Prof. Dr. Jörg Neugebauer

Computational Materials Design
Max-Planck-Institut für Eisenforschung

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  • A combined experimental and first-principles based assessment of finite-temperature thermodynamic properties of intermetallic al3sc
    Gupta, A. and Tas, B. and Korbmacher, D. and Dutta, B. and Neitzel, Y. and Grabowski, B. and Hickel, T. and Esin, V. and Divinski, S.V. and Wilde, G. and Neugebauer, J.
    Materials 14 (2021)
    We present a first-principles assessment of the finite-temperature thermodynamic properties of the intermetallic Al3Sc phase including the complete spectrum of excitations and compare the theoretical findings with our dilatometric and calorimetric measurements. While significant electronic contributions to the heat capacity and thermal expansion are observed near the melting temperature, anharmonic contributions, and electron–phonon coupling effects are found to be relatively small. On the one hand, these accurate methods are used to demonstrate shortcomings of empirical predictions of phase stabilities such as the Neumann–Kopp rule. On the other hand, their combination with elasticity theory was found to provide an upper limit for the size of Al3Sc nanoprecipitates needed to maintain coherency with the host matrix. The chemo-mechanical coupling being responsible for the coherency loss of strengthening precipitates is revealed by a combination of state-of-the-art simulations and dedicated experiments. These findings can be exploited to fine-tune the microstructure of Al-Sc-based alloys to approach optimum mechanical properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14081837
  • A fully automated approach to calculate the melting temperature of elemental crystals
    Zhu, L.-F. and Janssen, J. and Ishibashi, S. and Körmann, F. and Grabowski, B. and Neugebauer, J.
    Computational Materials Science 187 (2021)
    The interface method is a well established approach for predicting melting points of materials using interatomic potentials. However, applying the interface method is tedious and involves significant human intervention. The whole procedure involves several successive tasks: estimate a rough melting point, set up the interface structure, run molecular dynamic calculations and analyze the data. Loop calculations are necessary if the predicted melting point is different from the estimated one by more than a certain convergence criterion, or if full melting/solidification occurs. In this case monitoring the solid–liquid phase transition in the interface structure becomes critical. As different initial random seeds for the molecular dynamic simulations within the interface method induce slightly different melting points, a few ten or hundred interface method calculations with different random seeds are necessary for performing a statistical analysis on these melting points. Considering all these technical details, the work load for manually executing and combining the various involved scripts and programs quickly becomes prohibitive. To simplify and automatize the whole procedure, we have implemented the interface method into pyiron (http://pyiron.org). Our fully automatized procedure allows to efficiently and precisely predict melting points of stable unaries represented by arbitrary potentials with only two user-specified parameters (interatomic potential file and element). For metastable or dynamically unstable unary phases, the crystal structure needs to be provided as an additional parameter. We have applied our automatized approach on fcc Al, Ni, dynamically unstable bcc Ti and hcp Mg and employed a large set of available interatomic potentials. Melting points for classical interatomic potentials of these metals have been obtained with a numerical precision well below 1 K. © 2020 The Authors
    view abstract10.1016/j.commatsci.2020.110065
  • 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 abstract10.1103/PhysRevB.103.054506
  • 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 abstract10.1103/PhysRevMaterials.5.053803
  • 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 abstract10.1038/s41524-021-00502-y
  • Defect phases–thermodynamics and impact on material properties
    Korte-Kerzel, S. and Hickel, T. and Huber, L. and Raabe, D. and Sandlöbes-Haut, S. and Todorova, M. and Neugebauer, J.
    International Materials Reviews (2021)
    Two approaches in materials physics have proven immensely successful in alloy design: First, thermodynamic and kinetic descriptions for tailoring and processing alloys to achieve a desired microstructure. Second, crystal defect manipulation to control strength, formability and corrosion resistance. However, to date, the two concepts remain essentially decoupled. A bridge is needed between these powerful approaches to achieve a single conceptual framework. Considering defects and their thermodynamic state holistically as ‘defect phases’, provides a future materials design strategy by jointly treating the thermodynamic stability of both, the local crystalline structure and the distribution of elements at defects. Here, we suggest that these concepts are naturally linked by defect phase diagrams describing the coexistence and transitions of defect phases. Construction of these defect phase diagrams will require new quantitative descriptors. We believe such a framework will enable a paradigm shift in the description and design of future engineering materials. © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/09506608.2021.1930734
  • Dielectric Properties of Nanoconfined Water: A Canonical Thermopotentiostat Approach
    Deißenbeck, F. and Freysoldt, C. and Todorova, M. and Neugebauer, J. and Wippermann, S.
    Physical Review Letters 126 (2021)
    We introduce a novel approach to sample the canonical ensemble at constant temperature and applied electric potential. Our approach can be straightforwardly implemented into any density-functional theory code. Using thermopotentiostat molecular dynamics simulations allows us to compute the dielectric constant of nanoconfined water without any assumptions for the dielectric volume. Compared to the commonly used approach of calculating dielectric properties from polarization fluctuations, our thermopotentiostat technique reduces the required computational time by 2 orders of magnitude. © 2021 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevLett.126.136803
  • Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations
    Yoo, S.-H. and Lymperakis, L. and Neugebauer, J.
    Physical Review Materials 5 (2021)
    Removing artificial bands from the back side of surface slabs with pseudohydrogen atoms has become the method of choice to boost the convergence of density-functional theory (DFT) surface calculation with respect to slab thickness. In this paper we apply this approach to semipolar compound semiconductor surfaces, which have recently become attractive for device applications. We show that approaches employing saturation of dangling bonds by pseudohydrogen atoms alone are inadequate to properly passivate the surfaces, remove spurious surface states from the fundamental band gap, and achieve flat band conditions in the slab. We propose and successfully apply to technologically interesting semipolar wurtzite surfaces of III-N, III-V, and II-VI semiconductors a reconstruction-inspired passivation scheme that utilizes native anions to passivate cation dangling bonds and pseudohydrogen atoms to obey the electron counting rule and compensate for polarization-induced surface-bound charges. This scheme is generic and robust and can be straightforwardly implemented in DFT investigations of low-symmetry surfaces as well as in high-throughput and machine learning studies. © 2021 authors.
    view abstract10.1103/PhysRevMaterials.5.044605
  • Finite-size correction for slab supercell calculations of materials with spontaneous polarization
    Yoo, S.-H. and Todorova, M. and Wickramaratne, D. and Weston, L. and Walle, C.G.V. and Neugebauer, J.
    npj Computational Materials 7 (2021)
    The repeated slab approach has become a de facto standard to accurately describe surface properties of materials by density functional theory calculations with periodic boundary conditions. For materials exhibiting spontaneous polarization, we show that the conventional scheme of passivation with pseudo hydrogen is unable to realize a charge-neutral surface. The presence of a net surface charge induces via Gauss’s law a macroscopic electric field through the slab and results in poor size convergence with respect to the thickness of the slab. We propose a modified passivation method that accounts for the effect of spontaneous polarization, describes the correct bulk limits and boosts convergence with respect to slab thickness. The robustness, reliability, and superior convergence of energetics and electronic structure achieved by the proposed method are demonstrated using the example of polar ZnO surfaces. © 2021, The Author(s).
    view abstract10.1038/s41524-021-00529-1
  • Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials
    Gubaev, K. and Ikeda, Y. and Tasnádi, F. and Neugebauer, J. and Shapeev, A.V. and Grabowski, B. and Körmann, F.
    Physical Review Materials 5 (2021)
    An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect). © 2021 American Physical Society.
    view abstract10.1103/PhysRevMaterials.5.073801
  • Impact of Water Coadsorption on the Electrode Potential of H-Pt(1 1 1)-Liquid Water Interfaces
    Surendralal, S. and Todorova, M. and Neugebauer, J.
    Physical Review Letters 126 (2021)
    Density functional theory molecular dynamics simulations of H-covered Pt(111)-H2O interfaces reveal that, in contrast to common understanding, H2O coadsorption has a significant impact on the electrode potential of and plays a major role in determining the stability of H adsorbates under electrochemical conditions. Based on these insights, we explain the origin behind the experimentally observed upper limit of H coverage well below one monolayer and derive a chemically intuitive model for metal-water bonding that explains an unexpectedly large interaction between coadsorbed water and adsorbates. © 2021 authors.
    view abstract10.1103/PhysRevLett.126.166802
  • Revealing atomic-scale vacancy-solute interaction in nickel
    Morgado, F.F. and Katnagallu, S. and Freysoldt, C. and Klaes, B. and Vurpillot, F. and Neugebauer, J. and Raabe, D. and Neumeier, S. and Gault, B. and Stephenson, L.T.
    Scripta Materialia 203 (2021)
    It is widely accepted that the different types of crystalline imperfections, such as vacancies or dislocations, greatly influence a material's physical and mechanical properties. However, imaging individual vacancies in solids and revealing their atomic neighborhood remains one of the frontiers of microscopy and microanalysis. Here, we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of Ta with vacancies, supporting positive solute-vacancy interactions previously predicted by atomistic simulations. © 2021
    view abstract10.1016/j.scriptamat.2021.114036
  • Ab initio Description of Bond Breaking in Large Electric Fields
    Ashton, M. and Mishra, A. and Neugebauer, J. and Freysoldt, C.
    Physical Review Letters 124 (2020)
    Strong (1010 V/m) electric fields capable of inducing atomic bond breaking represent a powerful tool for surface chemistry. However, their exact effects are difficult to predict due to a lack of suitable tools to probe their associated atomic-scale mechanisms. Here we introduce a generalized dipole correction for charged repeated-slab models that controls the electric field on both sides of the slab, thereby enabling direct theoretical treatment of field-induced bond-breaking events. As a prototype application, we consider field evaporation from a kinked W surface. We reveal two qualitatively different desorption mechanisms that can be selected by the magnitude of the applied field. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevLett.124.176801
  • Anharmonic free energy of lattice vibrations in fcc crystals from a mean-field bond
    Swinburne, T.D. and Janssen, J. and Todorova, M. and Simpson, G. and Plechac, P. and Luskin, M. and Neugebauer, J.
    Physical Review B 102 (2020)
    It has recently been shown that the ab initio anharmonic free energy of fcc crystals can be approximated to meV/atom accuracy by a lattice of anharmonic nearest-neighbor bonds, where the bonding potential can be efficiently parametrized from the target system. We develop a mean-field approach for the free energy of a general bond lattice, analytically accounting for strong bond-bond correlations while enforcing material compatibility and thermodynamic self-consistency. Applying our fundamentally anharmonic model to fcc crystals yields free energies within meV/atom of brute force thermodynamic integration for core seconds of computational effort. Potential applications of this approach in computational materials science are discussed. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.100101
  • Atomic relaxation around defects in magnetically disordered materials computed by atomic spin constraints within an efficient Lagrange formalism
    Hegde, O. and Grabowski, M. and Zhang, X. and Waseda, O. and Hickel, T. and Freysoldt, C. and Neugebauer, J.
    Physical Review B 102 (2020)
    Lattice and magnetic degrees of freedom are strongly coupled in magnetic materials. We propose a consistent first-principles framework to explore the joint configurational space. For this, we define atomic spin moments from the projector augmented-wave formalism of density-functional theory and control them via Lagrangian constraints. We demonstrate our approach for vacancy formation and migration in collinear paramagnetic bcc iron by implementing a relaxation scheme based on spin-space averaged forces (SSA relaxation). Based on these results we discuss the impact of the magnetic state on vacancy formation energies, migration barriers, and relaxations. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.102.144101
  • 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 (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 abstract10.1016/j.actamat.2019.11.004
  • Atomistic deformation behavior of single and twin crystalline Cu nanopillars with preexisting dislocations
    Ko, W.-S. and Stukowski, A. and Hadian, R. and Nematollahi, A. and Jeon, J.B. and Choi, W.S. and Dehm, G. and Neugebauer, J. and Kirchlechner, C. and Grabowski, B.
    Acta Materialia 197 (2020)
    Molecular dynamics simulations are performed to investigate the impact of a coherent Σ3 (111) twin boundary on the plastic deformation behavior of Cu nanopillars. Our work reveals that the mechanical response of pillars with and without the twin boundary is decisively driven by the characteristics of initial dislocation sources. In the condition of comparably large pillar size and abundant initial mobile dislocations, overall yield and flow stresses are controlled by the longest, available mobile dislocation. An inverse correlation of the yield and flow stresses with the length of the longest dislocation is established and compared to experimental data. The experimentally reported subtle differences in yield and flow stresses between pillars with and without the twin boundary are likely related to the maximum lengths of the mobile dislocations. In the condition of comparably small pillar size, for which a reduction of mobile dislocations during heat treatment and mechanical loading occurs, the mechanical response of pillars with and without the twin boundary can be clearly distinguished. Dislocation starvation during deformation is more pronounced in pillars without the twin boundary than in pillars with the twin boundary because the twin boundary acts as a pinning surface for the dislocation network. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.07.029
  • Beyond Solid Solution High-Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition
    Rao, Z. and Dutta, B. and Körmann, F. and Lu, W. and Zhou, X. and Liu, C. and da Silva, A.K. and Wiedwald, U. and Spasova, M. and Farle, M. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D. and Li, Z.
    Advanced Functional Materials (2020)
    Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
    view abstract10.1002/adfm.202007668
  • 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 abstract10.1103/PhysRevMaterials.4.023608
  • 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 abstract10.1103/PhysRevB.102.045403
  • Impact of magnetism on the phase stability of rare-earth based hard magnetic materials
    Sözen, H.İ. and Hickel, T. and Neugebauer, J.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 68 (2020)
    In recent years, quantum-mechanically guided materials design has been used to identify candidate hard magnetic materials with a reduced content of rare earth elements. The focus of these studies was on optimal magnetic properties. In the present work we address the issue of thermodynamic stability of such materials. As prototype system we consider CeFe11Ti and focus on the impact of magnetism on the free energy. To this end, we use the magnetic model suggested by Gerhard Inden as a reference. The performance of this model is compared to Monte Carlo simulations for the magnetic entropy contribution. We conclude that despite the empirical nature of the Inden model, it provides a surprisingly accurate description of the magnetic contribution. Based on this approach we are able to faithfully predict the critical temperature for the decomposition of CeFe11Ti into competing Laves phases. We further show that the Inden model can be improved if the reduction of the magnetic moment at finite temperatures is taken into account. This is demonstrated for the hard magnetic phase Nd2Fe14B. In addition, the impact of magnetism on the lattice vibrations of relevant phases in the Ce–Fe–Ti system is analyzed. © 2019 Elsevier Ltd
    view abstract10.1016/j.calphad.2019.101731
  • Interplay of Chemistry and Faceting at Grain Boundaries in a Model Al Alloy
    Zhao, H. and Huber, L. and Lu, W. and Peter, N.J. and An, D. and De Geuser, F. and Dehm, G. and Ponge, D. and Neugebauer, J. and Gault, B. and Raabe, D.
    Physical Review Letters 124 (2020)
    The boundary between two crystal grains can decompose into arrays of facets with distinct crystallographic character. Faceting occurs to minimize the system's free energy, i.e., when the total interfacial energy of all facets is below that of the topologically shortest interface plane. In a model Al-Zn-Mg-Cu alloy, we show that faceting occurs at investigated grain boundaries and that the local chemistry is strongly correlated with the facet character. The self-consistent coevolution of facet structure and chemistry leads to the formation of periodic segregation patterns of 5-10 nm, or to preferential precipitation. This study shows that segregation-faceting interplay is not limited to bicrystals but exists in bulk engineering Al alloys and hence affects their performance. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevLett.124.106102
  • Mechanism of collective interstitial ordering in Fe–C alloys
    Zhang, X. and Wang, H. and Hickel, T. and Rogal, J. and Li, Y. and Neugebauer, J.
    Nature Materials 19 (2020)
    Collective interstitial ordering is at the core of martensite formation in Fe–C-based alloys, laying the foundation for high-strength steels. Even though this ordering has been studied extensively for more than a century, some fundamental mechanisms remain elusive. Here, we show the unexpected effects of two correlated phenomena on the ordering mechanism: anharmonicity and segregation. The local anharmonicity in the strain fields induced by interstitials substantially reduces the critical concentration for interstitial ordering, up to a factor of three. Further, the competition between interstitial ordering and segregation results in an effective decrease of interstitial segregation into extended defects for high interstitial concentrations. The mechanism and corresponding impact on interstitial ordering identified here enrich the theory of phase transitions in materials and constitute a crucial step in the design of ultra-high-performance alloys. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41563-020-0677-9
  • Performance of the standard exchange-correlation functionals in predicting melting properties fully from first principles: Application to Al and magnetic Ni
    Zhu, L.-F. and Körmann, F. and Ruban, A.V. and Neugebauer, J. and Grabowski, B.
    Physical Review B 101 (2020)
    We apply the efficient two-optimized references thermodynamic integration using Langevin dynamics method [Phys. Rev. B 96, 224202 (2017)2469-995010.1103/PhysRevB.96.224202] to calculate highly accurate melting properties of Al and magnetic Ni from first principles. For Ni we carefully investigate the impact of magnetism on the liquid and solid free energies including longitudinal spin fluctuations and the reverse influence of atomic vibrations on magnetic properties. We show that magnetic fluctuations are effectively canceling out for both phases and are thus not altering the predicted melting temperature. For both elements, the generalized gradient approximation (GGA) and the local-density approximation (LDA) are used for the exchange-correlation functional revealing a reliable ab initio confidence interval capturing the respective experimental melting point, enthalpy of fusion, and entropy of fusion. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.101.144108
  • Phase diagram of grain boundary facet and line junctions in silicon
    Alam, M. and Lymperakis, L. and Neugebauer, J.
