-temperature structural, elastic and thermodynamic properties of FeTi
    Zhu, L.-F. and Friák, M. and Udyansky, A. and Ma, D. and Schlieter, A. and Kühn, U. and Eckert, J. and Neugebauer, J.
    Intermetallics 45 11-17 (2014)
    We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.09.008
  • 2014 • 63 DFT-supported phase-field study on the effect of mechanically driven fluxes in Ni4Ti3 precipitation
    Kamachali, R.D. and Borukhovich, E. and Hatcher, N. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Formation of the Ni4Ti3 precipitate has a strong effect on the shape memory properties of NiTi alloys. In this work, growth of this precipitate is studied using phase-field modelling and density functional theory (DFT) calculations. Using first-principles calculations, the composition-dependent stability and elastic properties of the B2 phase are obtained. Composition-dependent elastic constants are incorporated into our phase-field model to investigate the interplay between stress and concentration fields around the precipitate. The model introduces a source of diffusion due to mechanical relaxation which is accompanied by local softening/hardening of the B2 phase. The results are discussed in light of previous experimental and simulation studies. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034003
  • 2014 • 62 Ductility improvement of Mg alloys by solid solution: Ab initio modeling, synthesis and mechanical properties
    Sandlöbes, S. and Pei, Z. and Friák, M. and Zhu, L.-F. and Wang, F. and Zaefferer, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 70 92-104 (2014)
    The I1 intrinsic stacking fault energy (I1 SFE) serves as an alloy design parameter for ductilizing Mg alloys. In view of this effect we have conducted quantum-mechanical calculations for Mg15X solid-solution crystals (X = Dy, Er, Gd, Ho, Lu, Sc, Tb, Tm, Nd, Pr, Be, Ti, Zr, Zn, Tc, Re, Co, Ru, Os, Tl). We find that Y, Sc and all studied lanthanides reduce the I1 SFE and render hexagonal closed-packed (hcp) and double hcp phases thermodynamically, structurally and elastically similar. Synthesis, experimental testing and characterization of some of the predicted key alloys (Mg-3Ho, Mg-3Er, Mg-3Tb, Mg-3Dy) indeed confirm reduced I1 SFEs and significantly improved room-temperature ductility by up to 4-5 times relative to pure Mg, a finding that is attributed to the higher activity of non-basal dislocation slip. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.02.011
  • 2014 • 61 Existence of minimizers in the geometrically non-linear 6-parameter resultant shell theory with drilling rotations
    Bîrsan, M. and Neff, P.
    Mathematics and Mechanics of Solids 19 376-397 (2014)
    This paper is concerned with the geometrically non-linear theory of 6-parametric elastic shells with drilling degrees of freedom. This theory establishes a general model for shells, which is characterized by two independent kinematic fields: the translation vector and the rotation tensor. Thus, the kinematical structure of 6-parameter shells is identical to that of Cosserat shells. We show the existence of global minimizers for the geometrically non-linear 2D equations of elastic shells. The proof of the existence theorem is based on the direct methods of the calculus of variations essentially using the convexity of the energy in the strain and curvature measures. Since our result is valid for general anisotropic shells, we analyze the particular cases of isotropic shells, orthotropic shells and composite shells separately. © The Author(s) 2013.
    view abstractdoi: 10.1177/1081286512466659
  • 2014 • 60 First-principles calculations for point defects in solids
    Freysoldt, C. and Grabowski, B. and Hickel, T. and Neugebauer, J. and Kresse, G. and Janotti, A. and Van De Walle, C.G.
    Reviews of Modern Physics 86 253-305 (2014)
    Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional (DFT+U, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials. © 2014 American Physical Society.
    view abstractdoi: 10.1103/RevModPhys.86.253
  • 2014 • 59 First-principles-based phase diagrams and thermodynamic properties of TCP phases in Re-X systems (X = Ta, V, W)
    Palumbo, M. and Fries, S.G. and Hammerschmidt, T. and Abe, T. and Crivello, J.-C. and Breidi, A.A.H. and Joubert, J.-M. and Drautz, R.
    Computational Materials Science 81 433-445 (2014)
    The structural stability of topologically close-packed phases in binary transition metal alloys is investigated with a combination of first-principles calculations based on density-functional theory and the Bragg-Williams-Gorsky approximation for the description of the configurational entropy. For a variety of different (i) exchange-correlation functionals, (ii) pseudopotentials, and (iii) relaxation schemes, for the relevant phases in Re-X (X = Ta, V, W) binary systems, we compare the energy of formation at T = 0 K, as well as the phase diagrams and site occupancies at finite temperatures. We confirm previous findings that the configurational entropy plays a stabilising role for complex phases in these systems at elevated temperatures. Small differences in the calculated energy of formation for different exchange-correlation functionals, pseudopotentials and relaxation schemes are expected, but give rise to qualitatively different phase diagrams. We employ these differences in order to estimate the order of magnitude of the standard deviation necessary in the qualitatively-reliable calculation of phase diagrams and site occupancies. In an attempt to determine the accuracy that is required to assure a qualitatively correct prediction of phase diagrams, we modify our first-principles results numerically by random variations with the determined standard deviation as maximum amplitude. Taking the order of site occupancies and the set of stable phases as simple criteria for a qualitatively correct prediction, we find that the accuracy required for the energy of formation of the individual configurations in these systems is approximately 5 meV/atom (≈0.5 kJ/mol at). © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2013.08.051
  • 2014 • 58 Hydrogen embrittlement of a carbon segregated σ5 (310) [001] symmetrical tilt grain boundary in α-Fe
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Materials Science and Engineering A 612 462-467 (2014)
    The physical and mechanical properties of a σ5 (310) [001] symmetrical tilt grain boundary (STGB) in body centred cubic (bcc) Fe are investigated by means of ab initio calculations with respect to the effect of a varying number of C and H atoms at the grain boundary. The obtained results show that with increasing number of C atoms the grain boundary energy is lowered, and the segregation energy remains negative up to a full coverage of the grain boundary with C. Thus, in a bcc Fe-C system with a sufficient amount of interstitial C, the C segregated state should be considered as the ground state of this interface. Ab initio uni-axial tensile tests of the grain boundary reveal that the work of separation as well as the theoretical strength of the σ5 (310) [001] STGB increases significantly with increasing C content. The improved cohesion due to C is mainly a chemical effect, but the mechanical contribution is also cohesion enhancing. The presence of hydrogen changes the cohesion enhancing mechanical contribution of C to an embrittling contribution, and also reduces the beneficial chemical contribution to the cohesion. When hydrogen is present together with C at the grain boundary, the reduction in strength amounts to almost 20% for the co-segregated case and to more than 25% if C is completely replaced by H. Compared to the strength of the STGB in pure iron, however, the influence of H is negligible. Hence, H embrittlement can only be understood in the three component Fe-C-H system. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.06.071
  • 2014 • 57 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 • 56 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 • 55 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 • 54 PC-SAFT parameters from ab initio calculations
    Umer, M. and Albers, K. and Sadowski, G. and Leonhard, K.
    Fluid Phase Equilibria 362 41-50 (2014)
    We use highly accurate ab initio calculations of binding enthalpies and entropies of gas phase clusters of alcohols to demonstrate how they can be used to obtain association parameters for PC-SAFT. The thermochemical results demonstrate that cooperativity effects and state dependent cluster distributions cause a strongly varying average enthalpy and entropy per bond as function of temperature and density for alcohols. In contrast to this, the two association parameters of PC-SAFT lead to density independent bond enthalpy and entropy and are thus effective parameters. Therefore, we choose to compute the cluster distribution at a universal state point and show that the thus obtained association parameters can be used to reduce the number of adjustable parameters from 5 to 3 with only a marginal loss of accuracy for most of the studied systems, and even an estimation of thermodynamic properties without adjusted parameters is possible. The ab initio calculations suggest that the 2B association scheme is more appropriate for 1-alkanols than the 3B one. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2013.08.037
  • 2014 • 53 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
  • 2014 • 52 Quantum-mechanical study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Friák, M. and Zhu, L.-F. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Key Engineering Materials 592-593 335-341 (2014)
    We use quantum-mechanical calculations to study single-crystalline elastic properties of (Ca,Mg)CO3 crystals with concentrations ranging from calcite CaCO3 to magnesite MgCO3. By analyzing results for a dense set of distributions of Ca and Mg atoms within 30-atom supercells, our theoretical study shows that those atomic configurations, that minimize the total energy for a given concentration, are characterized by elastic constants that either increase with the Mg content or remain nearly constants. Employing these ab initio calculated single-crystalline elastic parameters, the polycrystalline elastic properties of (Ca,Mg)CO3 aggregates are determined using a mean-field self-consistent homogenization method. The computed integral elastic moduli (bulk and shear) show a significant stiffening impact of Mg atoms on calcite crystals. Our analysis also demonstrates that it is not advantageous to use a granular two-phase composite of stoichiometric calcite and magnesite instead of substituting individual Ca and Mg atoms. Such two-phase aggregates are not significantly thermodynamically favorable and do not offer any strong additional stiffening effect. © (2014) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.592-593.335
  • 2014 • 51 Reliability evaluation of thermophysical properties from first-principles calculations
    Palumbo, M. and Fries, S.G. and Corso, A.D. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Journal of Physics Condensed Matter 26 (2014)
    Thermophysical properties, such as heat capacity, bulk modulus and thermal expansion, are of great importance for many technological applications and are traditionally determined experimentally. With the rapid development of computational methods, however, first-principles computed temperature-dependent data are nowadays accessible. We evaluate various computational realizations of such data in comparison to the experimental scatter. The work is focussed on the impact of different first-principles codes (quantum espresso and vasp), pseudopotentials (ultrasoft and projector augmented wave) as well as phonon determination methods (linear response and direct force constant method) on these properties. Based on the analysis of data for two pure elements, Cr and Ni, consequences for the reliability of temperature-dependent first-principles data in computational thermodynamics are discussed. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/33/335401
  • 2014 • 50 Stabilizing CuPc coordination networks on Ag(100) by Ag atoms
    Antczak, G. and Kamiński, W. and Morgenstern, K.