    Physical Review Materials 4 (2020)
    The presence of facets and line junctions connecting facets on grain boundaries (GBs) has a strong impact on the properties of structural, functional, and optoelectronic materials: They govern the mobility of interfaces, the segregation of impurities, as well the electronic properties. In the present paper, we employ density-functional theory and modified embedded atom method calculations to systematically investigate the energetics and thermodynamic stability of these defects. As a prototype system, we consider ς3 tilt GBs in Si. By analyzing the energetics of different faceted GBs, we derive a diagram that describes and predicts the reconstruction of these extended defects as a function of facet length and boundary inclination angle. The phase diagram sheds light upon the fundamental mechanisms causing GB faceting phenomena. It demonstrates that the properties of faceting are not determined solely by anisotropic GB energies but by a complex interplay between geometry and microstructure, boundary energies as well as long-range strain interactions. © 2020 authors. Published by the American Physical Society. Open access publication funded by the Max Planck Society.
    view abstract10.1103/PhysRevMaterials.4.083604
  • 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 abstract10.1103/PhysRevB.101.094201
  • 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 abstract10.1103/PhysRevMaterials.4.033601
  • Segregation-assisted spinodal and transient spinodal phase separation at grain boundaries
    Darvishi Kamachali, R. and Kwiatkowski da Silva, A. and McEniry, E. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D.
    npj Computational Materials 6 (2020)
    Segregation to grain boundaries affects their cohesion, corrosion, and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and low-dimensional phase separation at grain boundaries, here, we apply a density-based phase-field model. The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data, while the grain-boundary-density profile is obtained using atomistic simulations. To benchmark the performance of the model, Mn grain-boundary segregation in the Fe–Mn system is studied. 3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions. We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm. The jump corresponds to a spinodal phase separation at grain boundary. For alloy compositions above the jump, we reveal an interfacial transient spinodal phase separation. The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries. © 2020, The Author(s).
    view abstract10.1038/s41524-020-00456-7
  • 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 abstract10.1103/PhysRevMaterials.4.113802
  • Ab initio based method to study structural phase transitions in dynamically unstable crystals, with new insights on the β to ω transformation in titanium
    Korbmacher, D. and Glensk, A. and Duff, A.I. and Finnis, M.W. and Grabowski, B. and Neugebauer, J.
    Physical Review B 100 (2019)
    We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable β (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal ω phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111]β linear-chain defects that exhibit randomly disordered arrangements in the (111)β plane. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.104110
  • 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 abstract10.1103/PhysRevMaterials.3.084407
  • Ab initio vibrational free energies including anharmonicity for multicomponent alloys
    Grabowski, B. and Ikeda, Y. and Srinivasan, P. and Körmann, F. and Freysoldt, C. and Duff, A.I. and Shapeev, A. and Neugebauer, J.
    npj Computational Materials 5 (2019)
    The unique and unanticipated properties of multiple principal component alloys have reinvigorated the field of alloy design and drawn strong interest across scientific disciplines. The vast compositional parameter space makes these alloys a unique area of exploration by means of computational design. However, as of now a method to compute efficiently, yet with high accuracy the thermodynamic properties of such alloys has been missing. One of the underlying reasons is the lack of accurate and efficient approaches to compute vibrational free energies—including anharmonicity—for these chemically complex multicomponent alloys. In this work, a density-functional-theory based approach to overcome this issue is developed based on a combination of thermodynamic integration and a machine-learning potential. We demonstrate the performance of the approach by computing the anharmonic free energy of the prototypical five-component VNbMoTaW refractory high entropy alloy. © 2019, The Author(s).
    view abstract10.1038/s41524-019-0218-8
  • Deciphering Charge Transfer and Electronic Polarization Effects at Gold Nanocatalysts on Reduced Titania Support
    Yoo, S.-H. and Siemer, N. and Todorova, M. and Marx, D. and Neugebauer, J.
    Journal of Physical Chemistry C 123 (2019)
    Gold nanoparticles supported on reduced TiO2 (110) surfaces are widely used as catalysts for oxidation reactions. Despite extensive studies, the role of oxygen vacancies in such systems remains elusive and is controversially discussed. Combining ab initio molecular dynamics simulations with methods originally developed to describe defects in semiconductor physics we study how the electronic charge originally located at the vacancy modifies the charge on the cluster. Despite differences resulting from the employed level of density functional theory (namely semilocal/GGA, GGA + U, and hybrid functionals), we consistently find that the Au clusters remain either neutral or acquire a positive charge. The intuitively expected electron transfer from the oxygen vacancy to the gold cluster can be safely ruled out. Analyzing these findings, we discuss the role of the oxygen vacancy in the bonding between Au clusters and support and the catalytic activity of the system. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b12015
  • Discovery of Elusive K4O6, a Compound Stabilized by Configurational Entropy of Polarons
    Freysoldt, C. and Merz, P. and Schmidt, M. and Mohitkar, S. and Felser, C. and Neugebauer, J. and Jansen, M.
    Angewandte Chemie - International Edition 58 (2019)
    Synthesis of elusive K4O6 has disclosed implications of crucial relevance for new solid materials discovery. K4O6 forms in equilibrium from K2O2 and KO2, in an all-solid state, endothermic reaction at elevated temperature, undergoing back reaction upon cooling to ambient conditions. This tells that the compound is stabilized by entropy alone. Analyzing possible entropic contributions reveals that the configurational entropy of “localized” electrons, i.e., of polaronic quasi-particles, provides the essential contribution to the stabilization. We corroborate this assumption by measuring the relevant heats of transformation and tracking the origin of entropy of formation computationally. These findings challenge current experimental and computational approaches towards exploring chemical systems for new materials by searching the potential energy landscape: one would fail in detecting candidates that are crucially stabilized by the configurational entropy of localized polarons. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201809409
  • Elasticity of phases in Fe-Al-Ti superalloys: Impact of atomic order and anti-phase boundaries
    Friák, M. and Buršíková, V. and Pizúrová, N. and Pavlů, J. and Jirásková, Y. and Homola, V. and Miháliková, I. and Slávik, A. and Holec, D. and Všianská, M. and Koutná, N. and Fikar, J. and Janičkovič, D. and Šob, M. and Neugebauer, J.
    Crystals 9 (2019)
    We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe71Al22Ti7, we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into a matrix). The chemical composition of both phases, Fe66.2Al23.3Ti10.5 for cuboids and Fe81Al19 (with about 1% or less of Ti) for the matrix, was determined from an Energy-Dispersive X-ray Spectroscopy (EDS) analysis. The phase of cuboids is found to be a rather strongly off-stoichiometric (Fe-rich and Ti-poor) variant of Heusler Fe2TiAl intermetallic compound with the L21 structure. The phase of the matrix is a solid solution of Al atoms in a ferromagnetic body-centered cubic (bcc) Fe. Quantum-mechanical calculations were employed to obtain an insight into elastic properties of the two phases. Three distributions of chemical species were simulated for the phase of cuboids (A2, B2 and L21) in order to determine a sublattice preference of the excess Fe atoms. The lowest formation energy was obtained when the excess Fe atoms form a solid solution with the Ti atoms at the Ti-sublattice within the Heusler L21 phase (L21 variant). Similarly, three configurations of Al atoms in the phase of the matrix with different level of order (A2, B2 and D03) were simulated. The computed formation energy is the lowest when all the 1st and 2nd nearest-neighbor Al-Al pairs are eliminated (the D03 variant). Next, the elastic tensors of all phases were calculated. The maximum Young’s modulus is found to increase with increasing chemical order. Further we simulated an anti-phase boundary (APB) in the L21 phase of cuboids and observed an elastic softening (as another effect of the APB, we also predict a significant increase of the total magnetic moment by 140% when compared with the APB-free material). Finally, to validate these predicted trends, a nano-scale dynamical mechanical analysis (nanoDMA) was used to probe elasticity of phases. Consistent with the prediction, the cuboids were found stiffer. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/cryst9060299
  • Imaging individual solute atoms at crystalline imperfections in metals
    Katnagallu, S. and Stephenson, L.T. and Mouton, I. and Freysoldt, C. and Subramanyam, A.P.A. and Jenke, J. and Ladines, A.N. and Neumeier, S. and Hammerschmidt, T. and Drautz, R. and Neugebauer, J. and Vurpillot, F. and Raabe, D. and Gault, B.
    New Journal of Physics 21 (2019)
    Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials' properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni-Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab5cc4
  • 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 abstract10.1103/PhysRevMaterials.3.113603
  • Impact of lattice relaxations on phase transitions in a high-entropy alloy studied by machine-learning potentials
    Kostiuchenko, T. and Körmann, F. and Neugebauer, J. and Shapeev, A.
    npj Computational Materials 5 (2019)
    Recently, high-entropy alloys (HEAs) have attracted wide attention due to their extraordinary materials properties. A main challenge in identifying new HEAs is the lack of efficient approaches for exploring their huge compositional space. Ab initio calculations have emerged as a powerful approach that complements experiment. However, for multicomponent alloys existing approaches suffer from the chemical complexity involved. In this work we propose a method for studying HEAs computationally. Our approach is based on the application of machine-learning potentials based on ab initio data in combination with Monte Carlo simulations. The high efficiency and performance of the approach are demonstrated on the prototype bcc NbMoTaW HEA. The approach is employed to study phase stability, phase transitions, and chemical short-range order. The importance of including local relaxation effects is revealed: they significantly stabilize single-phase formation of bcc NbMoTaW down to room temperature. Finally, a so-far unknown mechanism that drives chemical order due to atomic relaxation at ambient temperatures is discovered. © 2019, The Author(s).
    view abstract10.1038/s41524-019-0195-y
  • 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 abstract10.1016/j.intermet.2019.106520
  • Phonon Lifetimes throughout the Brillouin Zone at Elevated Temperatures from Experiment and Ab Initio
    Glensk, A. and Grabowski, B. and Hickel, T. and Neugebauer, J. and Neuhaus, J. and Hradil, K. and Petry, W. and Leitner, M.
    Physical review letters 123 (2019)
    We obtain phonon lifetimes in aluminium by inelastic neutron scattering experiments, by ab initio molecular dynamics, and by perturbation theory. At elevated temperatures significant discrepancies are found between experiment and perturbation theory, which disappear when using molecular dynamics due to the inclusion of full anharmonicity and the correct treatment of the multiphonon background. We show that multiple-site interactions are small and that local pairwise anharmonicity dominates phonon-phonon interactions, which permits an efficient computation of phonon lifetimes.
    view abstract10.1103/PhysRevLett.123.235501
  • 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 (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 abstract10.1016/j.commatsci.2018.07.043
  • Role of hole confinement in the recombination properties of InGaN quantum structures
    Anikeeva, M. and Albrecht, M. and Mahler, F. and Tomm, J.W. and Lymperakis, L. and Chèze, C. and Calarco, R. and Neugebauer, J. and Schulz, T.
    Scientific Reports 9 (2019)
    We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions. © 2019, The Author(s).
    view abstract10.1038/s41598-019-45218-8
  • The basics of electronic structure theory for periodic systems
    Kratzer, P. and Neugebauer, J.
    Frontiers in Chemistry 7 (2019)
    When density functional theory is used to describe the electronic structure of periodic systems, the application of Bloch's theorem to the Kohn-Sham wavefunctions greatly facilitates the calculations. In this paper of the series, the concepts needed to model infinite systems are introduced. These comprise the unit cell in real space, as well as its counterpart in reciprocal space, the Brillouin zone. Grids for sampling the Brillouin zone and finite k-point sets are discussed. For metallic systems, these tools need to be complemented by methods to determine the Fermi energy and the Fermi surface. Various schemes for broadening the distribution function around the Fermi energy are presented and the approximations involved are discussed. In order to obtain an interpretation of electronic structure calculations in terms of physics, the concepts of bandstructures and atom-projected and/or orbital-projected density of states are useful. Aspects of convergence with the number of basis functions and the number of k-points need to be addressed specifically for each physical property. The importance of this issue will be exemplified for force constant calculations and simulations of finite-temperature properties of materials. The methods developed for periodic systems carry over, with some reservations, to less symmetric situations by working with a supercell. The chapter closes with an outlook to the use of supercell calculations for surfaces and interfaces of crystals. © 2019 Kratzer and Neugebauer.
    view abstract10.3389/fchem.2019.00106
  • Thermodynamics of grain boundary segregation, interfacial spinodal and their relevance for nucleation during solid-solid phase transitions
    Kwiatkowski da Silva, A. and Kamachali, R.D. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 168 (2019)
    Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.02.005
  • Transferability of interatomic potentials for molybdenum and silicon
    Lysogorskiy, Y. and Hammerschmidt, T. and Janssen, J. and Neugebauer, J. and Drautz, R.
    Modelling and Simulation in Materials Science and Engineering 27 (2019)
    Interatomic potentials are widely used in computational materials science, in particular for simulations that are too computationally expensive for density functional theory (DFT). Most interatomic potentials have a limited application range and often there is very limited information available regarding their performance for specific simulations. We carried out high-throughput calculations for molybdenum and silicon with DFT and a number of interatomic potentials. We compare the DFT reference calculations and experimental data to the predictions of the interatomic potentials. We focus on a large number of basic materials properties, including the cohesive energy, atomic volume, elastic coefficients, vibrational properties, thermodynamic properties, surface energies and vacancy formation energies, which enables a detailed discussion of the performance of the different potentials. We further analyze correlations between properties as obtained from DFT calculations and how interatomic potentials reproduce these correlations, and suggest a general measure for quantifying the accuracy and transferability of an interatomic potential. From our analysis we do not establish a clearcut ranking of the potentials as each potential has its strengths and weaknesses. It is therefore essential to assess the properties of a potential carefully before application of the potential in a specific simulation. The data presented here will be useful for selecting a potential for simulations of Mo or Si. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-651X/aafd13
  • 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 abstract10.1002/adma.201807142
  • A machine learning approach to model solute grain boundary segregation
    Huber, L. and Hadian, R. and Grabowski, B. and Neugebauer, J.
    npj Computational Materials 4 (2018)
    Even minute amounts of one solute atom per one million bulk atoms may give rise to qualitative changes in the mechanical response and fracture resistance of modern structural materials. These changes are commonly related to enrichment by several orders of magnitude of the solutes at structural defects in the host lattice. The underlying concept—segregation—is thus fundamental in materials science. To include it in modern strategies of materials design, accurate and realistic computational modelling tools are necessary. However, the enormous number of defect configurations as well as sites solutes can occupy requires models which rely on severe approximations. In the present study we combine a high-throughput study containing more than 1 million data points with machine learning to derive a computationally highly efficient framework which opens the opportunity to model this important mechanism on a routine basis. © 2018, The Author(s).
    view abstract10.1038/s41524-018-0122-7
  • Ab initio simulation of hydrogen-induced decohesion in cementite-containing microstructures
    McEniry, E.J. and Hickel, T. and Neugebauer, J.
    Acta Materialia 150 (2018)
    In high-strength carbon steels suitable for use in the automotive industry, hydrogen embrittlement (HE) is a potential barrier to the widespread application of these materials. The behaviour of hydrogen within the most prevalent carbide, namely cementite, has been investigated via ab initio simulation. In order to examine possible decohesion effects of hydrogen on carbon steels, the binding and diffusion of hydrogen at the interface between ferrite and cementite has been examined. In order to understand the effect of hydrogen on the mechanical properties of carbon steels, simulated ab initio tensile tests have been performed on the ferrite-cementite bicrystal. The results of the tensile tests can be combined with thermodynamic considerations in order to obtain the expected hydrogen concentrations at such ferrite-cementite phase boundaries. We find that the effect of hydrogen on the cohesion of the phase boundary may be significant, even when the bulk hydrogen concentration is low. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.03.005
  • Advanced data mining in field ion microscopy
    Katnagallu, S. and Gault, B. and Grabowski, B. and Neugebauer, J. and Raabe, D. and Nematollahi, A.
    Materials Characterization (2018)
    Field ion microscopy (FIM) allows to image individual surface atoms by exploiting the effect of an intense electric field. Widespread use of atomic resolution imaging by FIM has been hampered by a lack of efficient image processing/data extraction tools. Recent advances in imaging and data mining techniques have renewed the interest in using FIM in conjunction with automated detection of atoms and lattice defects for materials characterization. After a brief overview of existing routines, we review the use of machine learning (ML) approaches for data extraction with the aim to catalyze new data-driven insights into high electrical field physics. Apart from exploring various supervised and unsupervised ML algorithms in this context, we also employ advanced image processing routines for data extraction from large sets of FIM images. The outcomes and limitations of such routines are discussed, and we conclude with the possible application of energy minimization schemes to the extracted point clouds as a way of improving the spatial resolution of FIM. © 2018 Elsevier Inc.
    view abstract10.1016/j.matchar.2018.02.040
  • Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study
    Stockem, I. and Bergman, A. and Glensk, A. and Hickel, T. and Körmann, F. and Grabowski, B. and Neugebauer, J. and Alling, B.
    Physical Review Letters 121 (2018)
    We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.125902
  • Calculating free energies of point defects from ab initio
    Zhang, X. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Computational Materials Science 148 (2018)
    The formation and lifetime of point defects is governed by an interplay of kinetics and thermodynamic stability. To evaluate the stability under process conditions, empirical potentials and ab initio calculations at T=0K are often not sufficient. Therefore, various concepts to determine the full temperature dependence of the free energy of point defects with ab initio accuracy are reviewed. Examples for the importance of accurately describing defect properties include the stabilization of vacancies by impurities and the non-Arrhenius behaviour of vacancy formation energies due to anharmonic lattice vibrations. © 2018
    view abstract10.1016/j.commatsci.2018.02.042
  • Elastically frustrated rehybridization: Origin of chemical order and compositional limits in InGaN quantum wells
    Lymperakis, L. and Schulz, T. and Freysoldt, C. and Anikeeva, M. and Chen, Z. and Zheng, X. and Shen, B. and Chèze, C. and Siekacz, M. and Wang, X.Q. and Albrecht, M. and Neugebauer, J.
    Physical Review Materials 2 (2018)
    Nominal InN monolayers grown by molecular beam epitaxy on GaN(0001) are investigated combining in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and density functional theory (DFT). TEM reveals a chemical intraplane ordering never observed before. Employing DFT, we identify a novel surface stabilization mechanism elastically frustrated rehybridization, which is responsible for the observed chemical ordering. The mechanism also sets an incorporation barrier for indium concentrations above 25% and thus fundamentally limits the indium content in coherently strained layers. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.011601
  • 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 abstract10.1103/PhysRevLett.120.246801
  • First-principles calculations for charged defects at surfaces, interfaces, and two-dimensional materials in the presence of electric fields
    Freysoldt, C. and Neugebauer, J.
    Physical Review B 97 (2018)
    We present a methodology to calculate the formation energy of a charged defect at a surface, an interface, or a two-dimensional material in the presence of a macroscopic electric field. We demonstrate that the proposed formalism corrects for electrostatic artifacts in standard repeated-slab calculations and allows us to extract reliably the formation energy in the isolated defect limit independently of vacuum thickness, slab thickness, or lateral supercell size. The formalism does not enter the self-consistency loop of a density functional theory (DFT) calculation, but requires as input only the electrostatic potential of the converged calculation. Thus, employing the proposed scheme does not require any changes in the DFT code, but only a postprocessing module that we provide for various codes. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.205425
  • Impact of asymmetric martensite and austenite nucleation and growth behavior on the phase stability and hysteresis of freestanding shape-memory nanoparticles
    Ko, W.-S. and Grabowski, B. and Neugebauer, J.