    Journal of Physical Chemistry C 119 1442-1450 (2014)
    We demonstrate that Ag adatoms are capable of stabilizing negatively charged copper-phthalocyanine (CuPc) molecules in a Ag-CuPc network at room temperature. For this aim, the structure of the Ag-CuPc coordination network at different molecule-adatom densities is investigated experimentally by scanning tunneling microscopy and theoretically by firstprinciples calculations. The islands formed at saturation adatom density, close to the source of adatoms, consist of a closed-packed layer without voids. The islands formed at lower adatom density consist of an irregular arrangement of larger entities, named subunits, mainly (CuPc)4Ag and (CuPc)6Ag2, which are interconnected in the same fashion as the CuPc molecules in the closed-packed layer. Silver adatoms in the subunits and between them differ by the number of molecules they link. The Ag-CuPc networks are stabilized, because the adsorption energy of CuPc molecules increases due to the presence of adatoms. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp5103803
  • 2014 • 49 Surface termination of the metal-organic framework HKUST-1: A theoretical investigation
    Amirjalayer, S. and Tafipolsky, M. and Schmid, R.
    Journal of Physical Chemistry Letters 5 3206-3210 (2014)
    The surface morphology and termination of metal-organic frameworks (MOF) is of critical importance in many applications, but the surface properties of these soft materials are conceptually different from those of other materials like metal or oxide surfaces. Up to now, experimental investigations are scarce and theoretical simulations have focused on the bulk properties. The possible surface structure of the archetypal MOF HKUST-1 is investigated by a first-principles derived force field in combination with DFT calculations of model systems. The computed surface energies correctly predict the [111] surface to be most stable and allow us to obtain an unprecedented atomistic picture of the surface termination. Entropic factors are identified to determine the preferred surface termination and to be the driving force for the MOF growth. On the basis of this, reported strategies like employing "modulators" during the synthesis to tailor the crystal morphology are discussed. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jz5012065
  • 2014 • 48 Tribenzotriquinacene receptors for C60 fullerene rotors: Towards C3 symmetrical chiral stators for unidirectionally operating nanoratchets
    Bredenkötter, B. and Grzywa, M. and Alaghemandi, M. and Schmid, R. and Herrebout, W. and Bultinck, P. and Volkmer, D.
    Chemistry - A European Journal 20 9100-9110 (2014)
    The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (7) for C60 fullerene, possessing C3 point group symmetry, by threefold condensation of C2-symmetric 1,2-diketone synthons (5) and a hexaaminotribenzotriquinacene core (6) is described. The chiral diketone was synthesized in a five-step reaction sequence starting from C2h-symmetric 2,6-di-tert-butylanthracene. The highly diastereo-discriminating Diels-Alder reaction of 2,6-di-tert-butylanthracene with fumaric acid di(-)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by 1H and 13C NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7 % ee, based on the ee (99.9 %) of the chiral synthon (1). The absolute configuration of 7 could thus be established as all-S [(2S,7S,16S,21S,30S,35S)-(7)]. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with either pure fullerene (C60) or fullerene derivatives, such as rotor 1'-(4-nitrophenyl)-3'-(4-N,N- dimethylaminophenyl)-pyrazolino[4',5':1,2][60]fullerene (R). The complex stability constants of the complexes dissolved in CHCl3/CS 2 (1:1 vol. %) are K([C60-7])=319(±156) M -1 and K([R-7])=110(±50) M-1. With molecular dynamics simulations using a first-principles parameterized force field the asymmetry of the rotational potential for [R-7] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet. Going through the motions: The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (1) for C 60 fullerenes is described. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with fullerenes. Molecular dynamics simulations show the asymmetry of the rotational potential for [R-1], demonstrating the potential suitability of receptor 1 to act as a stator in a unidirectionally operating nanoratchet (see figure). © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201304980
  • 2014 • 47 Tunable emission properties by ferromagnetic coupling Mn(II) aggregates in Mn-doped CdS microbelts/nanowires
    Kamran, M.A. and Liu, R. and Shi, L.-J. and Li, Z.-A. and Marzi, T. and Schöppner, C. and Farle, M. and Zou, B.
    Nanotechnology 25 (2014)
    Tunable optical emission properties from ferromagnetic semiconductors have not been well identified yet. In this work, high-quality Mn(II)-doped CdS nanowires and micrometer belts were prepared using a controlled chemical vapor deposition technique. The Mn doping could be controlled with time, precursor concentration and temperature. These wires or belts can produce both tunable redshifted emissions and ferromagnetic responses simultaneously upon doping. The strong emission bands at 572, 651, 693, 712, 745, 768, 787 and 803 nm, due to the Mn(II) 4T1(4G) → 6A 1(6s) d-d transition, can be detected and accounted for by the aggregation of Mn ions at Cd sites in the CdS lattice at high temperature. These aggregates with ferromagnetism and shifted luminescence are related to the excitonic magnetic polaron (EMP) and localized EMP formations; this is verified by ab initio calculations. The correlation between aggregation-dependent optical emissions and ferromagnetic responses not only presents a new size effect for diluted magnetic semiconductors (DMSs), but also supplies a possible way to study or modulate the ferromagnetic properties of a DMS and to fabricate spin-related photonic devices in the future. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/38/385201
  • 2013 • 46 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 • 45 Ab initio calculation of traction separation laws for a grain boundary in molybdenum with segregated C impurites
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the Σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction-separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy or WoS of the grain boundary, while the strength of the Σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction-separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy-displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/075005
  • 2013 • 44 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 • 43 Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 296-304 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.01.030
  • 2013 • 42 Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations
    Song, W. and Von Appen, J. and Choi, P. and Dronskowski, R. and Raabe, D. and Bleck, W.
    Acta Materialia 61 7582-7590 (2013)
    Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260 C and only θ precipitates form in upper bainite isothermally held at 500 C. ε and θ precipitate under paraequilibrium condition at 260 C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 C. In order to theoretically study ε and θ precipitation and the ε → θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol-1 at room temperature and still by 4 kJ mol-1 at 500 C. Hence, the thermodynamic predictions agree well with the atom probe tomography results. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.08.051
  • 2013 • 41 Band offsets in complex-oxide thin films and heterostructures of SrTiO 3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy
    Conti, G. and Kaiser, A.M. and Gray, A.X. and Nemšák, S. and Pálsson, G.K. and Son, J. and Moetakef, P. and Janotti, A. and Bjaalie, L. and Conlon, C.S. and Eiteneer, D. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw...
    Journal of Applied Physics 113 (2013)
    The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν &gt; 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO 3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4795612
  • 2013 • 40 Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Arróyave, R. and Singh, N. and Talapatra, A. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Albertini, F. and Righi, L. and Chernenko, V.A.
    European Physical Journal B 86 (2013)
    First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1140/epjb/e2012-30936-9
  • 2013 • 39 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 • 38 DFT+ U study of arsenate adsorption on FeOOH surfaces: Evidence for competing binding mechanisms
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Journal of Physical Chemistry C 117 15571-15582 (2013)
    On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites - directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1-As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2-As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1-As = 3.26-3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2-As = 3.31-3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with d Fe1-As = 3.55-3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2-As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2-As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe-As distance. Our results show that the Fe-As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe-As distance cannot be used as a unique criterion to discriminate between adsorption modes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp400649m
  • 2013 • 37 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 • 36 Interaction of phase transformation and magnetic properties of heusler alloys: A density functional theory study
    Entel, P. and Gruner, M.E. and Comtesse, D. and Wuttig, M.
    JOM 65 1540-1549 (2013)
    The structural, electronic, and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn, and Sb) Heusler alloys are studied by first-principles and Monte Carlo tools. The ab initio calculations give a basic understanding of the underlying physics that are associated with the complex magnetic behavior arising from the competition of ferromagnetic and antiferromagnetic interactions with increasing chemical disorder in the super cell. This complex magnetic ordering is the driving mechanism of structural transformations. It also essentially determines the multifunctional properties of the Heusler alloys such as magnetic shape-memory and magnetocaloric effects. The thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. The experimental entropy and specific heat changes across the magnetostructural transition are accurately reproduced by the Monte Carlo simulations. The predictive power of the first-principles calculations allows one to optimize the functional features by choosing optimal compositions. © 2013 The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-013-0757-2
  • 2013 • 35 Molecular tweezers with varying anions: A comparative study
    Dutt, S. and Wilch, C. and Gersthagen, T. and Talbiersky, P. and Bravo-Rodriguez, K. and Hanni, M. and Sánchez-García, E. and Ochsenfeld, C. and Klärner, F.-G. and Schrader, T.