    Physical Review Materials 2 (2018)
    Martensitic transformations in nanoscaled shape-memory alloys exhibit characteristic features absent for the bulk counterparts. Detailed understanding is required for applications in micro- and nanoelectromechanical systems, and experimental limitations render atomistic simulation an important complementary approach. Using a recently developed, accurate potential we investigate the phase transformation in freestanding Ni-Ti shape-memory nanoparticles with molecular-dynamics simulations. The results confirm that the decrease in the transformation temperature with decreasing particle size is correlated with an overstabilization of the austenitic surface energy over the martensitic surface energy. However, a detailed atomistic analysis of the nucleation and growth behavior reveals an unexpected difference in the mechanisms determining the austenite finish and martensite start temperature. While the austenite finish temperature is directly affected by a contribution of the surface energy difference, the martensite start temperature is mostly affected by the transformation strain, contrary to general expectations. This insight not only explains the reduced transformation temperature but also the reduced thermal hysteresis in freestanding nanoparticles. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.030601
  • Impact of chemical fluctuations on stacking fault energies of CrCoNi and CrMnFeCoNi high entropy alloys from first principles
    Ikeda, Y. and Körmann, F. and Tanaka, I. and Neugebauer, J.
    Entropy 20 (2018)
    Medium and high entropy alloys (MEAs and HEAs) based on 3d transition metals, such as face-centered cubic (fcc) CrCoNi and CrMnFeCoNi alloys, reveal remarkable mechanical properties. The stacking fault energy (SFE) is one of the key ingredients that controls the underlying deformation mechanism and hence the mechanical performance of materials. Previous experiments and simulations have therefore been devoted to determining the SFEs of various MEAs and HEAs. The impact of local chemical environment in the vicinity of the stacking faults is, however, still not fully understood. In this work, we investigate the impact of the compositional fluctuations in the vicinity of stacking faults for two prototype fcc MEAs and HEAs, namely CrCoNi and CrMnFeCoNi by employing first-principles calculations. Depending on the chemical composition close to the stacking fault, the intrinsic SFEs vary in the range of more than 150 mJ/m2 for both the alloys, which indicates the presence of a strong driving force to promote particular types of chemical segregations towards the intrinsic stacking faults in MEAs and HEAs. Furthermore, the dependence of the intrinsic SFEs on local chemical fluctuations reveals a highly non-linear behavior, resulting in a non-trivial interplay of local chemical fluctuations and SFEs. This sheds new light on the importance of controlling chemical fluctuations via tuning, e.g., the annealing condition to obtain the desired mechanical properties for MEAs and HEAs. © 2018 by the authors.
    view abstract10.3390/e20090655
  • Impact of Co and Fe Doping on the Martensitic Transformation and the Magnetic Properties in Ni-Mn-Based Heusler Alloys
    Dutta, B. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 255 (2018)
    Doping with Co and Fe is a promising strategy to enhance the performance of Ni-Mn-based magnetocaloric materials. We investigate here the impact of these two elements on the magnetic properties and the martensitic transformations in Mn-rich Ni-Mn-Al Heusler alloy. Based on ab initio ground-state energies, we explain the martensitic transformation temperatures in these alloys. The magnetic ground states of the austenite and the martensite phases are discussed on the basis of magnetic exchange parameters. A thorough comparison of the transition temperatures and the change in saturation moments during martensitic transformation brings into light the relative potential of the two substitutional elements for future applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700455
  • Impact of local electrostatic field rearrangement on field ionization
    Katnagallu, S. and Dagan, M. and Parviainen, S. and Nematollahi, A. and Grabowski, B. and Bagot, P.A.J. and Rolland, N. and Neugebauer, J. and Raabe, D. and Vurpillot, F. and Moody, M.P. and Gault, B.
    Journal of Physics D: Applied Physics 51 (2018)
    Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aaaba6
  • Migration mechanisms of a faceted grain boundary
    Hadian, R. and Grabowski, B. and Finnis, M.W. and Neugebauer, J.
    Physical Review Materials 2 (2018)
    We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the Σ7 symmetric tilt boundary of the type {123} in aluminum. When minimized in energy at 0K, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries. © 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevMaterials.2.043601
  • Modeling of phase equilibria in Ni-H: Bridging the atomistic with the continuum scale
    Korbmacher, D. and Von Pezold, J. and Brinckmann, S. and Neugebauer, J. and Hüter, C. and Spatschek, R.
    Metals 8 (2018)
    In this paper, we present a model which allows bridging the atomistic description of two-phase systems to the continuum level, using Ni-H as a model system. Considering configurational entropy, an attractive hydrogen–hydrogen interaction, mechanical deformations and interfacial effects, we obtained a fully quantitative agreement in the chemical potential, without the need for any additional adjustable parameter. We find that nonlinear elastic effects are crucial for a complete understanding of constant volume phase coexistence, and predict the phase diagram with and without elastic effects. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met8040280
  • 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 abstract10.3390/ma11091732
  • Precipitate-induced nonlinearities of diffusion along grain boundaries in Al-based alloys
    Gupta, A. and Kulitcki, V. and Kavakbasi, B.T. and Buranova, Y. and Neugebauer, J. and Wilde, G. and Hickel, T. and Divinski, S.V.
    Physical Review Materials 2 (2018)
    The formation of precipitates in metallic alloys is determined by a two-way chemomechanical coupling. While the dependence on solute diffusion is apparent, the opposite effect, namely the impact of nanoprecipitates on the diffusion, is highlighted in the present paper. Using severe plastic deformation and post-deformation annealing of an Al-based alloy, different microstructures and sizes of Al3Sc-based nanoprecipitates are produced. The enhanced diffusion along grain boundaries in the so-called C-type kinetic regime is used as a probe to ensure a well-defined distance of the diffusion path from the precipitates and to prohibit their evolution during measurements. Tracer measurements with the radioisotope Co57 reveal a remarkable nonmonotonic dependence of the diffusion rates on the annealing temperature. It has been fully explained by an ab initio informed phenomenological model that considers the elastic stress around the coherent and noncoherent precipitates. © 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevMaterials.2.073801
  • Selective Solvent-Induced Stabilization of Polar Oxide Surfaces in an Electrochemical Environment
    Yoo, S.-H. and Todorova, M. and Neugebauer, J.
    Physical Review Letters 120 (2018)
    The impact of an electrochemical environment on the thermodynamic stability of polar oxide surfaces is investigated for the example of ZnO(0001) surfaces immersed in water using density functional theory calculations. We show that solvation effects are highly selective: They have little effect on surfaces showing a metallic character, but largely stabilize semiconducting structures, particularly those that have a high electrostatic penalty in vacuum. The high selectivity is shown to have direct consequences for the surface phase diagram and explains, e.g., why certain surface structures could be observed only in an electrochemical environment. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.120.066101
  • Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.015702
  • Temperature dependence of the Gibbs energy of vacancy formation of fcc Ni
    Gong, Y. and Grabowski, B. and Glensk, A. and Körmann, F. and Neugebauer, J. and Reed, R.C.
    Physical Review B 97 (2018)
    Quantum-mechanical calculations are used to determine the temperature dependence of the Gibbs energy of vacancy formation in nickel. Existing data reveal a discrepancy between the high-temperature estimates from experiments and low-temperature approximations from density functional theory. Our finite-temperature calculations - which include the effects of magnetism and fully interacting phonon vibrations - demonstrate that this discrepancy is mostly caused by the previously neglected explicit anharmonic contribution. © 2018 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.97.214106
  • Temperature dependence of the stacking-fault Gibbs energy for Al, Cu, and Ni
    Zhang, X. and Grabowski, B. and Körmann, F. and Ruban, A.V. and Gong, Y. and Reed, R.C. and Hickel, T. and Neugebauer, J.
    Physical Review B 98 (2018)
    The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements. © 2018 authors.
    view abstract10.1103/PhysRevB.98.224106
  • Temperature-dependent phonon spectra of magnetic random solid solutions
    Ikeda, Y. and Körmann, F. and Dutta, B. and Carreras, A. and Seko, A. and Neugebauer, J. and Tanaka, I.
    npj Computational Materials 4 (2018)
    A first-principles-based computational tool for simulating phonons of magnetic random solid solutions including thermal magnetic fluctuations is developed. The method takes fluctuations of force constants due to magnetic excitations as well as due to chemical disorder into account. The developed approach correctly predicts the experimentally observed unusual phonon hardening of a transverse acoustic mode in Fe-Pd an Fe-Pt Invar alloys with increasing temperature. This peculiar behavior, which cannot be explained within a conventional harmonic picture, turns out to be a consequence of thermal magnetic fluctuations. The proposed methodology can be straightforwardly applied to a wide range of materials to reveal new insights into physical behaviors and to design materials through computation, which were not accessible so far. © 2018 The Author(s).
    view abstract10.1038/s41524-018-0063-1
  • Tetragonal fcc-Fe induced by κ -carbide precipitates: Atomic scale insights from correlative electron microscopy, atom probe tomography, and density functional theory
    Liebscher, C.H. and Yao, M. and Dey, P. and Lipińska-Chwalek, M. and Berkels, B. and Gault, B. and Hickel, T. and Herbig, M. and Mayer, J. and Neugebauer, J. and Raabe, D. and Dehm, G. and Scheu, C.
    Physical Review Materials 2 (2018)
    Correlative scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations resolve the correlation between elastic strain fields and local impurity concentrations on the atomic scale. The correlative approach is applied to coherent interfaces in a κ-carbide strengthened low-density steel and establishes a tetragonal distortion of fcc-Fe. An interfacial roughness of ∼1nm and a localized carbon concentration gradient extending over ∼2-3nm is revealed, which originates from the mechano-chemical coupling between local strain and composition. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.023804
  • {110} planar faults in strained bcc metals: Origins and implications of a commonly observed artifact of classical potentials
    Möller, J.J. and Mrovec, M. and Bleskov, I. and Neugebauer, J. and Hammerschmidt, T. and Drautz, R. and Elsässer, C. and Hickel, T. and Bitzek, E.
    Physical Review Materials 2 (2018)
    Large-scale atomistic simulations with classical potentials can provide valuable insights into microscopic deformation mechanisms and defect-defect interactions in materials. Unfortunately, these assets often come with the uncertainty of whether the observed mechanisms are based on realistic physical phenomena or whether they are artifacts of the employed material models. One such example is the often reported occurrence of stable planar faults (PFs) in body-centered cubic (bcc) metals subjected to high strains, e.g., at crack tips or in strained nano-objects. In this paper, we study the strain dependence of the generalized stacking fault energy (GSFE) of {110} planes in various bcc metals with material models of increasing sophistication, i.e., (modified) embedded atom method, angular-dependent, Tersoff, and bond-order potentials as well as density functional theory. We show that under applied tensile strains the GSFE curves of many classical potentials exhibit a local minimum which gives rise to the formation of stable PFs. These PFs do not appear when more sophisticated material models are used and have thus to be regarded as artifacts of the potentials. We demonstrate that the local GSFE minimum is not formed for reasons of symmetry and we recommend including the determination of the strain-dependent (110) GSFE as a benchmark for newly developed potentials. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.093606
  • 100 years public-private partnership in metallurgical and materials science research
    Raabe, D. and Dehm, G. and Neugebauer, J. and Rohwerder, M.
    Materials Today 20 (2017)
    view abstract10.1016/j.mattod.2017.02.005
  • A rare-earth free magnesium alloy with improved intrinsic ductility
    Sandlöbes, S. and Friák, M. and Korte-Kerzel, S. and Pei, Z. and Neugebauer, J. and Raabe, D.
    Scientific Reports 7 (2017)
    Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm3, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints. © 2017 The Author(s).
    view abstract10.1038/s41598-017-10384-0
  • 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 (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 abstract10.1016/j.actamat.2017.07.023
  • Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C κ carbides
    Dey, P. and Nazarov, R. and Dutta, B. and Yao, M. and Herbig, M. and Friák, M. and Hickel, T. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In this work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nanosized κ carbides (Fe,Mn)3AlC by atom probe tomography in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magnetic disorder in these carbides. Second, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures. © 2017 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevB.95.104108
  • 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 (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 abstract10.1016/j.actamat.2017.04.024
  • Accurate electronic free energies of the 3 d,4 d, and 5 d transition metals at high temperatures
    Zhang, X. and Grabowski, B. and Körmann, F. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at T=0 K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all 3d,4d, and 5d transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical d band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.95.165126
  • Adsorption and desorption of hydrogen at nonpolar GaN(1 1 00) surfaces: Kinetics and impact on surface vibrational and electronic properties
    Lymperakis, L. and Neugebauer, J. and Himmerlich, M. and Krischok, S. and Rink, M. and Kröger, J. and Polyakov, V.M.
    Physical Review B 95 (2017)
    The adsorption of hydrogen at nonpolar GaN(1100) surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of H2 and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1100) surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga-N surface dimer buckling. The signatures of the Ga-H and N-H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean GaN(1100) surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.95.195314
  • Atomic scale processes of phase transformations in nanocrystalline NiTi shape-memory alloys
    Ko, W.-S. and Maisel, S.B. and Grabowski, B. and Jeon, J.B. and Neugebauer, J.
    Acta Materialia 123 (2017)
    Molecular dynamics simulations are performed to investigate temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed insights into the origins of the experimentally reported characteristics of phase transformations at the nanoscale, such as the decrease of the transformation temperature with grain size and the disappearance of the plateau in the stress-strain response. The relevant atomic scale processes, such as nucleation, growth, and twinning are analyzed and explained. We suggest that a single, unified mechanism—dominated by the contribution of a local transformation strain—explains the characteristics of both temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.10.019
  • Atomic structures of twin boundaries in hexagonal close-packed metallic crystals with particular focus on Mg
    Pei, Z. and Zhang, X. and Hickel, T. and Friák, M. and Sandlöbes, S. and Dutta, B. and Neugebauer, J.
    npj Computational Materials 3 (2017)
    We have investigated twin boundaries in double-lattice hexagonal close-packed metallic materials, focusing on their atomic geometry. Combining accurate ab-initio methods and large-scale atomistic simulations we address the following two fundamental questions: (i) What are the possible intrinsic twin boundary structures in hcp crystals? (ii) Are these structures stable against small distortions? In order to help end a decade-long controversy over the experimental observations of the atomic structures of twin boundaries, we have determined the energetics, spectra, and transition mechanisms of the twin boundaries. Our results confirm that the mechanical stability controls structures which are observed. © 2017 The Author(s).
    view abstract10.1038/s41524-017-0010-6
  • Atomistic Modeling-Based Design of Novel Materials
    Holec, D. and Zhou, L. and Riedl, H. and Koller, C.M. and Mayrhofer, P.H. and Friák, M. and Šob, M. and Körmann, F. and Neugebauer, J. and Music, D. and Hartmann, M.A. and Fischer, F.D.
    Advanced Engineering Materials 19 (2017)
    Modern materials science increasingly advances via a knowledge-based development rather than a trial-and-error procedure. Gathering large amounts of data and getting deep understanding of non-trivial relationships between synthesis of materials, their structure and properties is experimentally a tedious work. Here, theoretical modeling plays a vital role. In this review paper we briefly introduce modeling approaches employed in materials science, their principles and fields of application. We then focus on atomistic modeling methods, mostly quantum mechanical ones but also Monte Carlo and classical molecular dynamics, to demonstrate their practical use on selected examples. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201600688
  • Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.11.013
  • Efficient approach to compute melting properties fully from ab initio with application to Cu
    Zhu, L.-F. and Grabowski, B. and Neugebauer, J.
    Physical Review B 96 (2017)
    Applying thermodynamic integration within an ab initio-based free-energy approach is a state-of-the-art method to calculate melting points of materials. However, the high computational cost and the reliance on a good reference system for calculating the liquid free energy have so far hindered a general application. To overcome these challenges, we propose the two-optimized references thermodynamic integration using Langevin dynamics (TOR-TILD) method in this work by extending the two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) method, which has been originally developed to obtain anharmonic free energies of solids, to the calculation of liquid free energies. The core idea of TOR-TILD is to fit two empirical potentials to the energies from density functional theory based molecular dynamics runs for the solid and the liquid phase and to use these potentials as reference systems for thermodynamic integration. Because the empirical potentials closely reproduce the ab initio system in the relevant part of the phase space the convergence of the thermodynamic integration is very rapid. Therefore, the proposed approach improves significantly the computational efficiency while preserving the required accuracy. As a test case, we apply TOR-TILD to fcc Cu computing not only the melting point but various other melting properties, such as the entropy and enthalpy of fusion and the volume change upon melting. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and the local-density approximation (LDA) are used. Using both functionals gives a reliable ab initio confidence interval for the melting point, the enthalpy of fusion, and entropy of fusion. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.96.224202
  • Fermi-level pinning and intrinsic surface states of Al 1 - X in x N (10 1 - 0) surfaces
    Portz, V. and Schnedler, M. and Lymperakis, L. and Neugebauer, J. and Eisele, H. and Carlin, J.-F. and Butté, R. and Grandjean, N. and Dunin-Borkowski, R.E. and Ebert, P.
    Applied Physics Letters 110 (2017)
    The electronic structure of Al1-xInxN(101-0) surfaces is investigated by cross-sectional scanning tunneling spectroscopy and density functional theory calculations. The surface exhibits empty Al and/or In-derived dangling bond states, which are calculated to be within the fundamental bulk band gap for In compositions smaller than 60%. The energy of the lowest empty In-derived surface state is extracted from the tunnel spectra for lattice-matched Al1- xInxN with In compositions of x = 0.19 and x = 0.20 to be EC - 1.82 ± 0.41 and EC - 1.80 ± 0.56 eV, respectively, in good agreement with the calculated energies. Under growth conditions, the Fermi level is hence pinned (unpinned) for In compositions smaller (larger) than 60%. The analysis of the tunnel spectra suggests an electron affinity of ∼3.5 eV for nonpolar lattice-matched Al1- xInxN cleavage surfaces, which is large compared to linearly interpolated values of polar AlN and InN (0001) surfaces. © 2017 Author(s).
    view abstract10.1063/1.4973765
  • Hydrogen behaviour at twist {110} grain boundaries in α-Fe
    McEniry, E.J. and Hickel, T. and Neugebauer, J.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2017)
    The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-Throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron-hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-Terminated twist grain boundaries in α-Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a 'glue' for the boundary, increasing both the energetic barrier and the elongation to rupture. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rsta.2016.0402
  • Low-temperature features in the heat capacity of unary metals and intermetallics for the example of bulk aluminum and Al3 Sc
    Gupta, A. and Kavakbasi, B.T. and Dutta, B. and Grabowski, B. and Peterlechner, M. and Hickel, T. and Divinski, S.V. and Wilde, G. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    We explore the competition and coupling of vibrational and electronic contributions to the heat capacity of Al and Al3Sc at temperatures below 50 K, combining experimental calorimetry with highly converged finite-temperature density functional theory calculations. We find that semilocal exchange-correlation functionals accurately describe the rich feature set observed for these temperatures, including electron-phonon coupling. Using different representations of the heat capacity, we are therefore able to identify and explain deviations from the Debye behavior in the low-temperature limit and in the temperature regime 30-50 K as well as the reduction of these features due to the addition of Sc. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.95.094307
  • Magnetic properties of the CrMnFeCoNi high-entropy alloy
    Schneeweiss, O. and Friák, M. and Dudová, M. and Holec, D. and Šob, M. and Kriegner, D. and Holý, V. and Beran, P. and George, E.P. and Neugebauer, J. and Dlouhý, A.
    Physical Review B 96 (2017)
    We present experimental data showing that the equiatomic CrMnFeCoNi high-entropy alloy undergoes two magnetic transformations at temperatures below 100 K while maintaining its fcc structure down to 3 K. The first transition, paramagnetic to spin glass, was detected at 93 K and the second transition of the ferromagnetic type occurred at 38 K. Field-assisted cooling below 38 K resulted in a systematic vertical shift of the hysteresis curves. Strength and direction of the associated magnetization bias was proportional to the strength and direction of the cooling field and shows a linear dependence with a slope of 0.006±0.001 emuT. The local magnetic moments of individual atoms in the CrMnFeCoNi quinary fcc random solid solution were investigated by ab initio (electronic density functional theory) calculations. Results of the numerical analysis suggest that, irrespective of the initial configuration of local magnetic moments, the magnetic moments associated with Cr atoms align antiferromagnetically with respect to a cumulative magnetic moment of their first coordination shell. The ab initio calculations further showed that the magnetic moments of Fe and Mn atoms remain strong (between 1.5 and 2μB), while the local moments of Ni atoms effectively vanish. These results indicate that interactions of Mn- and/or Fe-located moments with the surrounding magnetic structure account for the observed macroscopic magnetization bias. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.014437
  • Modelling of grain boundary dynamics using amplitude equations
    Hüter, C. and Neugebauer, J. and Boussinot, G. and Svendsen, B. and Prahl, U. and Spatschek, R.
    Continuum Mechanics and Thermodynamics 29 (2017)
    We discuss the modelling of grain boundary dynamics within an amplitude equations description, which is derived from classical density functional theory or the phase field crystal model. The relation between the conditions for periodicity of the system and coincidence site lattices at grain boundaries is investigated. Within the amplitude equations framework, we recover predictions of the geometrical model by Cahn and Taylor for coupled grain boundary motion, and find both (Formula presented.) and (Formula presented.) coupling. No spontaneous transition between these modes occurs due to restrictions related to the rotational invariance of the amplitude equations. Grain rotation due to coupled motion is also in agreement with theoretical predictions. Whereas linear elasticity is correctly captured by the amplitude equations model, open questions remain for the case of nonlinear deformations. © 2015 Springer-Verlag Berlin Heidelberg
    view abstract10.1007/s00161-015-0424-7
  • Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hickel, T. and Roldan Cuenya, B.
    Journal of the American Chemical Society 139 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b07902
  • 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 abstract10.1103/PhysRevLett.118.236101
  • The role of κ-carbides as hydrogen traps in high-Mn steels
    Timmerscheidt, T.A. and Dey, P. and Bogdanovski, D. and von Appen, J. and Hickel, T. and Neugebauer, J. and Dronskowski, R.
    Metals 7 (2017)
    Since the addition of Al to high-Mn steels is known to reduce their sensitivity to hydrogen-induced delayed fracture, we investigate possible trapping effects connected to the presence of Al in the grain interior employing density-functional theory (DFT). The role of Al-based precipitates is also investigated to understand the relevance of short-range ordering effects. So-called E21-Fe3AlC κ-carbides are frequently observed in Fe-Mn-Al-C alloys. Since H tends to occupy the same positions as C in these precipitates, the interaction and competition between both interstitials is also investigated via DFT-based simulations. While the individual H–H/C–H chemical interactions are generally repulsive, the tendency of interstitials to increase the lattice parameter can yield a net increase of the trapping capability. An increased Mn content is shown to enhance H trapping due to attractive short-range interactions. Favorable short-range ordering is expected to occur at the interface between an Fe matrix and the E21-Fe3AlC κ-carbides, which is identified as a particularly attractive trapping site for H. At the same time, accumulation of H at sites of this type is observed to yield decohesion of this interface, thereby promoting fracture formation. The interplay of these effects, evident in the trapping energies at various locations and dependent on the H concentration, can be expressed mathematically, resulting in a term that describes the hydrogen embrittlement. © 2017 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met7070264
  • A QM/MM approach for low-symmetry defects in metals
    Huber, L. and Grabowski, B. and Militzer, M. and Neugebauer, J. and Rottler, J.
    Computational Materials Science 118 (2016)
    Concurrent multiscale coupling is a powerful tool for obtaining quantum mechanically (QM) accurate material behavior in a small domain while still capturing long range stress fields using a molecular mechanical (MM) description. We outline an improved scheme for QM/MM coupling in metals which permits the QM treatment of a small region chosen from a large, arbitrary MM domain to calculate total system energy and relaxed geometry. In order to test our improved method, we compute solute-vacancy binding in bulk Al as well as the binding of Mg and Pb to a symmetric Σ5 grain boundary. Results are calculated with and without our improvement to the QM/MM scheme and compared to periodic QM results for the same systems. We find that our scheme accurately and efficiently reproduces periodic QM target values in these test systems and therefore can be expected to perform well using more general geometries. © 2016 Published by Elsevier B.V.
    view abstract10.1016/j.commatsci.2016.03.028
  • Ab Initio Determined Phase Diagram of Clean and Solvated Muscovite Mica Surfaces
    Vatti, A.K. and Todorova, M. and Neugebauer, J.
    Langmuir 32 (2016)
    Focusing on muscovite mica, the most significant phyllosilicate in the mica series, we determine its surface phase diagram employing density functional theory. Surfaces in vacuum and in more realistic environmental conditions, that is, the surface in contact with water or an ionic liquid, are considered. These results naturally explain experimental observations such as the swelling of mica when it comes into contact with water. © 2016 American Chemical Society.
    view abstract10.1021/acs.langmuir.5b04087
  • Ab initio Prediction of Martensitic and Intermartensitic Phase Boundaries in Ni-Mn-Ga
    Dutta, B. and Çaklr, A. and Giacobbe, C. and Al-Zubi, A. and Hickel, T. and Acet, M. and Neugebauer, J.
    Physical Review Letters 116 (2016)
    Despite the importance of martensitic transformations of Ni-Mn-Ga Heusler alloys for their magnetocaloric and shape-memory properties, the martensitic part of their phase diagrams is not well determined. Using an ab initio approach that includes the interplay of lattice and vibrational degrees of freedom we identify an intermartensitic transformation between a modulated and a nonmodulated phase as a function of excess Ni and Mn content. Based on an evaluation of the theoretical findings and experimental x-ray diffraction data for Mn-rich alloys, we are able to predict the phase diagram for Ni-rich alloys. In contrast to other mechanisms discussed for various material systems in the literature, we herewith show that the intermartensitic transformation can be understood solely using thermodynamic concepts. © 2016 American Physical Society.
    view abstract10.1103/PhysRevLett.116.025503
  • Ab initio-guided design of twinning-induced plasticity steels
    Raabe, D. and Roters, F. and Neugebauer, J. and Gutierrez-Urrutia, I. and Hickel, T. and Bleck, W. and Schneider, J.M. and Wittig, J.E. and Mayer, J.
    MRS Bulletin 41 (2016)
    The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms. Copyright © 2016 Materials Research Society.
    view abstract10.1557/mrs.2016.63
  • Atomistic migration mechanisms of atomically flat, stepped, and kinked grain boundaries
    Hadian, R. and Grabowski, B. and Race, C.P. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt (111) Σ7 grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of the boundaries during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.165413
  • Autofluorescence imaging in recurrent oral squamous cell carcinoma
    Scheer, M. and Fuss, J. and Derman, M.A. and Kreppel, M. and Neugebauer, J. and Rothamel, D. and Drebber, U. and Zoeller, J.E.
    Oral and Maxillofacial Surgery 20 (2016)
    The survival of patients with oral cancer is decreased by locoregional recurrence after an initial multimodal treatment. In order to identify lesions in the oral cavity for a possible recurrence, clinical evaluation as well as MRI or CT scanning is advised. The evaluation of mucosa lesions is hampered by changes related to radio- and chemotherapy as well as reconstruction with tissue flaps. Several techniques for easier identification of tissue abnormalities in the oral cavity have been advocated as adjuncts in order to facilitate identification. Especially methods using altered tissue fluorescence have gained much interest during the last decade. The aim of our prospective study was to evaluate fluorescence properties of undiagnosed mucosa lesions with the VELscope device in patients with multimodal treated oral cancer prior to histological confirmation. In total, 41 patients with a history of oral squamous cell carcinomas (OSCC) (19 females and 22 males) with undiagnosed mucosa lesions where included in the study. After clinical evaluation, examination and documentation using the VELscope® device were performed. Then, an incisional biopsy was performed. An autofluorescence loss indicating a malignant or dysplastic mucosa condition could be detected in six patients (14.6 %); however, only one OSCC and one SIN revealed a complete autofluorescence loss. In four patients, OSCC was present in lesions with retained autofluorescence. Sensitivity and specificity for the VELscope® examination to identify malignant oral lesions by autofluorescence were 33.3 and 88.6 %, respectively. The positive and negative predictive values were 33.3 and 88.6 %, respectively. No statistical correlation between gender and lesion appearance versus autofluorescence loss could be detected. In contrast to mucosa lesions in patients with no prior treatment, the autofluorescence evaluation with the VELscope reveals no additional information in our analysis. Accordingly, invasive biopsies as gold standard are still needed to get sufficient evidence regarding potential malignancy in patients after multimodal treatment for oral cancer. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s10006-015-0520-7
  • Changes in volume during the four months’ remodelling period of iliac crest grafts in reconstruction of the alveolar ridge
    Dreiseidler, T. and Kaunisaho, V. and Neugebauer, J. and Zöller, J.E. and Rothamel, D. and Kreppel, M.
    British Journal of Oral and Maxillofacial Surgery 54 (2016)
    Our aim was to assess the four months’ resorption rates of onlay iliac crest grafts in atrophic jaws prospectively, and to identify factors that influence them. Twenty-four patients had reconstructions of the alveolar ridge with iliac crest onlay grafts at 30 sites on the mandibles and maxillas. The augmentation volumes were measured on cone-beam computed tomographic (CT) data-sets directly after augmentation (V1), and after four months’ remodelling (V2). Statistical analysis allowed identification of potential influences from the recipient sites, volume of the graft, and the patients’ smoking behaviour. The mean (range) initial onlay graft volume (V1) was 2.82 (0.66 to 6.41) ml. After four months, the mean measured onlay graft volume (V2) was 2.39 (0.47 to 6.21) ml. Mean iliac crest onlay graft volume resorption after four months of remodelling was 0.43 (-0.15 - 1.78) ml (15%). We found no significant differences in the resorption rates of iliac crest onlay grafts between different recipient sites (maxilla and mandible) or in dependence on the volume of iliac crest grafts. Smokers tended to have a higher rate of resorption, but not significantly so (p = 0.056). The results of this study indicate the most favourable resorption rates for iliac crest onlay grafts that we know have seen published to date. © 2016 The British Association of Oral and Maxillofacial Surgeons
    view abstract10.1016/j.bjoms.2016.04.024
  • Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.01.007
  • Deformation-Induced Martensite: A New Paradigm for Exceptional Steels
    Djaziri, S. and Li, Y. and Nematollahi, G.A. and Grabowski, B. and Goto, S. and Kirchlechner, C. and Kostka, A. and Doyle, S. and Neugebauer, J. and Raabe, D. and Dehm, G.
    Advanced Materials 28 (2016)
    view abstract10.1002/adma.201601526
  • Electron and chemical reservoir corrections for point-defect formation energies
    Freysoldt, C. and Lange, B. and Neugebauer, J. and Yan, Q. and Lyons, J.L. and Janotti, A. and Van De Walle, C.G.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    Point-defect formation energies calculated within the framework of density functional theory often depend on the choice of the exchange and correlation (xc) functional. We show that variations between the local density approximation (LDA), generalized gradient approximation (GGA), and hybrid functionals mainly arise from differences in the position of the bulk valence-band maximum, as well as in the reference energies for the chemical potential obtained with distinct xc functionals. We demonstrate for point defects relevant for p-type GaN that these differences can be accounted for by corrections, reducing the maximum disagreement between the different functionals from more than 2 eV to below 0.2 eV. Our correction scheme should be useful for performing high-throughput calculations in cases where full hybrid functional calculations are prohibitively expensive. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.165206
  • First-principles calculation of the elastic dipole tensor of a point defect: Application to hydrogen in α -zirconium
    Nazarov, R. and Majevadia, J.S. and Patel, M. and Wenman, M.R. and Balint, D.S. and Neugebauer, J. and Sutton, A.P.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The elastic dipole tensor is a fundamental quantity relating the elastic field and atomic structure of a point defect. We review three methods in the literature to calculate the dipole tensor and apply them to hydrogen in α-zirconium using density functional theory (DFT). The results are compared with the dipole tensor deduced from earlier experimental measurements of the λ tensor for hydrogen in α-zirconium. There are significant errors with all three methods. We show that calculation of the λ tensor, in combination with experimentally measured elastic constants and lattice parameters, yields dipole tensor components that differ from experimental values by only 10%-20%. There is evidence to suggest that current state-of-the-art DFT calculations underestimate bonding between hydrogen and α-zirconium. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.241112
  • First-principles investigation of hydrogen interaction with TiC precipitates in α -Fe
    Di Stefano, D. and Nazarov, R. and Hickel, T. and Neugebauer, J. and Mrovec, M. and Elsässer, C.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    A correct description of hydrogen diffusion and trapping is the prerequisite for an understanding of the phenomenon of hydrogen embrittlement. In this study, we carried out extensive first-principles calculations based on density functional theory to investigate the interaction of H with TiC precipitates that are assumed to be efficient trapping agents mitigating HE in advanced high-strength steels. We found that there exists a large variety of possible trapping sites for H associated with different types of interfaces between the TiC particle and the Fe matrix, with misfit dislocations and other defects at these interfaces, and with carbon vacancies in TiC. The most efficient trapping by more than 1 eV occurs at carbon vacancies in the interior of TiC particles. However, these traps are difficult to populate at ambient temperatures since the energy barrier for H entering the particles is high. H trapping at the semicoherent interfaces between the TiC particles and the Fe matrix is moderate, ranging from 0.3 to 0.5 eV. However, a sufficiently large concentration of the carbide particles can significantly reduce the amount of H segregated at dislocation cores in the Fe matrix. A systematic comparison of the obtained theoretical results with available experimental observations reveals a consistent picture of hydrogen trapping at the TiC particles that is expected to be qualitatively valid also for other carbide precipitates with the rock-salt crystal structure. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.184108
  • From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.04.059
  • Functional adaptation of crustacean exoskeletal elements through structural and compositional diversity: A combined experimental and theoretical study
    Fabritius, H.-O. and Ziegler, A. and Friák, M. and Nikolov, S. and Huber, J. and Seidl, B.H.M. and Ruangchai, S. and Alagboso, F.I. and Karsten, S. and Lu, J. and Janus, A.M. and Petrov, M. and Zhu, L.-F. and Hemzalová, P. and Hild, S. and Raabe, D. and Neugebauer, J.
    Bioinspiration and Biomimetics 11 (2016)
    The crustacean cuticle is a composite material that covers the whole animal and forms the continuous exoskeleton. Nano-fibers composed of chitin and protein molecules form most of the organic matrix of the cuticle that, at the macroscale, is organized in up to eight hierarchical levels. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate and phosphate. The high number of hierarchical levels and the compositional diversity provide a high degree of freedom for varying the physical, in particular mechanical, properties of the material. This makes the cuticle a versatile material ideally suited to form a variety of skeletal elements that are adapted to different functions and the eco-physiological strains of individual species. This review presents our recent analytical, experimental and theoretical studies on the cuticle, summarising at which hierarchical levels structure and composition are modified to achieve the required physical properties. We describe our multi-scale hierarchical modeling approach based on the results from these studies, aiming at systematically predicting the structure-composition-property relations of cuticle composites from the molecular level to the macro-scale. This modeling approach provides a tool to facilitate the development of optimized biomimetic materials within a knowledge-based design approach. © 2016 IOP Publishing Ltd.
    view abstract10.1088/1748-3190/11/5/055006
  • Impact of local magnetism on stacking fault energies: A first-principles investigation for fcc iron
    Bleskov, I. and Hickel, T. and Neugebauer, J. and Ruban, A.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    A systematic ab initio study of the influence of local magnetism on the generalized stacking fault energy (GSFE) surface in pure fcc iron at 0 K has been performed. In the calculations we considered ferro- and antiferro- (single- and double-layer) magnetic order of local moments as well as their complete disorder, corresponding to paramagnetic (PM) state. We have shown that local magnetism is one of the most important factors stabilizing austenitic structure in iron (with respect to more stable at 0 K hcp) and that the perturbation of magnetic structure by the formation of stacking fault is a short-range effect. Local magnetism also strongly influences the GSFE surface topology and, therefore, the material's plasticity by reducing the energetic barriers that need to be overcome to form the intrinsic stacking fault (ISF) or return from the ISF structure to fcc. The influence of atomic relaxations on such barriers is moderate and does not exceed 15%. In addition, a methodology to evaluate the PM ISF energy using a superposition of the ISF energies obtained for ordered magnetic structures is proposed to overcome computational impediments arising when dealing with disorder in the PM state. The complications of the proposed methodology together with the ways to overcome them are also discussed. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.214115
  • Impact of magnetic fluctuations on lattice excitations in fcc nickel
    Körmann, F. and Ma, P.-W. and Dudarev, S.L. and Neugebauer, J.
    Journal of Physics Condensed Matter 28 (2016)
    The spin-space averaging formalism is applied to compute atomic forces and phonon spectra for magnetically excited states of fcc nickel. Transverse and longitudinal magnetic fluctuations are taken into account by a combination of magnetic special quasi random structures and constrained spin-density-functional theory. It turns out that for fcc Ni interatomic force constants and phonon spectra are almost unaffected by both kinds of spin fluctuations. Given the computational expense to simulate coupled magnetic and atomic fluctuations, this insight facilitates computational modeling of magnetic alloys such as Ni-based superalloys. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/28/7/076002
  • 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 (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 abstract10.1016/j.cossms.2015.06.001
  • Interplay between interstitial displacement and displacive lattice transformations
    Zhang, X. and Hickel, T. and Rogal, J. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    Diffusionless displacive lattice rearrangements, which include martensitic transformations, are in real materials often accompanied by a displacive drag of interstitials. The interplay of both processes leads to a particular atomistic arrangement of the interstitials in the product phase, which is decisive for its performance. An archetype example is the martensitic transformation in Fe-C alloys. One of the puzzles for this system is that the deviation from the cubic symmetry (i.e., the tetragonality) in the martensite resulting from this interplay is lower than what thermodynamics dictates. In our ab initio approach, the relative motion of C in the transforming lattice is studied with the nudged elastic band method. We prove that an atomic shearlike shuffle mechanism of adjacent (112) Fe layers along the ±[111]bcc directions is essential to achieve a redistribution of C atoms during the fcc → bcc transition, which fully explains the abnormal behavior. Furthermore, the good agreement with experiment validates our method to treat a diffusionless redistribution of interstitials and a displacive rearrangement of the host lattice simultaneously. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.104109
  • Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires
    Nematollahi, Gh.A. and Grabowski, B. and Raabe, D. and Neugebauer, J.