    Journal of Organic Chemistry 78 6721-6734 (2013)
    Selective binding of the phosphate-substituted molecular tweezer 1a to protein lysine residues was suggested to explain the inhibition of certain enzymes and the aberrant aggregation of amyloid petide Aβ42 or α-synuclein, which are assumed to be responsible for Alzheimer's and Parkinson's disease, respectively. In this work we systematically investigated the binding of four water-soluble tweezers 1a-d (substituted by phosphate, methanephosphonate, sulfate, or O-methylenecarboxylate groups) to amino acids and peptides containing lysine or arginine residues by using fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry (ITC). The comparison of the experimental results with theoretical data obtained by a combination of QM/MM and ab initio1H NMR shift calculations provides clear evidence that the tweezers 1a-c bind the amino acid or peptide guest molecules by threading the lysine or arginine side chain through the tweezers' cavity, whereas in the case of 1d the guest molecule is preferentially positioned outside the tweezer's cavity. Attractive ionic, CH-π, and hydrophobic interactions are here the major binding forces. The combination of experiment and theory provides deep insight into the host-guest binding modes, a prerequisite to understanding the exciting influence of these tweezers on the aggregation of proteins and the activity of enzymes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jo4009673
  • 2013 • 34 Optimization of smart Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Chakrabarti, A. and Barman, S.R. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Journal of Alloys and Compounds 577 S107-S112 (2013)
    The strong magnetoelastic interaction in ternary X2YZ Heusler alloys is reponsible for the appearance of magnetostructural phase transitions and related functional properties such as the magnetocaloric and magnetic shape-memory effects. Here, X and Y are transition metal elements and Z is usually an element from the III-V group. In order to discuss possibilities to optimize the multifunctional effects, we use density functional theory calculations from which the martensitic driving forces of the magnetic materials can be derived. We find that the electronic contribution arising from the band Jahn-Teller effect is one of the major driving forces. The ab initio calculations also give a hint of how to design new intermetallics with higher martensitic transformation temperatures compared to the prototype alloy system Ni-Mn-Ga. As an example, we discuss quarternary PtxNi 2-xMnGa alloys which have properties very similar to Ni-Mn-Ga but exhibit a higher maximal eigenstrain of 14%. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.03.005
  • 2013 • 33 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
  • 2012 • 32 A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
    Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
    Advanced Engineering Materials 14 530-546 (2012)
    The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200063
  • 2012 • 31 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 • 30 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 • 29 Competition between ordering, twinning, and segregation in binary magnetic 3d-5d nanoparticles: A supercomputing perspective
    Gruner, M.E. and Entel, P.
    International Journal of Quantum Chemistry 112 277-288 (2012)
    The benefit of massively parallel supercomputers for technologically relevant applications in the field of materials science is demonstrated at the example of first-principles total energy calculations of magnetic binary transition metal nanoparticles containing up to 1415 3d and 5d transition metal atoms. The simulations, which take into account structural optimizations without symmetry constraints, reveal the size-dependent evolution of the energetic order of single crystalline and multiply twinned Fe-Pt nanoparticles up to 4 nm in diameter, which are discussed as promising building blocks for future ultra-high density data recording media. Although at small diameters, multiply twinned morphologies are preferred, we can show that an energetic crossover to a single crystalline, ordered arrangement can be expected at diameters around four nanometers. The comparison with Co-Pt indicates that the contributions of the interfaces in multiply twinned structures are of similar importance as the surface and cannot be neglected especially for small particle sizes. The results imply that for Co-Pt particles segregation of Pt to the surface and the formation of a Pt-depleted subsurface layer is also dominant for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, whereas for Fe-Pt multiple twinning is the most important equilibrium mechanism for small particle sizes. Hybrid combinations of the most favorable ordering motifs, that is, L10-type ordering in the particle core in combination with segregation in the outer shells, may thus lead to highly stable morphologies, which could dominate the growth process. © 2011 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/qua.23254
  • 2012 • 28 Disentangling the physical contributions to the electrical resistance in magnetic domain walls: A multiscale study
    Seemann, K.M. and Garcia-Sanchez, F. and Kronast, F. and Miguel, J. and Kákay, A. and Schneider, C. M. and Hertel, R. and Freimuth, F. and Mokrousov, Y. and Blügel, S.