    Acta Materialia 111 (2016)
    A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.03.052
  • Multiscale modeling of hydrogen enhanced homogeneous dislocation nucleation
    Leyson, G.P.M. and Grabowski, B. and Neugebauer, J.
    Acta Materialia 107 (2016)
    A multiscale approach is proposed to predict how the presence of hydrogen influences the onset of homogeneous dislocation nucleation (HDN) and thus of plasticity. The model takes inputs that can be solely obtained from atomistic calculations, such as dislocation core structure, stacking fault energy and hydrogen-hydrogen interaction. The equilibrium hydrogen concentration around the dislocation loop is calculated using a recently developed self-consistent iterative method [1]. The complex nature of the dislocation field, as well as the equilibrium hydrogen concentration around the loops, is taken into account. The onset of HDN as a function of bulk hydrogen concentration and temperature is quantitatively predicted and is consistent with nano-indentation experiments on hydrogen loaded samples. Applying the approach to Ni, we find that even low hydrogen concentrations of about 1 at-% result in largely reduced HDN energy barriers and thus largely reduce the critical shear stress. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.01.036
  • Nonlinear elastic effects in phase field crystal and amplitude equations: Comparison to ab initio simulations of bcc metals and graphene
    Hüter, C. and Friák, M. and Weikamp, M. and Neugebauer, J. and Goldenfeld, N. and Svendsen, B. and Spatschek, R.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We investigate nonlinear elastic deformations in the phase field crystal model and derived amplitude equation formulations. Two sources of nonlinearity are found, one of them is based on geometric nonlinearity expressed through a finite strain tensor. This strain tensor is based on the inverse right Cauchy-Green deformation tensor and correctly describes the strain dependence of the stiffness for anisotropic and isotropic behavior. In isotropic one- and two-dimensional situations, the elastic energy can be expressed equivalently through the left deformation tensor. The predicted isotropic low-temperature nonlinear elastic effects are directly related to the Birch-Murnaghan equation of state with bulk modulus derivative K′=4 for bcc. A two-dimensional generalization suggests K2D′=5. These predictions are in agreement with ab initio results for large strain bulk deformations of various bcc elements and graphene. Physical nonlinearity arises if the strain dependence of the density wave amplitudes is taken into account and leads to elastic weakening. For anisotropic deformation, the magnitudes of the amplitudes depend on their relative orientation to the applied strain. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.214105
  • 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 (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 abstract10.1016/j.actamat.2015.09.010
  • 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 (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 abstract10.1007/s11669-015-0426-y
  • Scale bridging description of coherent phase equilibria in the presence of surfaces and interfaces
    Spatschek, R. and Gobbi, G. and Hüter, C. and Chakrabarty, A. and Aydin, U. and Brinckmann, S. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We investigate phase separation including elastic coherency effects in the bulk and at surfaces and find a reduction of the solubility limit in the presence of free surfaces. This mechanism favors phase separation near free surfaces even in the absence of external stresses. We apply the theory to hydride formation in nickel, iron, and niobium and obtain a reduction of the solubility limit by up to two orders of magnitude at room temperature in the presence of free surfaces. We develop in particular a scale bridging description of the solubility limit in the low-temperature regime, where the long-ranged elastic effects are expressed through a geometrical solubility modification factor, which expresses the difference to bulk systems. This expression allows to include elastic coherency effects near surfaces, e.g., in ab initio simulations. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.134106
  • Strong impact of lattice vibrations on electronic and magnetic properties of paramagnetic Fe revealed by disordered local moments molecular dynamics
    Alling, B. and Körmann, F. and Grabowski, B. and Glensk, A. and Abrikosov, I.A. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We study the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron at finite temperature, employing the disordered local moments molecular dynamics (DLM-MD) method. Vibrations strongly affect the distribution of local magnetic moments at finite temperature, which in turn correlates with the local atomic volumes. Without the explicit consideration of atomic vibrations, the mean local magnetic moment and mean field derived magnetic entropy of paramagnetic bcc Fe are larger compared to paramagnetic fcc Fe, which would indicate that the magnetic contribution stabilizes the bcc phase at high temperatures. In the present study we show that this assumption is not valid when the coupling between vibrations and magnetism is taken into account. At the γ-δ transition temperature (1662 K), the lattice distortions cause very similar magnetic moments of both bcc and fcc structures and hence magnetic entropy contributions. This finding can be traced back to the electronic densities of states, which also become increasingly similar between bcc and fcc Fe with increasing temperature. Given the sensitive interplay of the different physical excitation mechanisms, our results illustrate the need for an explicit consideration of vibrational disorder and its impact on electronic and magnetic properties to understand paramagnetic Fe. Furthermore, they suggest that at the γ-δ transition temperature electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.224411
  • The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/28/38/385001
  • The structure and dynamics of chitin nanofibrils in an aqueous environment revealed by molecular dynamics simulations
    Strelcova, Z. and Kulhanek, P. and Friak, M. and Fabritius, H. O. and Petrov, M. and Neugebauer, J. and Koca, J.
    Rsc Advances 6 (2016)
    Chitin is one of the most abundant structural biomolecules in nature, where it occurs in the form of nanofibrils that are the smallest building blocks for many biological structural materials, such as the exoskeleton of Arthropoda. Despite this fact, little is known about the structural properties of these nanofibrils. Here, we present a theoretical study of a single chitin molecule and 10 alpha-chitin nanofibrils with different numbers of chains in an aqueous environment that mimics the conditions in natural systems during self-assembly. Our extensive analysis of the molecular dynamics trajectories, including free energy calculations for every model system, reveals not only the structural properties of the nanofibrils, but also provides insight into the principles of nanofibril formation. We identified the fundamental phenomena occurring in the chitin nanofibrils such as their hydrogen bonding pattern and resulting helical shape. With increasing size, the nanofibrils become increasingly stable and their structural properties approach those of crystalline alpha-chitin if they consist of more than 20 chains. Interestingly, this coincides with the typical size of chitin nanofibrils observed in natural systems, suggesting that their evolutionary success was at least partially driven by these specific structure-property relations.
    view abstract10.1039/c6ra00107f
  • A first principles investigation of zinc induced embrittlement at grain boundaries in bcc iron
    Bauer, K.-D. and Todorova, M. and Hingerl, K. and Neugebauer, J.
    Acta Materialia 90 (2015)
    Using density-functional theory calculations we study the embrittling behaviour of liquid Zn in the Σ3[111]60° and the Σ5[100]36.8° symmetric tilt grain boundaries in bcc iron (ferrite). Investigating Zn induced changes in energetics of the grain boundaries and their associated free surfaces we utilise both a canonical and a grand-canonical thermodynamic model to discuss the embrittling behaviour of liquid zinc. Our results show that Zn wetting significantly destabilises the grain boundaries, but the conclusion which GB is more susceptible to embrittlement depends on the mechanism, as well as on the concentration. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.02.018
  • 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 (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 abstract10.1016/j.actamat.2015.07.054
  • Ab initio thermodynamics of the CoCrFeMnNi high entropy alloy: Importance of entropy contributions beyond the configurational one
    Ma, D. and Grabowski, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 100 (2015)
    We investigate the thermodynamic properties of the prototype equi-atomic high entropy alloy (HEA) CoCrFeMnNi by using finite-temperature ab initio methods. All relevant free energy contributions are considered for the hcp, fcc, and bcc structures, including electronic, vibrational, and magnetic excitations. We predict the paramagnetic fcc phase to be most stable above room temperature in agreement with experiment. The corresponding thermal expansion and bulk modulus agree likewise well with experimental measurements. A careful analysis of the underlying entropy contributions allows us to identify that the originally postulated dominance of the configurational entropy is questionable. We show that vibrational, electronic, and magnetic entropy contributions must be considered on an equal footing to reliably predict phase stabilities in HEA systems. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.08.050
  • Ab initio-based bulk and surface thermodynamics of InGaN alloys: Investigating the effects of strain and surface polarity
    Duff, A.I. and Lymperakis, L. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 252 (2015)
    The growth of high In content InGaN with sufficiently high crystal quality is challenging due to the differences in the GaN and InN thermodynamics. The surprisingly different thermodynamics is due to a complex competition between strain and chemistry and mediated by the different indium and gallium atomic radii as well as their different bonding enthalpies with nitrogen. In the present work, we investigate bulk and surface thermodynamics of molecular beam epitaxial (MBE) growth of In<inf>x</inf>Ga<inf>1-x</inf>N for the technologically relevant (0001) and (0001-) growth planes by means of density functional theory calculations. Our calculations confirm that coherent growth fully suppresses phase separation through spinodal decomposition. However, the biaxial strain is found to have a marginal effect on the critical temperatures for In<inf>x</inf>Ga<inf>1-x</inf>N decomposition. Furthermore, the thermal stability of excess indium is found to be remarkably higher on N-polar surfaces than on the Ga-polar surfaces. Phase diagram of In covered (a) Ga-polar and (b) N-polar surfaces as function of In pressure. InGaN alloys are key materials for solid state lighting applications in the green region of the optical spectrum. This Feature Article provides an overview of recent advances in ab initio-based InGaN bulk and surface thermodynamics calculations and elucidates the effects of strain and surface polarity on indium incorporation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201451687
  • Comparison of design and torque measurements of various manual wrenches
    Neugebauer, J. and Petermöller, S. and Scheer, M. and Happe, A. and Faber, F.-J. and Zoeller, J.E.
    International Journal of Oral and Maxillofacial Implants 30 (2015)
    Purpose: Accurate torque application and determination of the applied torque during surgical and prosthetic treatment is important to reduce complications. A study was performed to determine and compare the accuracy of manual wrenches, which are available in different designs with a large range of preset torques. Materials and Methods: Thirteen different wrench systems with a variety of preset torques ranging from 10 to 75 Ncm were evaluated. Three different designs were available, with a spring-in-coil or toggle design as an active mechanism or a beam as a passive mechanism, to select the preset torque. To provide a clinically relevant analysis, a total of 1,170 torque measurements in the range of 10 to 45 Ncm were made in vitro using an electronic torque measurement device. Results: The absolute deviations in Ncm and percent deviations across all wrenches were small, with a mean of -0.24 ± 2.15 Ncm and -0.84% ± 11.72% as a shortfall relative to the preset value. The greatest overage was 8.2 Ncm (82.5%), and the greatest shortfall was 8.47 Ncm (46%). However, extreme values were rare, with 95th-percentile values of -1.5% (lower value) and -0.16% (upper value). A comparison with respect to wrench design revealed significantly higher deviations for coil and toggle-style wrenches than for beam wrenches. Conclusion: Beam wrenches were associated with a lower risk of rare extreme values thanks to their passive mechanism of achieving the selected preset torque, which minimizes the risk of harming screw connections. © 2016 by Quintessence Publishing Co Inc.
    view abstract10.11607/jomi.3733
  • 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 (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 abstract10.1016/j.actamat.2014.10.044
  • Connecting semiconductor defect chemistry with electrochemistry: Impact of the electrolyte on the formation and concentration of point defects in ZnO
    Todorova, M. and Neugebauer, J.
    Surface Science 631 (2015)
    We study the effect an aqueous electrolyte can have on the concentration and electronic character of native point defects in a semiconducting electrode by utilising a recently derived grand canonical approach. Constructing defect phase diagrams which show the majority defect species as function of applied bias and chemical potential we identify which native point defects of ZnO become important when this semiconductor comes into contact with an aqueous electrolyte at varying pH and bias. Our results show that in thermodynamic equilibrium Zn rather than O vacancies are stable under electrochemical conditions. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2014.07.023
  • Development and application of a Ni-Ti interatomic potential with high predictive accuracy of the martensitic phase transition
    Ko, W.-S. and Grabowski, B. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Phase transitions in nickel-titanium shape-memory alloys are investigated by means of atomistic simulations. A second nearest-neighbor modified embedded-atom method interatomic potential for the binary nickel-titanium system is determined by improving the unary descriptions of pure nickel and pure titanium, especially regarding the physical properties at finite temperatures. The resulting potential reproduces accurately the hexagonal-close-packed to body-centered-cubic phase transition in Ti and the martensitic B2-B19′ transformation in equiatomic NiTi. Subsequent large-scale molecular-dynamics simulations validate that the developed potential can be successfully applied for studies on temperature- and stress-induced martensitic phase transitions related to core applications of shape-memory alloys. A simulation of the temperature-induced phase transition provides insights into the effect of sizes and constraints on the formation of nanotwinned martensite structures with multiple domains. A simulation of the stress-induced phase transition of a nanosized pillar indicates a full recovery of the initial structure after the loading and unloading processes, illustrating a superelastic behavior of the target system. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.134107
  • Difference in linear polarization of biaxially strained i nx G a1-x N alloys on nonpolar a -plane and m -plane GaN
    Zhang, S. and Cui, Y. and Griffiths, J.T. and Fu, W.Y. and Freysoldt, C. and Neugebauer, J. and Humphreys, C.J. and Oliver, R.A.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    InxGa1-xN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m-plane than a-plane InxGa1-xN. Using density functional theory, we study the response of InxGa1-xN random alloys to finite biaxial strains on both nonpolar planes. The calculated m-plane InxGa1-xN valence band splitting is larger than that of the a plane, due to a greater degree of structural relaxation in a-plane InxGa1-xN. We provide a parametrization of the valence band splitting of InxGa1-xN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.245202
  • Effects of various chair-side surface treatment methods on dental restorative materials with respect to contact angles and surface roughness
    Sturz, C.R.C. and Faber, F.-J. and Scheer, M. and Rothamel, D. and Neugebauer, J.
    Dental Materials Journal 34 (2015)
    Available chair-side surface treatment methods may adversely affect prosthetic materials and promote plaque accumulation. This study investigated the effects of treatment procedures on three resin restorative materials, zirconium-dioxide and polyetheretherketone in terms of surface roughness and hydrophobicity. Treatments were grinding with silicon carbide paper or white Arkansas stone, blasting with prophylaxis powder and polishing with diamond paste. Surface roughness was assessed using confocal laser scanning. Hydrophobicity as measured by water contact angle was determined by computerized image analysis using the sessile drop technique. All of the specific surface treatments performed led to significant changes in contact angle values and surface roughness (Ra) values. Median contact angle values ranged from 51.6° to 114°. Ra values ranged from 0.008 μm to 2.917 μm. Air-polishing as well as other polishing procedures increased surface roughness values in all materials except zirconium dioxide. Polyetheretherketone displayed greatest change in contact angle values after air-polishing treatment. © 2015, Japanese Society for Dental Materials and Devices. All rights reserved.
    view abstract10.4012/dmj.2014-098
  • Errautm: From generalized stacking fault energies to dislocation properties: Five-energy-point approach and solid solution effects in magnesium (Physical Review B - Condensed Matter and Materials Physics (2015) 92 (064107))
    Pei, Z. and Ma, D. and Friák, M. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    view abstract10.1103/PhysRevB.92.179901
  • From generalized stacking fault energies to dislocation properties: Five-energy-point approach and solid solution effects in magnesium
    Pei, Z. and Ma, D. and Friák, M. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Using ab initio calculations and symmetrized plane waves, we analyze the basal-plane generalized stacking fault energies in pure Mg and Mg-Y alloys and show that the knowledge of energies of only five specific points is sufficient to accurately predict the core structures and Peierls stresses of (a)-type edge dislocations in these alloys. Our five-point approach substantially reduces the computational cost related to the Peierls-Nabarro (PN) model and allows for a high-throughput application of the PN model to study Peierls stress changes in Mg upon alloying. We employ our approach to study Mg binary alloys containing nine rare-earth (RE) and 11 other solutes. Based on the Peierls stresses of these 20 Mg alloys calculated from the Peierls-Nabarro model, the solutes are divided into three groups: (i) the first group, consisting of Be, Zn, Tl, Tc, Os, Ru, Re, and Co, when added as solutes into Mg, lead to more compact dislocation core structures and larger Peierls stresses than found for pure Mg. (ii) Elements in the second group, including Ti, Nd, Lu, Zr, Hf, La, and Pr change the core widths and Peierls stresses moderately. (iii) The solutes in the third group containing Y, Er, Tm, Ho, and Sc extend the stacking fault width, and the resulting Peierls stresses are generally very low. Based on an error analysis, we conclude that the first group has a clear solute strengthening effect and the third group has a clear solute softening effect, while the effects of the elements in the second group are too small to be resolved by the present approach. © 2015 American Physical Society. ©2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.064107
  • From wetting to melting along grain boundaries using phase field and sharp interface methods
    Sai Pavan Kumar Bhogireddy, V. and Hüter, C. and Neugebauer, J. and Shchyglo, O. and Steinbach, I. and Spatschek, R.
    Computational Materials Science 108 (2015)
    Abstract We investigate the ability of a multi-order parameter phase field model with obstacle potentials to describe grain boundary premelting in equilibrium situations. In agreement with an energetic picture we find that the transition between dry and wet grain boundaries at the bulk melting point is given by the threshold 2σsl=σgb, with σsl being the solid-melt interfacial energy and σgb the energy of a dry grain boundary. The predictions for premelting are confirmed by simulations using the phase field package OpenPhase. For the prediction of the kinetics of melting along grain boundaries in pure materials, taking into account the short ranged interactions which are responsible for the grain boundary premelting, a sharp interface theory is developed. It confirms that for overheated grain boundaries the melting velocity is reduced (increased) for non-wetting (wetting) grain boundaries. Numerical steady state predictions are in agreement with a fully analytical solution in a subset of the parameter space. Phase field simulations confirm the predictions of the sharp interface theory. © 2015 Elsevier B.V.
    view abstract10.1016/j.commatsci.2015.02.040
  • Identification of bulk oxide defects in an electrochemical environment
    Todorova, M. and Neugebauer, J.