    Physical Review Letters 108 (2012)
    We analyze the origin of the electrical resistance arising in domain walls of perpendicularly magnetized materials by considering a superposition of anisotropic magnetoresistance and the resistance implied by the magnetization chirality. The domain wall profiles of L1 0-FePd and L1 0-FePt are determined by micromagnetic simulations based on which we perform first-principles calculations to quantify electron transport through the core and closure region of the walls. The wall resistance, being twice as high in L1 0-FePd than in L1 0-FePt, is found to be clearly dominated in both cases by a high gradient of magnetization rotation, which agrees well with experimental observations. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.108.077201
  • 2012 • 27 Elastic properties of face-centred cubic Fe-Mn-C studied by nanoindentation and ab initio calculations
    Reeh, S. and Music, D. and Gebhardt, T. and Kasprzak, M. and Jäpel, T. and Zaefferer, S. and Raabe, D. and Richter, S. and Schwedt, A. and Mayer, J. and Wietbrock, B. and Hirt, G. and Schneider, J.M.
    Acta Materialia 60 6025-6032 (2012)
    We have studied experimentally and theoretically the influence of C and Mn content on the Young's modulus of Fe-Mn-C alloys. Combinatorial thin film and bulk samples were characterized regarding their structure, texture and Young's modulus. The following chemical composition range was investigated: 1.5-3.0 at.% C, 28.0-37.5 at.% Mn and 60.6-69.8 at.% Fe. The experimental lattice parameters change marginally within 3.597-3.614 Å with the addition of C and are consistent with ab initio calculations. The Young's modulus data are in the range of 185 ± 12-251 ± 59 GPa for the bulk samples and the thin film, respectively. C has no significant effect on the Young's modulus of these alloys within the composition range studied here. The ab initio calculations are 15-22% larger than the average Young's modulus values of the as-deposited and polished thin film at 3 at.% C. The comparison of thin film and bulk samples results reveals similar elastic properties for equivalent compositions, indicating that the applied research strategy consisting of the combinatorial thin film approach in conjunction with ab initio calculations is useful to study the composition dependence of the structure and elastic properties of Fe-Mn-C alloys. The very good agreement between the presented calculations and the experimentally determined lattice parameters and Young's modulus values implies that the here-adopted simulation strategy yields a reliable description of carbon in Fe-Mn alloys, important for future alloy design. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.038
  • 2012 • 26 Enhancing magnetocrystalline anisotropy of the Fe 70Pd 30 magnetic shape memory alloy by adding Cu
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Schultz, L. and Ludwig, Al. and Fähler, S.
    Acta Materialia 60 6920-6930 (2012)
    Fe-Pd-Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe-Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe 70Pd 30-xCu x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a bct &gt; 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K 1 at room temperature, which reaches a maximum of -2.4 × 10 5 J m -3 around c/a bct = 1.33. This value exceeds those of binary Fe 70Pd 30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K 1 represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuator applications with significantly improved work output. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.08.001
  • 2012 • 25 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 • 24 First-principles study of the thermodynamic and elastic properties of eutectic Fe-Ti alloys
    Zhu, L.-F. and Friák, M. and Dick, A. and Grabowski, B. and Hickel, T. and Liot, F. and Holec, D. and Schlieter, A. and Kühn, U. and Eckert, J. and Ebrahimi, Z. and Emmerich, H. and Neugebauer, J.
    Acta Materialia 60 1594-1602 (2012)
    Ti-Fe alloys covering a broad range of Ti concentrations are studied using quantum-mechanical calculations. Employing density functional theory, we correctly reproduce selected key features of the experimental Fe-Ti phase diagram. Analyzing the electronic structure of the stable phases in detail provides an explanation for the thermodynamic stability in terms of the strong correlation between the composition and density of states at the Fermi energy (DOS(EF)). Based on this insight, we extend our study on both single-crystalline and polycrystalline elasticity of various Fe-Ti alloys by computing the compositional dependence of homogenized elastic constants. These quantities and their compositional dependence provide a direct explanation for the origin of the ductility and softness of the β-Ti(Fe) phase. Specifically, we find that this phase has an Fe concentration close to a threshold value connected with the onset of mechanical instability. By interlinking thermodynamic and mechanical stabilities we explain the softness and ductility of the β-Ti(Fe) in terms of a reduced mechanical stability that is connected with an increased DOS(EF) in the β-Ti(Fe). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.046
  • 2012 • 23 Synthesis, structure, tautomerism, and reactivity of methanetrisamidines
    Gutschank, B. and Schulz, S. and Marcinkowski, M. and Jansen, G. and Bandmann, H. and Bläser, D. and Wölper, C.
    Angewandte Chemie - International Edition 51 10893-10897 (2012)
    Tout au contraire: Both tautomeric forms of a methanetrisamidine were structurally characterized for the first time by X-ray diffraction and by ab initio calculations (see structures; gray C, red H, blue N). Their reactivity as proton acceptors and multianionic ligands was demonstrated. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201205030
  • 2012 • 22 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 • 21 Understanding the magnetic shape memory system Fe-Pd-X by thin film experiments and first principle calculations
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Buschbeck, J. and Ludwig, Al. and Schultz, L. and Fähler, S.