    Faraday Discussions 180 (2015)
    We discuss how electronic-structure calculations can be used to identify the dominant point defects that control the growth and dissolution of the oxide barrier layer formed if a metal comes into contact with a corrosive environment. Using the example of the Zn/ZnO/H<inf>2</inf>O interface we develop and apply a theoretical approach that is firmly based on ab initio computed defect formation energies and that unifies concepts of semiconductor defect chemistry with electrochemical concepts. Employing this approach we find that the commonly invoked and chemically intuitive defects such as the doubly negatively charged oxygen vacancy in electrochemically formed ZnO films may not be present. Rather, hitherto not discussed defects such as the oxygen interstitial or unexpected charge states, such as the neutral oxygen vacancy, are found. These new defect types will be shown to critically impact our understanding of fundamental corrosion mechanisms and to provide new insight into strategies to develop alloys with higher corrosion resistance. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4fd00238e
  • Importance of coordination number and bond length in titanium revealed by electronic structure investigations
    Huang, L.-F. and Grabowski, B. and Mceniry, E. and Trinkle, D.R. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 252 (2015)
    We study the influence of coordination number and bond length on the phase stability and orbital occupation in Ti using density functional theory. In particular, Ti under a wide range of conditions (equilibrium state, hydrostatic pressure, anisotropic strain, and phase transformations) is systematically investigated allowing us to derive generic energetic and electronic trends. Our analysis of the correlations between electronic structure and the atomic geometry reveals that the most suitable descriptors of the system are an effective coordination number and an effective bond length. Utilizing these descriptors, we show that (i) the phase stability of Ti increases with coordination number, because of the increased number of interatomic bonds; (ii) the occupation number of the d (s and p) orbital decreases (increases) with increasing the bond length, because of the localized (delocalized) character of the d (p and s) orbital. These dependencies are particularly evident after applying a simple harmonic strain correction to the energy and an electron-transfer correction within the ω phase. The physical picture derived from pure Ti is used to explain the phase stability and orbital occupation of Ti-Nb and Ti-Zr alloys, which reveals the underlying mechanisms for various experimental phenomena in Ti alloys. The lattice coordination number determines the bonding states and phase stability of Ti, which explains the energy-pressure and energy-strain relationships, energy variations in phase transformations, and phase stabilities of Ti-transition metal alloys. The bond length influences the occupation numbers of s, p, and d orbitals in pure Ti and Ti alloys by changing the effective pressures on their electron clouds. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201552280
  • Improved method of calculating ab initio high-temperature thermodynamic properties with application to ZrC
    Duff, A.I. and Davey, T. and Korbmacher, D. and Glensk, A. and Grabowski, B. and Neugebauer, J. and Finnis, M.W.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    Thermodynamic properties of ZrC are calculated up to the melting point (Tmelt≈3700K), using density functional theory (DFT) to obtain the fully anharmonic vibrational contribution, and including electronic excitations. A significant improvement is found in comparison to results calculated within the quasiharmonic approximation. The calculated thermal expansion is in better agreement with experiment and the heat capacity reproduces rather closely a CALPHAD estimate. The calculations are presented as an application of a development of the upsampled thermodynamic integration using Langevin dynamics (UP-TILD) approach. This development, referred to here as two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD), is the inclusion of tailored interatomic potentials to characterize an intermediate reference state of anharmonic vibrations on a two-stage path of thermodynamic integration between the original DFT quasiharmonic free energy and the fully anharmonic DFT free energy. This approach greatly accelerates the convergence of the calculation, giving a factor of improvement in efficiency of ∼50 in the present case compared to the original UP-TILD approach, and it can be applied to a wide range of materials. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.214311
  • 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 (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 abstract10.1002/pssr.201510070
  • Mechanisms and kinetics of the migration of grain boundaries containing extended defects
    Race, C.P. and Hadian, R. and Von Pezold, J. and Grabowski, B. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    A full understanding of the basic processes of grain boundary migration is a fundamental prerequisite for predictive models of microstructural evolution in polycrystalline materials in processing and in service. In a detailed study of the kinetics of a [111] Σ7 symmetric tilt boundary, we have previously shown that defect-free, flat grain boundaries, below their roughening temperature, can be strictly immobile in the experimental limit. Here we present the results of molecular dynamics simulations of grain boundaries containing a variety of "defects." These simulations show that the presence of some of these defects restores the mobility of flat boundaries, even well below the roughening transition temperature. These defects fundamentally alter the mesoscale mechanism of grain boundary migration from one involving homogeneous nucleation to a heterogenous process. At the atomistic level, the crystal lattice reorients via coordinated shuffling of groups of atoms. In the case of flat boundaries, these shuffles must accumulate to form critically stable nuclei, but in the case of boundaries containing defects the shuffling of a small number of atoms at the defects can be sufficient, fundamentally altering the mechanism and kinetics of migration. ©2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.174115
  • Multiscale description of dislocation induced nano-hydrides
    Leyson, G.P.M. and Grabowski, B. and Neugebauer, J.
    Acta Materialia 89 (2015)
    The interaction of hydrogen with the core and the strain field of edge dislocations is studied using a multiscale approach. We have therefore developed a combined thermodynamic and analytical model with full atomistic resolution that allows to quantify the local hydrogen concentration around the dislocation core as a function of temperature and hydrogen chemical potential. This model takes, as input, information from atomistic calculations, such as hydrogen-hydrogen interaction and the dislocation core structure, and faithfully reproduces results from a computationally much more expensive fully atomistic approach that combines the Embedded Atom Method with Monte Carlo simulations. The onset of nano-hydride formation and with it the activation of hydrogen enhanced local plasticity (HELP) is predicted through a parametric study of the hydride size as a function of temperature and bulk hydrogen concentration. The study reveals a sharp transition between hydride forming and non-hydride forming regimes. The transition between these two regimes corresponds to a critical hydrogen chemical potential μHc related to the nano-hydride nucleus of the system. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.01.057
  • Random phase approximation up to the melting point: Impact of anharmonicity and nonlocal many-body effects on the thermodynamics of Au
    Grabowski, B. and Wippermann, S. and Glensk, A. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    Application of the generalized gradient corrected functional within standard density-functional theory results in a dramatic failure for Au, leading to divergent thermodynamic properties well below the melting point. By combining the upsampled thermodynamic integration using Langevin dynamics technique with the random phase approximation, we show that inclusion of nonlocal many-body effects leads to a stabilization and to an excellent agreement with experiment. © Published by the American Physical Society.
    view abstract10.1103/PhysRevB.91.201103
  • 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 abstract10.1088/1367-2630/17/9/093009
  • Role of biaxial strain and microscopic ordering for structural and electronic properties of InxGa1-x N
    Cui, Y. and Lee, S. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    The structural and electronic properties of InxGa1-xN alloys are studied as a function of c-plane biaxial strain and In ordering by density functional theory with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. A nonlinear variation of the c lattice parameter with In content is observed in biaxial strain and should be taken into account when deducing In content from interplanar distances. From compressive to tensile strain, the character of the top valence-band state changes, leading to a nonlinear variation of the band gap in InxGa1-xN. Interestingly, the well-known bowing of the InxGa1-xN band gap is largely removed for alloys grown strictly coherently on GaN, while the actual values for band gaps at x<0.33 are hardly affected by strain. Ordering plays a minor role for lattice constants but may induce changes of the band gap up to 0.15 eV. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.085204
  • Structural transformations among austenite, ferrite and cementite in Fe-C alloys: A unified theory based on ab initio simulations
    Zhang, X. and Hickel, T. and Rogal, J. and Fähler, S. and Drautz, R. and Neugebauer, J.
    Acta Materialia 99 (2015)
    Structural transformations in Fe-C alloys are decisive for the mechanical properties of steels, but their modeling remains a challenge due to the simultaneous changes in Fe lattice and redistribution of C. With a combination of the orientation relationships between austenite, ferrite and cementite, we identify a metastable intermediate structure (MIS), which can serve as a link between the three phases. Based on this framework, different mechanisms depending on the local conditions (C concentration, strain, magnetism) are revealed from ab initio nudged elastic band simulations, which allow us to construct a unified theory for the structural transformations among austenite, ferrite and cementite. © 2015 Acta Materialia Inc. All rights reserved.
    view abstract10.1016/j.actamat.2015.07.075
  • 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 abstract10.1088/1367-2630/17/9/093004
  • The influence of low level pre- and perinatal exposure to PCDD/Fs, PCBs, and lead on attention performance and attention-related behavior among German school-aged children: Results from the Duisburg Birth Cohort Study
    Neugebauer, J. and Wittsiepe, J. and Kasper-Sonnenberg, M. and Schoneck, N. and Scholmerich, A. and Wilhelm, M.
    International Journal of Hygiene and Environmental Health 218 (2015)
    Background: Prenatal exposure to polychlorinated biphenyls (PCBs) and lead are thought to be risk factors for attention-deficit hyperactivity disorder (ADHD), whereas the prenatal influence of polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) on attention performance has been less studied. Objectives: Within the Duisburg Birth Cohort Study, we investigated low-level exposure to these compounds in relation to children's attention. Methods: We measured blood levels of PCDD/Fs, PCBs and lead from pregnant mothers (32nd week of pregnancy) and PCDD/Fs and PCBs in breast milk (2 weeks after delivery). The attention of school-aged children (N= 117) was investigated with a computer-based test battery of attention performance (KITAP) and a parent rating questionnaire of behaviors related to ADHD (FBB-ADHS). Influences of the exposure on attention were analyzed by multiple regression analyses. Results: Increasing prenatal PCDD/F and PCB concentrations were significantly (p< 0.05) associated with a higher number of omission errors in the subtest Divided Attention (47% and 42%; 95% confidence intervals (95%-CI): 1.08-2.00 and 1.07-1.89, respectively). Prenatal lead concentrations had few significant associations with attention performance (e.g., a 23% higher number of omission errors in the subtest Distractibility; 95%-CI: 1.00-1.51), whereas ADHD-related behavior (questionnaire based) was increased with increasing lead exposure (Overall-ADHD: 9%; 95%-CI: 1.01-1.17). ADHD-related behavior was negatively associated with prenatal PCDD/F or PCB exposures (e.g., for PCB exposure: -10%; 95%-CI: 0.82-0.99). Conclusions: Pre- and perinatal PCDD/F and PCB exposure may have subtle influences on attention performance in healthy children at low environmental levels, while behavior changes are negatively associated. Furthermore, we provide additional evidence for the impact of prenatal lead exposure on attention deficits. (C) 2014 Elsevier GmbH. All rights reserved.
    view abstract10.1016/j.ijheh.2014.09.005
  • 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 abstract10.1103/PhysRevLett.114.195901
  • A generalized plane-wave formulation of k · p formalism and continuum-elasticity approach to elastic and electronic properties of semiconductor nanostructures
    Marquardt, O. and Boeck, S. and Freysoldt, C. and Hickel, T. and Schulz, S. and Neugebauer, J. and O'Reilly, E.P.
    Computational Materials Science 95 (2014)
    We present a generalized and flexible plane-wave based implementation of the multiband k·p formalism to study the electronic properties of semiconductor nanostructures. All ingredients of the modeling process, namely the Hamiltonian, the nanostructure's geometry and the required material parameters, are defined in human-readable input files that can be easily generated and modified. The generalized k·p model can contain an arbitrary number of directly treated bands as well as strain, piezoelectric, and external potentials. All calculations can be performed for arbitrary crystal structures. The nanostructure is described in terms of a real-space composition map that may contain an arbitrary number of base compounds and alloys. We demonstrate the applicability and flexibility of our implementation for the example of (111)-oriented, site-controlled InGaAs quantum dots, where a rotated eight-band k·p Hamiltonian is employed. As a second example, a 14-band k·p model that captures the bulk inversion asymmetry of the zinc-blende lattice is applied for the case of a pyramidal (0 0 1)-oriented InAs/GaAs quantum dot. Here we show that the explicit treatment of 14 bands removes the well known shortcoming of eight-band k·p models for (0 0 1)-oriented zinc-blende quantum dots which leads to artificially degenerate p-like electron states. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2014.06.047
  • 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 (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 abstract10.1016/j.intermet.2013.09.008
  • Ab initio based understanding of the segregation and diffusion mechanisms of hydrogen in steels
    Hickel, T. and Nazarov, R. and McEniry, E.J. and Leyson, G. and Grabowski, B. and Neugebauer, J.
    JOM 66 (2014)
    A microscopic understanding of the processes that lead to hydrogen embrittlement is of critical importance for developing new generations of high-strength steels. With this article, we provide an overview of insights that can be gained from ab initio based methods when investigating the segregation and diffusion mechanisms of hydrogen in steels. We first discuss the solubility and diffusion behavior of hydrogen in the ferrite, austenite, and martensite phases. We consider not only defect-free bulk phases but also the influence of alloying elements and geometric defects such as vacancies and grain boundaries. In the second part, the behavior of hydrogen in the presence of precipitates, the solubility, the surface absorption, and the influence of hydrogen on the interface cohesion are studied. Finally, we provide simulation results for the interaction of hydrogen with dislocations. For all these applications, we will comment on advantages and shortcomings of ab initio methods and will demonstrate how the obtained data and insights can complement experimental approaches to extract general trends and to identify causes of hydrogen embrittlement. © 2014 The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-014-1055-3
  • Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
    Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 35 (2014)
    The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.
    view abstract10.1007/s11669-014-0342-6
  • Ab initio study of H-vacancy interactions in fcc metals: Implications for the formation of superabundant vacancies
    Nazarov, R. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Hydrogen solubility and interaction with vacancies and divacancies are investigated in 12 fcc metals by density functional theory. We show that in all studied fcc metals, vacancies trap H very efficiently and multiple H trapping is possible. H is stronger trapped by divacancies and even stronger by surfaces. We derive a condition for the maximum number of trapped H atoms as a function of the H chemical potential. Based on this criterion, the possibility of a dramatic increase of vacancy concentration (superabundant vacancy formation) in the studied metals is discussed. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.144108
  • Ab initio study of point defects in NiTi-based alloys
    Holec, D. and Friák, M. and Dlouhý, A. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Changes in temperature or stress state may induce reversible B2↔(R)↔B19′ martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite ↔B19′ martensite transformation path. In this paper, we employ density functional theory to study properties of Ar, He, and H interstitials in B2 austenite and B19′ martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich NiTi-based alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range 51-52at.% is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to ≈6at.%. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19′ phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.014110
  • Breakdown of the arrhenius law in describing vacancy formation energies: The importance of local anharmonicity revealed by Ab initio thermodynamics
    Glensk, A. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Physical Review X 4 (2014)
    We study the temperature dependence of the Gibbs energy of vacancy formation in Al and Cu from T = 0 K up to the melting temperature, fully taking into account anharmonic contributions. Our results show that the formation entropy of vacancies is not constant as often assumed but increases almost linearly with temperature. The resulting highly nonlinear temperature dependence in the Gibbs formation energy naturally explains the differences between positron annihilation spectroscopy and differential dilatometry data and shows that nonlinear thermal corrections are crucial to extrapolate high-temperature experimental data to T = 0 K. Employing these corrections-rather than the linear Arrhenius extrapolation that is commonly assumed in analyzing experimental data-revised formation enthalpies are obtained that differ up to 20% from the previously accepted ones. Using the revised experimental formation enthalpies, we show that a large part of the discrepancies between DFT-GGA and unrevised experimental vacancy formation energies disappears. The substantial shift between previously accepted and the newly revised T = 0 K formation enthalpies also has severe consequences in benchmarking ab initio methods against experiments, e.g., in deriving corrections that go beyond commonly used LDA and GGA exchangecorrelation functionals such as the AM05 functional.
    view abstract10.1103/PhysRevX.4.011018
  • Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.05.016
  • 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 (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 abstract10.1016/j.actamat.2014.02.011
  • Extending the concept of defect chemistry from semiconductor physics to electrochemistry
    Todorova, M. and Neugebauer, J.
    Physical Review Applied 1 (2014)
    We present a method by which first-principles calculations are linked quite naturally to experimental observables, which determine and characterize the state of an electrochemical system. The method is based on the formation energies of ions in solution and utilizes a grand canonical framework. This is particularly well suited to use in conjunction with ab initio calculations, since it allows charged defects to be treated individually, rather than requiring the consideration of full charge-neutral reactions. It allows the identification of a region of water stability, the pH scale, and the electrode potential. We discuss and demonstrate the possibilities and capabilities of the approach by the extraction of ion-hydration energies from the Nernst equation and the construction of a potential/pH diagram for zinc by an alternative route deviating from the standard approach used to obtain Pourbaix diagrams. © 2014 American Physical Society.
    view abstract10.1103/PhysRevApplied.1.014001
  • 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 (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 abstract10.1103/RevModPhys.86.253
  • Genome-wide association study identifies 25 known breast cancer susceptibility loci as risk factors for triple-negative breast cancer
    Purrington, K. S. and Slager, S. and Eccles, D. and Yannoukakos, D. and Fasching, P. A. and Miron, P. and Carpenter, J. and Chang-Claude, J. and Martin, N. G. and Montgomery, G. W. and Kristensen, V. and Anton-Culver, H. and Goodfellow, P. and Tapper, W. J. and Rafiq, S. and Gerty, S. M. and Durcan, L. and Konstantopoulou, I. and Fostira, F. and Vratimos, A. and Apostolou, P. and Konstanta, I. and Kotoula, V. and Lakis, S. and Dimopoulos, M. A. and Skarlos, D. and Pectasides, D. and Fountzilas, G. and Beckmann, M. W. and Hein, A. and Ruebner, M. and Ekici, A. B. and Hartmann, A. and Schulz-Wendtland, R. and Renner, S. P. and Janni, W. and Rack, B. and Scholz, C. and Neugebauer, J. and Andergassen, U. and Lux, M. P. and Haeberle, L. and Clarke, C. and Pathmanathan, N. and Rudolph, A. and Flesch-Janys, D. and Nickels, S. and Olson, J. E. and Ingle, J. N. and Olswold, C. and Slettedahl, S. and Eckel-Passow, J. E. and Anderson, S. K. and Visscher, D. W. and Cafourek, V. L. and Sicotte, H. and Prodduturi, N. and Weiderpass, E. and Bernstein, L. and Ziogas, A. and Ivanovich, J. and Giles, G. G. and Baglietto, L. and Southey, M. and Kosma, V. M. and Fischer, H. P. and Reed, M. W. R. and Cross, S. S. and Deming-Halverson, S. and Shrubsole, M. and Cai, Q. Y. and Shu, X. O. and Daly, M. and Weaver, J. and Ross, E. and Klemp, J. and Sharma, P. and Torres, D. and Rudiger, T. and Wolfing, H. and Ulmer, H. U. and Forsti, A. and Khoury, T. and Kumar, S. and Pilarski, R. and Shapiro, C. L. and Greco, D. and Heikkila, P. and Aittomaki, K. and Blomqvist, C. and Irwanto, A. and Liu, J. J. and Pankratz, V. S. and Wang, X. S. and Severi, G. and Mannermaa, A. and Easton, D. and Hall, P. and Brauch, H. and Cox, A. and Zheng, W. and Godwin, A. K. and Hamann, U. and Ambrosone, C. and Toland, A. E. and Nevanlinna, H. and Vachon, C. M. and Fergus, J.