    Advanced Engineering Materials 14 724-749 (2012)
    The magnetic shape memory (MSM) alloy Fe 70Pd 30 is of particular interest for novel microactuator and sensor applications. This review summarizes the underlying physical and material science concepts for this MSM alloy system. First-principles calculations of the electronic and crystallographic structure together with combinatorial and epitaxial film studies are presented. By these complementary methods we can address the open key questions of MSM alloys and microsystems: Which are the driving forces for a martensitic transformation and how does this transformation proceed? How is it possible to improve the MSM properties by adding third elements? What is the role of external interfaces and which routes allow the preparation of freestanding epitaxial films? Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200052
  • 2011 • 20 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 • 19 Ab-initio modeling of Fe-Mn based alloys and nanoclusters
    Entel, P. and Comtesse, D. and Herper, H.C. and Gruner, M.E. and Siewert, M. and Sahoo, S. and Hucht, A.
    Materials Research Society Symposium Proceedings 1296 80-91 (2011)
    New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1449
  • 2011 • 18 Atomistically informed continuum model for body centered cubic iron
    Koester, A. and Ma, A. and Hartmaier, A.
    Materials Research Society Symposium Proceedings 1296 47-55 (2011)
    Plastic deformation in body centered cubic iron is dominated by glide of screw dislocations with non-planar dislocation cores. This causes a strong strain rate and temperature dependence of flow stress, the breakdown of Schmid's law and a dependence of dislocation mobility on shear stress components that do not contribute to the mechanical driving force for dislocation glide. Based on the framework of crystal plasticity, we developed a constitutive plasticity model that takes all these phenomena into account. To parameterize this continuum plasticity model molecular statics simulations using a semi-empirical potential have been performed. These atomistic calculations yielded quantitative relationships for the influence of all components of the local stress tensor on dislocation mobility. Together with experimental data from the literature on the kinetics of screw dislocations in bcc iron the constitutive relation presented here has been developed. As application example of the model, we calculated the tension compression asymmetry and the strain rate dependence of the hardening behavior within a bcc iron crystal. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1445
  • 2011 • 17 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 • 16 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 • 15 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 • 14 First-principles investigation of the effect of carbon on the stacking fault energy of Fe-C alloys
    Abbasi, A. and Dick, A. and Hickel, T. and Neugebauer, J.
    Acta Materialia 59 3041-3048 (2011)
    The intrinsic stacking fault energy (SFE) is a critical parameter that defines the type of plasticity mechanisms in austenitic high-Mn steels. We have performed ab initio investigations to study the effect of interstitial carbon atoms on the SFE of face-centred cubic (fcc) Fe-C alloys. Simulating the stacking fault explicitly, we observe a strong dependence of the SFE on the position of carbon atoms with respect to the stacking-fault layer and the carbon concentration. To determine the SFE for realistic carbon distributions we consider two scenarios, assuming (i) an equilibration of the carbon atoms in response to the stacking fault formation and (ii) a homogeneous distribution of the C atoms when creating the stacking fault (i.e. diffusion is suppressed). This distinction allows us to interpret two sets of apparently contradicting experimental data sets, where some find an almost negligible dependence on the carbon concentration while others report a large carbon dependence. In particular, our results for the second scenario show a significant increase in the SFE as a function of carbon concentration. These trends are consistently found for the explicit calculations as well as for the computationally much more efficient axial next-nearest-neighbour Ising approach. They will be decisive for the selection of specific plasticity mechanisms in steels (such as twin formation, martensitic transformations and dislocation gliding). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.01.044
  • 2011 • 13 Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD
    Krumme, B. and Herper, H.C. and Erb, D. and Weis, C. and Antoniak, C. and Warland, A. and Westerholt, K. and Entel, P. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/41/415004
  • 2011 • 12 Isotopic effect on the vibrational lifetime of the carbon-deuterium stretch excitation on graphene
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 135 (2011)
    The relaxation of vibrational energy in the H and D stretch modes has been studied on the graphene surface using ab initio calculations. The dissipation of the vibrational energy stored in the stretching modes proceeds through vibration-phonon coupling, while the dissipation through electronic excitations makes only minor contributions. Recently, we reported the fast relaxation of the H stretch energy on graphene [S. Sakong and P. Kratzer, J. Chem. Phys. 133, 054505 (2010)]10.1063/1.3474806. Interestingly, we predict the lifetime of the D stretch to be markedly longer compared to the relaxation of the H stretch. This is unexpected since the vibrational amplitudes at carbon atoms in the joint C-D vibrational modes are larger than in the joint C-H modes, due to the mass ratio mDmC mHmC. However, the vibrational relaxation rate for the D stretch is smaller than for the H stretch, because the energy is dissipated to an acoustic phonon of graphene in the case of C-D rather than an optical phonon as is the case in C-H, and hence, the corresponding phonon density of states is lower in the C-D case. To rationalize our findings, we propose a general scheme for estimating vibrational lifetimes of adsorbates based on four factors: the density of states of the phonons that mediates the transitions, the vibration-phonon coupling strength, the anharmonic coupling between local modes, and the number of quanta involved in the transitions. Mainly the first two of these factors are responsible for the differences in the lifetimes of the C-H and C-D stretches. The possible role of the other factors is illustrated in the context of vibrational lifetimes in other recently studied systems. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3637040
  • 2011 • 11 Magnetic nanostructures by adaptive twinning in strained epitaxial films
    Kauffmann-Weiss, S. and Gruner, M.E. and Backen, A. and Schultz, L. and Entel, P. and Fähler, S.