    Carcinogenesis 35 (2014)
    In a genome-wide scan, we show that 30 variants in 25 genomic regions are associated with risk of TN breast cancer. Women carrying many of the risk variants may have 4-fold increased risk relative to women with few variants.Triple-negative (TN) breast cancer is an aggressive subtype of breast cancer associated with a unique set of epidemiologic and genetic risk factors. We conducted a two-stage genome-wide association study of TN breast cancer (stage 1: 1529 TN cases, 3399 controls; stage 2: 2148 cases, 1309 controls) to identify loci that influence TN breast cancer risk. Variants in the 19p13.1 and PTHLH loci showed genome-wide significant associations (P < 5 x 10(-) (8)) in stage 1 and 2 combined. Results also suggested a substantial enrichment of significantly associated variants among the single nucleotide polymorphisms (SNPs) analyzed in stage 2. Variants from 25 of 74 known breast cancer susceptibility loci were also associated with risk of TN breast cancer (P < 0.05). Associations with TN breast cancer were confirmed for 10 loci (LGR6, MDM4, CASP8, 2q35, 2p24.1, TERT-rs10069690, ESR1, TOX3, 19p13.1, RALY), and we identified associations with TN breast cancer for 15 additional breast cancer loci (P < 0.05: PEX14, 2q24.1, 2q31.1, ADAM29, EBF1, TCF7L2, 11q13.1, 11q24.3, 12p13.1, PTHLH, NTN4, 12q24, BRCA2, RAD51L1-rs2588809, MKL1). Further, two SNPs independent of previously reported signals in ESR1 [rs12525163 odds ratio (OR) = 1.15, P = 4.9 x 10(-) (4)] and 19p13.1 (rs1864112 OR = 0.84, P = 1.8 x 10(-) (9)) were associated with TN breast cancer. A polygenic risk score (PRS) for TN breast cancer based on known breast cancer risk variants showed a 4-fold difference in risk between the highest and lowest PRS quintiles (OR = 4.03, 95% confidence interval 3.46-4.70, P = 4.8 x 10(-) (69)). This translates to an absolute risk for TN breast cancer ranging from 0.8% to 3.4%, suggesting that genetic variation may be used for TN breast cancer risk prediction.
    view abstract10.1093/carcin/bgt404
  • 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 (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 abstract10.1002/jcc.23742
  • Impact of nanodiffusion on the stacking fault energy in high-strength steels
    Hickel, T. and Sandlöbes, S. and Marceau, R.K.W. and Dick, A. and Bleskov, I. and Neugebauer, J. and Raabe, D.
    Acta Materialia 75 (2014)
    A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.04.062
  • Influence of short-range forces on melting along grain boundaries
    Hüter, C. and Twiste, F. and Brener, E.A. and Neugebauer, J. and Spatschek, R.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    We investigate a model which couples diffusional melting and nanoscale structural forces via a combined nano-mesoscale description. Specifically, we obtain analytic and numerical solutions for melting processes at grain boundaries influenced by structural disjoining forces in the experimentally relevant regime of small deviations from the melting temperature. Though spatially limited to the close vicinity of the tip of the propagating melt finger, the influence of the disjoining forces is remarkable and leads to a strong modification of the penetration velocity. The problem is represented in terms of a sharp interface model to capture the wide range of relevant length scales, predicting the growth velocity and the length scale describing the pattern, depending on temperature, grain boundary energy, strength, and length scale of the exponential decay of the disjoining potential. Close to equilibrium the short-range effects near the triple junctions can be expressed through a contact angle renormalization in a mesoscale formulation. For higher driving forces strong deviations are found, leading to a significantly higher melting velocity than predicted from a purely mesoscopic description. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.224104
  • Influence of the dislocation core on the glide of the 〈1 1 〉{1 1 0} edge dislocation in bcc-iron: An embedded atom method study
    Hafez Haghighat, S.M. and Von Pezold, J. and Race, C.P. and Körmann, F. and Friák, M. and Neugebauer, J. and Raabe, D.
    Computational Materials Science 87 (2014)
    Four commonly used embedded atom method potentials for bcc-Fe by Ackland et al. (1997), Mendelev et al. (2003), Chiesa et al. (2009) and Malerba et al. (2010) are critically evaluated with respect to their description of the edge dislocation core structure and its dynamic behavior. Our results allow us to quantify the transferability of the various empirical potentials in the study of the 〈1 1 〉{1 1 0} edge dislocation core structure and kinetics. Specifically, we show that the equilibrium dislocation core structure is a direct consequence of the shape of the extended gamma surface. We further find that there is a strong correlation between the structure of the edge dislocation core and its glide stress. An in depth analysis of the dislocation migration results reveals that the dominant migration mechanism is via progressing straight line segments of the dislocation. This is further confirmed by the excellent qualitative agreement of nudged elastic band calculations of the Peierls barrier with the dynamically determined critical shear stresses. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2014.02.031
  • Macroscopic elastic properties of textured ZrN-AlN polycrystalline aggregates: From ab initio calculations to grain-scale interactions
    Holec, D. and Tasnádi, F. and Wagner, P. and Friák, M. and Neugebauer, J. and Mayrhofer, P.H. and Keckes, J.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    Despite the fast development of computational material modeling, the theoretical description of macroscopic elastic properties of textured polycrystalline aggregates starting from basic principles remains a challenging task. In this study we use a supercell-based approach to obtain the elastic properties of a random solid solution cubic Zr1-xAlxN system as a function of the metallic sublattice composition and texture descriptors. The employed special quasirandom structures are optimized not only with respect to short-range-order parameters, but also to make the three cubic directions [100], [010], and [001] as similar as possible. In this way, only a small spread of elastic constant tensor components is achieved and an optimum trade-off between modeling of chemical disorder and computational limits regarding the supercell size and calculational time is proposed. The single-crystal elastic constants are shown to vary smoothly with composition, yielding x≈0.5 an alloy constitution with an almost isotropic response. Consequently, polycrystals with this composition are suggested to have Young's modulus independent of the actual microstructure. This is indeed confirmed by explicit calculations of polycrystal elastic properties, both within the isotropic aggregate limit and with fiber textures with various orientations and sharpness. It turns out that for low AlN mole fractions, the spread of the possible Young's modulus data caused by the texture variation can be larger than 100 GPa. Consequently, our discussion of Young's modulus data of cubic Zr1-xAlxN contains also the evaluation of the texture typical for thin films. © 2014 Published by the American Physical Society.
    view abstract10.1103/PhysRevB.90.184106
  • Negatively charged ions on Mg(0001) surfaces: Appearance and origin of attractive adsorbate-adsorbate interactions
    Cheng, S.-T. and Todorova, M. and Freysoldt, C. and Neugebauer, J.
    Physical Review Letters 113 (2014)
    Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.136102
  • Ordering phenomena and formation of nanostructures in Inx Ga1-x N layers coherently grown on GaN(0001)
    Lee, S. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    We study the impact that local strain effects have on the spatial distribution of In in coherent InxGa1-xN grown epitaxially on GaN(0001) using an effective crystal growth modeling technique that combines a semi-grand-canonical Monte Carlo simulation with an ab initio parametrized empirical force field. Our calculations show that InxGa1-xN epitaxial layers exhibit a strong tendency towards ordering, as highlighted by the formation of a vertical stack of the 3×3 patterned layers along the ©c direction. The ordering phenomena are identified as a key factor that determines lateral phase separation in InxGa1-xN epitaxial layers at the nanometer scale. Consequences of this nanophase separation for the enhanced radiative emission through carrier localization in InxGa1-xN of x<1/3 are discussed. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.245301
  • Origin of the unusually strong luminescence of a -type screw dislocations in GaN
    Albrecht, M. and Lymperakis, L. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    Based on luminescence studies and density functional theory calculations we identify the origin of the unusually strong luminescence of a-type screw dislocations in GaN. In contrast to previous models where only a localization of the holes was considered, density functional theory calculations show a localization of both electrons and holes in the dislocation strain field. This strain field causes a mixing of the s-type state at the conduction band minimum with the next highest state that has p character and is thus susceptible to the shear strain induced by the dislocation. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.241201
  • Perspectives on point defect thermodynamics
    Rogal, J. and Divinski, S.V. and Finnis, M.W. and Glensk, A. and Neugebauer, J. and Perepezko, J.H. and Schuwalow, S. and Sluiter, M.H.F. and Sundman, B.
    Physica Status Solidi (B) Basic Research 251 (2014)
    We review and discuss methods for including the role of point defects in calculations of the free energy, composition and phase stability of elements and compounds. Our principle aim is to explain and to reconcile, with examples, the perspectives on this problem that are often strikingly different between exponents of CALPHAD, and others working in the overlapping fields of physics, chemistry and materials science. Current methodologies described here include the compound energy formalism of CALPHAD, besides the rather different but related canonical and grand-canonical formalisms. We show how the calculation of appropriate defect formation energies should be formulated, how they are included in the different formalisms and in turn how these yield equilibrium defect concentrations and their contribution to free energies and chemical potentials. Furthermore, we briefly review the current state-of-the-art and challenges in determining point defect properties from first-principles calculations as well as from experimental measurements. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201350155
  • Phase-field modeling of grain-boundary premelting using obstacle potentials
    Bhogireddy, V.S.P.K. and Hüter, C. and Neugebauer, J. and Steinbach, I. and Karma, A. and Spatschek, R.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    We investigate the multiorder parameter phase field model of Steinbach and Pezzolla [Physica D 134, 385 (1999)PDNPDT0167-278910.1016/S0167-2789(99)00129-3] concerning its ability to describe grain boundary premelting. For a single order parameter situation solid-melt interfaces are always attractive, which allows us to have (unstable) equilibrium solid-melt-solid coexistence above the bulk melting point. The temperature-dependent melt layer thickness and the disjoining potential, which describe the interface interaction, are affected by the choice of the thermal coupling function and the measure to define the amount of the liquid phase. Due to the strictly finite interface thickness the interaction range also is finite. For a multiorder parameter model we find either purely attractive or purely repulsive finite-ranged interactions. The premelting transition is then directly linked to the ratio of the grain boundary and solid-melt interfacial energy. © 2014 American Physical Society.
    view abstract10.1103/PhysRevE.90.012401
  • 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 (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 abstract10.4028/www.scientific.net/KEM.592-593.335
  • 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 abstract10.1088/0953-8984/26/33/335401
  • Role of the mesoscale in migration kinetics of flat grain boundaries
    Race, C.P. and Von Pezold, J. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Classical molecular dynamics simulations of bicrystalline systems are a commonly used tool for exploring the migration of grain boundaries. Most simulation work to date has focused on measuring the mobility of grain boundaries, assuming it to be an intrinsic property of a boundary of a given geometry. Here we present results from simulations of the migration of a typical high-angle grain boundary that show that the concept of intrinsic mobility fails for defect-free, flat boundaries of the type frequently simulated and that key assumptions often made in analyzing the kinetics of migration do not hold. Our dynamical simulations of grain boundary migration show that the grain boundary velocity is not simply proportional to the driving force for grain boundary motion, as commonly assumed, and shows a strong and complex dependence on the system size. By analyzing the migration mechanism at the larger mesoscale we show that defect-free, flat boundaries must migrate via the homogeneous nucleation and growth of islands of transformed crystal volume on the grain boundary surface. We present a detailed analysis of the kinetics of this process, which only emerges in simulations of large grain boundary areas. An island-based mesoscale mechanism implies an energy barrier for migration that is inversely proportional to the driving force for migration - in the experimental (zero-force) limit such boundaries must be immobile. This calls into question the concept of an intrinsic mobility for defect-free, flat grain boundaries and suggests that mobility of real boundaries at low temperatures is rather a function of their morphology and defect content and at high temperatures is a result of thermal roughening. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.214110
  • Scale bridging between atomistic and mesoscale modelling: Applications of amplitude equation descriptions
    Hüter, C. and Nguyen, C.-D. and Spatschek, R. and Neugebauer, J.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Amplitude equations are discussed as an extension of phase field models, which contain atomic resolution and allow one to describe polycrystalline structures, lattice deformations and defects. The interaction of adjacent grains, which are separated by a thin melt layer, leads to structural interactions if the grains are slightly misplaced, similar to the concept of γ-surfaces. We are able to predict these interactions essentially analytically, leading to a superposition of short-ranged interaction terms related to the individual density waves. Deviations from the analytical predictions are found only at short distances between the grains and are most pronounced in situations with different ranges of the contributions. Furthermore, we demonstrate the ability of the amplitude equation model to predict dislocation pairing transitions at high temperatures, which supports earlier findings using molecular dynamics and phase field crystal simulations. To effectively perform the numerical simulations, we present a way to implement the model on graphics cards. An enormous acceleration of the code in comparison to a single CPU code by up to two orders of magnitude is reached. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0965-0393/22/3/034001
  • Separating strain from composition in unit cell parameter maps obtained from aberration corrected high resolution transmission electron microscopy imaging
    Schulz, T. and Duff, A. and Remmele, T. and Korytov, M. and Markurt, T. and Albrecht, M. and Lymperakis, L. and Neugebauer, J. and Cheze, C. and Skierbiszewski, C.
    Journal of Applied Physics 115 (2014)
    Based on the evaluation of lattice parameter maps in aberration corrected high resolution transmission electron microscopy images, we propose a simple method that allows quantifying the composition and disorder of a semiconductor alloy at the unit cell scale with high accuracy. This is realized by considering, next to the out-of-plane, also the in-plane lattice parameter component allowing to separate the chemical composition from the strain field. Considering only the out-of-plane lattice parameter component not only yields large deviations from the true local alloy content but also carries the risk of identifying false ordering phenomena like formations of chains or platelets. Our method is demonstrated on image simulations of relaxed supercells, as well as on experimental images of an In0.20Ga0.80N quantum well. Principally, our approach is applicable to all epitaxially strained compounds in the form of quantum wells, free standing islands, quantum dots, or wires. (C) 2014 AIP Publishing LLC.
    view abstract10.1063/1.4862736
  • Structural stability and thermodynamics of CrN magnetic phases from ab initio calculations and experiment
    Zhou, L. and Körmann, F. and Holec, D. and Bartosik, M. and Grabowski, B. and Neugebauer, J. and Mayrhofer, P.H.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The dynamical and thermodynamic phase stabilities of the stoichiometric compound CrN including different structural and magnetic configurations are comprehensively investigated using a first-principles density functional theory (DFT) plus U (DFT+U) approach in conjunction with experimental measurements of the thermal expansion. Comparing DFT and DFT+U results with experimental data reveals that the treatment of electron correlations using methods beyond standard DFT is crucial. The nonmagnetic face-centered cubic B1-CrN phase is both elastically and dynamically unstable, even under high pressure, while CrN phases with nonzero local magnetic moments are predicted to be dynamically stable within the framework of the DFT+U scheme. Furthermore, the impact of different treatments for the exchange-correlation (xc)-functional is investigated by carrying out all computations employing the local density approximation and generalized gradient approximation. To address finite-temperature properties, both magnetic and vibrational contributions to the free energy have been computed employing our recently developed spin-space averaging method. The calculated phase transition temperature between low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN variants is in excellent agreement with experimental values and reveals the strong impact of the choice of the xc-functional. The temperature-dependent linear thermal expansion coefficient of CrN is experimentally determined by the wafer curvature method from a reactive magnetron sputter deposited single-phase B1-CrN thin film with dense film morphology. A good agreement is found between experimental and ab initio calculated linear thermal expansion coefficients of PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity, have been computed as well and compared to previous experimental data. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.184102
  • Temperature dependent magnon-phonon coupling in bcc Fe from theory and experiment
    Körmann, F. and Grabowski, B. and Dutta, B. and Hickel, T. and Mauger, L. and Fultz, B. and Neugebauer, J.
    Physical Review Letters 113 (2014)
    An ab initio based framework for quantitatively assessing the phonon contribution due to magnon-phonon interactions and lattice expansion is developed. The theoretical results for bcc Fe are in very good agreement with high-quality phonon frequency measurements. For some phonon branches, the magnon-phonon interaction is an order of magnitude larger than the phonon shift due to lattice expansion, demonstrating the strong impact of magnetic short-range order even significantly above the Curie temperature. The framework closes the previous simulation gap between the ferro- and paramagnetic limits. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.165503
  • Understanding and controlling indium incorporation and surface segregation on in x Ga 1 - X N surfaces: An ab initio approach
    Duff, A.I. and Lymperakis, L. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The incorporation of In into the technologically relevant (0001) (Ga-polar) and (0001̄) (N-polar) surfaces of In0.25Ga0.75N is investigated using density functional theory. The cases of coherent pseudomorphic growth on GaN and on lattice-matched heterointerfaces are considered. For pseudomorphic growth on GaN, In incorporation into the {0001} surface layers is limited to a tiny growth window corresponding to extreme In-rich growth conditions and at the In-rich/Ga-poor region of the metal chemical potentials. Lattice-matched growth, however, allows for a wider growth window. Surface phase diagrams are constructed as a function of growth conditions and reveal similarities between the two polar growth planes. However, a strong driving force is found for segregation of In atoms to the first III-N layer for Ga-polar growth, but not for N-polar growth. The former was found to be mainly due to chemical effects (stronger Ga-N as compared to In-N bonds), absent in the case of N-polar growth. Furthermore, finite-temperature calculations show that In incorporated into the first III-N layer is stable to ≈150 K higher temperatures in the N-polar surface than in the Ga-polar surface, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.085307
  • 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 abstract10.1088/1367-2630/15/4/043020
  • Ab Initio Based conformational study of the crystalline α-chitin
    Petrov, M. and Lymperakis, L. and Friák, M. and Neugebauer, J.
    Biopolymers 99 (2013)
    The equilibrium structure including the network of hydrogen bonds of an α-chitin crystal is determined combining density-functional theory (DFT), self-consistent DFT-based tight-binding (SCC-DFTB), and empirical forcefield molecular dynamics (MD) simulations. Based on the equilibrium geometry several possible crystal conformations (local energy minima) have been identified and related to hydrogen bond patterns. Our results provide new insight and allow to resolve the contradicting α-chitin structural models proposed by various experiments. © 2012 Wiley Periodicals, Inc. Copyright © 2012 Wiley Periodicals, Inc.
    view abstract10.1002/bip.22131
  • Ab Initio Guided Design of Materials
    Friák, M. and Raabe, D. and Neugebauer, J.