    Physical Review Letters 107 (2011)
    We exploit the intrinsic structural instability of the Fe 70Pd30 magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.107.206105
  • 2011 • 10 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 • 9 Tight-binding simulation of transition-metal alloys
    McEniry, E.J. and Madsen, G.K.H. and Drain, J.F. and Drautz, R.
    Journal of Physics Condensed Matter 23 (2011)
    In order to perform atomistic simulations of steel, it is necessary to have a detailed understanding of the complex interatomic interactions in transition metals and their alloys. The tight-binding approximation provides a computationally efficient, yet accurate, method to investigate such interactions. In the present work, an orthogonal tight-binding model for Fe, Mn and Cr, with the explicit inclusion of magnetism, has been parameterized from ab initio density-functional calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/27/276004
  • 2010 • 8 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 • 7 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 • 6 Fundamental aspects of magnetic shape memory alloys: Insights from ab initio and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Dannenberg, A. and Siewert, M. and Nayak, S.K. and Herper, H.C. and Buchelnikov, V.D.
    Materials Science Forum 635 3-12 (2010)
    Ferromagnetic Heusler alloys like Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb), which undergo a martensitic phase transformation, are on the edge of being used in technological applications involving actuator and magnetocaloric devices. The other class of ferromagnetic full Heusler alloys like Co-Mn-Z (Z = Al, Si, Ga, Ge, Sn) not undergoing a structural phase transition, are half-metals (in contrast to the Ni-based systems) with high spin polarization at the Fermi level and are of potential importance for future spintronics devices. On the basis of recent ab initio calculations, we highlight the main differences between the two classes of Heusler based materials. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/MSF.635.3
  • 2010 • 5 Intramolecular electronic interactions between nonconjugated arene and quinone chromophores
    Jansen, G. and Kahlert, B. and Klärner, F.-G. and Boese, R. and Bläser, D.
    Journal of the American Chemical Society 132 8581-8592 (2010)
    The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result from broad absorption bands in the visible range (1, λ<inf>max</inf> = 537 nm and 2, λ<inf>max</inf> = 423 and λ<inf>shoulder</inf> =515 nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-substituted duroquinone 31, even though the clips 1 and 2 only contain insulated π systems as chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3, the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones 4, 5, and 6, the so-called "half-clips". The "half-clips" 6 and 5 show a similar color change and the same trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already rules out that the color of these systems is a result of "through-space" π-π interactions between the aromatic sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra of the clip quinones 2, 3, and 1 and the "half- clip" quinones 4, 5, and 6 with an increasing number of rings in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration interaction between a n → π* excitation of the quinoid component and a π → π* excitation with intramolecular charge transfer (CT) character. The initial π orbitals involved here and in higher lying transitions mainly stem from through-space interactions between π orbitals of the aromatic sidewalls and π orbitals of the quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1-3, the "half-clips" 4-6, and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well with the experimental spectra. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja910362j
  • 2010 • 4 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 • 3 The ferromagnetic shape memory system Fe-Pd-Cu
    Hamann, S. and Gruner, M.E. and Irsen, S. and Buschbeck, J. and Bechtold, C. and Kock, I. and Mayr, S.G. and Savan, A. and Thienhaus, S. and Quandt, E. and Fähler, S. and Entel, P. and Ludwig, Al.
    Acta Materialia 58 5949-5961 (2010)
    A new ferromagnetic shape memory thin film system, Fe-Pd-Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc-fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe-Pd-Cu samples showed an enhanced stability of the disordered transforming Fe70Pd30 phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc-fct transformation temperature. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.011
  • 2010 • 2 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 • 1 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