    Structural Materials and Processes in Transportation (2013)
    view abstract10.1002/9783527649846.ch15
  • Ab initio identified design principles of solid-solution strengthening in Al
    Ma, D. and Friák, M. and Pezold, J.V. and Raabe, D. and Neugebauer, J.
    Science and Technology of Advanced Materials 14 (2013)
    Solid-solution strengthening in six Al-X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute-volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al-X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch. © 2013 National Institute for Materials Science.
    view abstract10.1088/1468-6996/14/2/025001
  • 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 (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 abstract10.1016/j.jmbbm.2013.01.030
  • Ab initio study of thermodynamic, electronic, magnetic, structural, and elastic properties of Ni4N allotropes
    Hemzalová, P. and Friák, M. and Šob, M. and Ma, D. and Udyansky, A. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    We have employed parameter-free density functional theory calculations to study the thermodynamic stability and structural parameters as well as elastic and electronic properties of Ni4N in eight selected crystallographic phases. In agreement with the experimental findings, the cubic structure with Pearson symbol cP5, space group Pm3̄m (221) is found to be the most stable and it is also the only thermodynamically stable structure at T=0 K with respect to decomposition to the elemental Ni crystal and N2 gas phase. We determine structural parameters, bulk moduli, and their pressure derivatives for all eight allotropes. The thermodynamic stability and bulk modulus is shown to be anticorrelated. Comparing ferromagnetic and nonmagnetic states, we find common features between the magnetism of elemental Ni and studied ferromagnetic Ni4N structures. For the ground-state Ni4N structure and other two Ni4N cubic allotropes, we predict a complete set of single-crystalline elastic constants (in the equilibrium and under hydrostatic pressure), the Young and area moduli, as well as homogenized polycrystalline elastic moduli obtained by different homogenization methods. We demonstrate that the elastic anisotropy of the ground-state Ni4N is qualitatively opposite to that in the elemental Ni, i.e., these materials have hard and soft crystallographic directions interchanged. Moreover, one of the studied metastable cubic phases is found auxetic, i.e., exhibiting negative Poisson ratio. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.174103
  • Band offsets at zincblende-wurtzite GaAs nanowire sidewall surfaces
    Capiod, P. and Xu, T. and Nys, J.P. and Berthe, M. and Patriarche, G. and Lymperakis, L. and Neugebauer, J. and Caroff, P. and Dunin-Borkowski, R.E. and Ebert, P. and Grandidier, B.
    Applied Physics Letters 103 (2013)
    The band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires are investigated by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p-i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4821293
  • Basal and non-basal dislocation slip in Mg–Y
    Sandlöbes, S. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science and Engineering A 576 (2013)
    The activation of non-basal slip systems is of high importance for the ductility in hcp Mg and its alloys. In particular, for Mg–Y alloys where a higher activation of pyramidal dislocation slip causes an increased ductility detailed characterization of the activated slip systems is essential to understand and describe plasticity in these alloys. In this study a detailed analysis of the activated dislocations and slip systems via post-mortem TEM and SEM-EBSD based slip band analysis in 3% deformed Mg–3 wt% Y is presented. The analysis reveals a substantial activity of pyramidal <c+a> dislocations with different Burgers vectors. The obtained dislocation densities and active slip systems are discussed with respect to atomistic simulations of non-basal dislocations in hcp Mg. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.03.006
  • Blocking growth by an electrically active subsurface layer: The effect of si as an antisurfactant in the growth of GaN
    Markurt, T. and Lymperakis, L. and Neugebauer, J. and Drechsel, P. and Stauss, P. and Schulz, T. and Remmele, T. and Grillo, V. and Rotunno, E. and Albrecht, M.
    Physical Review Letters 110 (2013)
    Combining aberration corrected high resolution transmission electron microscopy and density functional theory calculations we propose an explanation of the antisurfactant effect of Si in GaN growth. We identify the atomic structure of a Si delta-doped layer (commonly called SiNx mask) as a SiGaN3 monolayer that resembles a √3×√3 R30 surface reconstruction containing one Si atom, one Ga atom, and a Ga vacancy (V Ga) in its unit cell. Our density functional theory calculations show that GaN growth on top of this SiGaN3 layer is inhibited by forming an energetically unfavorable electrical dipole moment that increases with layer thickness and that is caused by charge transfer between cation dangling bonds at the surface to VGa bound at subsurface sites. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.110.036103
  • Dangling-bond defect in a-Si:H: Characterization of network and strain effects by first-principles calculation of the EPR parameters
    Pfanner, G. and Freysoldt, C. and Neugebauer, J. and Inam, F. and Drabold, D. and Jarolimek, K. and Zeman, M.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The performance of hydrogenated amorphous silicon (a-Si:H) solar cells is severely affected by the light-induced formation of metastable defects in the material (Staebler-Wronski effect). The common notion is that the dangling-bond (db) defect, a threefold coordinated silicon atom, plays a key role in the underlying mechanisms. To support the characterization of this defect by electron paramagnetic resonance (EPR), we present in this work a first-principles study of the EPR parameters for a structural ensemble of the db defect. We show that the a-Si:H dangling bond is a network defect for which charge and spin localization substantially depend on the actual coordination of the db atom and the local geometric and electronic structure of the immediate surrounding. It consequently differs by its very nature from its crystalline counterpart, which is typically related to the presence of a vacancy. The application of hydrostatic strain to our models yields further insights into the dependence of the hyperfine interaction on the structural characteristics of the defect. The observed trends are shown to result from the interplay between delocalization and sp hybridization. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.125308
  • Density functional theory in materials science
    Neugebauer, J. and Hickel, T.
    Wiley Interdisciplinary Reviews: Computational Molecular Science 3 (2013)
    Materials science is a highly interdisciplinary field. It is devoted to the understanding of the relationship between (a) fundamental physical and chemical properties governing processes at the atomistic scale with (b) typically macroscopic properties required of materials in engineering applications. For many materials, this relationship is not only determined by chemical composition, but strongly governed by microstructure. The latter is a consequence of carefully selected process conditions (e.g., mechanical forming and annealing in metallurgy or epitaxial growth in semiconductor technology). A key task of computational materials science is to unravel the often hidden composition-structure-property relationships using computational techniques. The present paper does not aim to give a complete review of all aspects of materials science. Rather, we will present the key concepts underlying the computation of selected material properties and discuss the major classes of materials to which they are applied. Specifically, our focus will be on methods used to describe single or polycrystalline bulk materials of semiconductor, metal or ceramic form. © 2013 John Wiley & Sons, Ltd.
    view abstract10.1002/wcms.1125
  • GaN(0001) surface states: Experimental and theoretical fingerprints to identify surface reconstructions
    Himmerlich, M. and Lymperakis, L. and Gutt, R. and Lorenz, P. and Neugebauer, J. and Krischok, S.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The surface electronic structure of GaN(0001) surfaces is characterized by photoelectron spectroscopy. Depending on the surface preparation conditions, such as cooldown in nitrogen plasma after growth and additional vacuum annealing or Ga deposition, different surface states are observed at the valence-band edge and inside the band gap of GaN, while the surface Fermi-level position EF was found to be independent at 2.9-3.0 eV, indicative of unoccupied surface states that pin EF. The experimental results are combined with band structures of different 2×2 reconstructed surfaces calculated by density-functional theory. Comparing the experimental results with the theoretical density of surface states allows an identification of the microscopic origin of these states and an assignment of the related surface structure. The presence of a 2×2 nitrogen adatom structure after growth is found that can be identified by its fingerprint surface states ≈0.9 eV above and ≈0.6 eV below the valence-band maximum (VBM). For Ga vacancy and adatom structures a similar agreement is found, revealing surface states ≈0.3 and ≈1.4 or ≈1.6 eV above the VBM, respectively. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.125304
  • Hidden surface states at non-polar GaN (101̄0) facets: Intrinsic pinning of nanowires
    Lymperakis, L. and Weidlich, P.H. and Eisele, H. and Schnedler, M. and Nys, J.-P. and Grandidier, B. and Stiévenard, D. and Dunin-Borkowski, R.E. and Neugebauer, J. and Ebert, Ph.
    Applied Physics Letters 103 (2013)
    We investigate the electronic structure of the GaN (10 1 ̄ 0) prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean (10 1 ̄ 0) sidewall facets are intrinsically pinned. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4823723
  • Interfacial structure and chemistry of GaN on Ge(111)
    Zhang, S. and Zhang, Y. and Cui, Y. and Freysoldt, C. and Neugebauer, J. and Lieten, R.R. and Barnard, J.S. and Humphreys, C.J.
    Physical Review Letters 111 (2013)
    The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5:4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5:4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5:4 lattice-matching growth of GaN. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.256101
  • Interplay between Coulomb interaction and quantum-confined Stark-effect in polar and nonpolar wurtzite InN/GaN quantum dots
    Barthel, S. and Schuh, K. and Marquardt, O. and Hickel, T. and Neugebauer, J. and Jahnke, F. and Czycholl, G.
    European Physical Journal B 86 (2013)
    In this paper we systematically analyze the electronic structures of polar and nonpolar wurtzite-InN/GaN quantum dots and their modification due to the quantum-confined Stark effect caused by intrinsic fields. This is achieved by combining continuum elasticity theory with an effective-bond orbital model to describe the elastic and single-particle electronic properties in these nitride systems. Based on these results, a many-body treatment is used to determine optical absorption spectra. The efficiency of optical transitions depends on the interplay between the Coulomb interaction and the quantum-confined Stark effect. We introduce an effective confinement potential which represents the electronic structure under the influence of the intrinsic polarization fields and calculate the needed strength of Coulomb interaction to diminish the separation of electrons and holes. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstract10.1140/epjb/e2013-40542-0
  • Polarization effects due to thickness fluctuations in nonpolar InGaN/GaN quantum wells
    Marquardt, O. and Hickel, T. and Neugebauer, J. and Van De Walle, C.G.
    Applied Physics Letters 103 (2013)
    We have employed continuum elasticity theory and an eight band k·p model to study the influence of thickness fluctuations in In 0.2Ga0.8N quantum wells grown along the [11 2 ̄ 0] direction in GaN. Such fluctuations are the origin of polarization potentials that may spatially separate electrons and holes in the vicinity of a thickness fluctuation and therefore reduce the efficiency of light emitters. Our calculations reveal that even shallow fluctuations of one or two monolayers can induce a significant spatial separation of electrons and holes, in particular, if the lateral extent of such a fluctuation is large. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4818752
  • Self-consistent scale-bridging approach to compute the elasticity of multi-phase polycrystalline materials
    Titrian, H. and Aydin, U. and Friák, M. and Ma, D. and Raabe, D. and Neugebauer, J.
    Materials Research Society Symposium Proceedings 1524 (2013)
    A necessary prerequisite for a successful theory-guided up-scale design of materials with application-driven elastic properties is the availability of reliable homogenization techniques. We report on a new software tool that enables us to probe and analyze scale-bridging structure-property relations in the elasticity of materials. The newly developed application, referred to as SC-EMA (Self-consistent Calculations of Elasticity of Multi-phase Aggregates) computes integral elastic response of randomly textured polycrystals. The application employs a Python modular library that uses single-crystalline elastic constants Cij as input parameters and calculates macroscopic elastic moduli (bulk, shear, and Young's) and Poisson ratio of both single-phase and multi-phase aggregates. Crystallites forming the aggregate can be of cubic, tetragonal, hexagonal, orthorhombic, or trigonal symmetry. For cubic polycrystals the method matches the Hershey homogenization scheme. In case of multi-phase polycrystalline composites, the shear moduli are computed as a function of volumetric fractions of phases present in aggregates. Elastic moduli calculated using the analytical self-consistent method are computed together with their bounds as determined by Reuss, Voigt and Hashin-Shtrikman homogenization schemes. The library can be used as (i) a toolkit for a forward prediction of macroscopic elastic properties based on known single-crystalline elastic characteristics, (ii) a sensitivity analysis of macro-scale output parameters as function of input parameters, and, in principle, also for (iii) an inverse materials-design search for unknown phases and/or their volumetric ratios. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.41
  • Thermodynamic modeling of chromium: Strong and weak magnetic coupling
    Körmann, F. and Grabowski, B. and Söderlind, P. and Palumbo, M. and Fries, S.G. and Hickel, T. and Neugebauer, J.
    Journal of Physics Condensed Matter 25 (2013)
    As chromium is a decisive ingredient for stainless steels, a reliable understanding of its thermodynamic properties is indispensable. Parameter-free first-principles methods have nowadays evolved to a state allowing such thermodynamic predictions. For materials such as Cr, however, the inclusion of magnetic entropy and higher order contributions such as anharmonic entropy is still a formidable task. Employing state-of-the-art ab initio molecular dynamics simulations and statistical concepts, we compute a set of thermodynamic properties based on quasiharmonic, anharmonic, electronic and magnetic free energy contributions from first principles. The magnetic contribution is modeled by an effective nearest-neighbor Heisenberg model, which itself is solved numerically exactly by means of a quantum Monte Carlo method. We investigate two different scenarios: a weak magnetic coupling scenario for Cr, as usually presumed in empirical thermodynamic models, turns out to be in clear disagreement with experimental observations. We show that instead a mixed Hamiltonian including weak and strong magnetic coupling provides a consistent picture with good agreement to experimental thermodynamic data. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/25/42/425401
  • Thermodynamics of carbon solubility in ferrite and vacancy formation in cementite in strained pearlite
    Nematollahi, G.A. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 61 (2013)
    In order to investigate the thermodynamic driving force for the experimentally observed accumulation of C in ferritic layers of severely plastically deformed pearlitic wires, the stabilities of C interstitials in ferrite and of C vacancies in cementite are investigated as a function of uniaxial stain, using density-functional theory. In the presence of an applied strain along [1 1 0] or [1 1 1], the C interstitial in ferrite is significantly stabilized, while the C vacancy in cementite is moderately destabilized by the corresponding strain states in cementite [1 0 0] and ([0 1 0]). The enhanced stabilization of the C interstitial gives rise to an increase in the C concentration within the ferritic layers by up to two orders of magnitude. Our results thus suggest that in addition to the generally assumed non-equilibrium, dislocation-based mechanism, there is also a strain-induced thermodynamic driving force for the experimentally observed accumulation of C in ferrite. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.12.001
  • Three-Dimensional Diagnosis and Treatment Planning of Dentoalveolar Problems
    Ritter, L. and Neugebauer, J. and Mischkowski, R. and Scheer, M. and Zöller, J.E.
    Three-Dimensional Imaging for Orthodontics and Maxillofacial Surgery (2013)
    view abstract10.1002/9781118786642.ch5
  • A flexible, plane-wave based multiband k ·p model
    Marquardt, O. and Schulz, S. and Freysoldt, C. and Boeck, S. and Hickel, T. and O'Reilly, E.P. and Neugebauer, J.
    Optical and Quantum Electronics 44 (2012)
    In this work, we present a highly generalized implementation of multiband k · p models. We have achieved a high efficiency of our approach by incorporating it in a planewave framework within the Density Functional Theory package S/PHI/nX. To demonstrate the flexibility and applicability of our code, we have chosen two example studies that are directly accessible with the standard eight-band k · p model. By employing a 14-band k · p model for the description of pyramidal InAs/GaAs quantum dots (QDs), we show that this model is able to accomodate for the correct symmetry of the underlying zincblende lattice, which is not reflected in the standard eight-band model. Our second example provides a description of site-controlled (111)-oriented InGaAs/GaAs QDs. The extremely small aspect ratio of these QDs makes a description using conventional k · p Hamiltonians computationally highly expensive.We have therefore rotated the standard eight-band Hamiltonian, to suit the description of these systems. The studies of electronic properties of the above mentioned model systems demonstrate the efficiency and flexibility of our approach. © Springer Science+Business Media, LLC. 2011.
    view abstract10.1007/s11082-011-9506-3
  • Ab initio EPR parameters for dangling-bond defect complexes in silicon: Effect of Jahn-Teller distortion
    Pfanner, G. and Freysoldt, C. and Neugebauer, J. and Gerstmann, U.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    A dangling bond (db) is an important point defect in silicon. It is realized in crystalline silicon by defect complexes of the monovacancy V with impurities. In this work, we present spin-polarized density-functional theory calculations of EPR parameters (g and hyperfine tensors) within the GIPAW formalism for two kinds of db defect complexes. The first class characterizes chemically saturated db systems, where three of the four dangling bonds of the isolated vacancy are saturated by hydrogen (VH 3) or hydrogen and oxygen (hydrogen-oxygen complex, VOH). The second kind of db consists of systems with a Jahn-Teller distortion, where the vacancy includes either a substitutional phosphorus atom (the E center, VP) or a single hydrogen atom (VH). For all systems we obtain excellent agreement with available experimental data, and we are therefore able to quantify the effect of the Jahn-Teller distortion on the EPR parameters. Furthermore we study the influence of strain to obtain further insights into the structural and electronic characteristics of the considered defects. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.195202
  • Ab initio-based prediction of phase diagrams: Application to magnetic shape memory alloys
    Hickel, T. and Uijttewaal, M. and Al-Zubi, A. and Dutta, B. and Grabowski, B. and Neugebauer, J.
    Advanced Engineering Materials 14 (2012)
    An ultimate goal of material scientists is the prediction of the thermodynamics of tailored materials solely based on first principles methods. The present work reviews recent methodological developments and advancements providing thereby an up-to-date basis for such an approach. Key ideas and the performance of these methods are discussed with respect to the Heusler alloy Ni-Mn-Ga - a prototype magnetic shape-memory alloy of great technological interest for various applications. Ni-Mn-Ga shows an interesting and complex sequence of phase transitions, rendering it a significant theoretical challenge for any first principles approach. The primary goal of this investigation is to determine the composition dependence of the martensitic transition temperature in these alloys. Quasiharmonic phonons and the magnetic exchange interactions as well as the delicate interplay of vibrational and magnetic excitations are taken into account employing density functional theory. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200092
  • Advancing density functional theory to finite temperatures: Methods and applications in steel design
    Hickel, T. and Grabowski, B. and Körmann, F. and Neugebauer, J.
    Journal of Physics Condensed Matter 24 (2012)
    The performance of materials such as steels, their high strength and formability, is based on an impressive variety of competing mechanisms on the microscopic/atomic scale (e.g. dislocation gliding, solid solution hardening, mechanical twinning or structural phase transformations). Whereas many of the currently available concepts to describe these mechanisms are based on empirical and experimental data, it becomes more and more apparent that further improvement of materials needs to be based on a more fundamental level. Recent progress for methods based on density functional theory (DFT) now makes the exploration of chemical trends, the determination of parameters for phenomenological models and the identification of new routes for the optimization of steel properties feasible. A major challenge in applying these methods to a true materials design is, however, the inclusion of temperature-driven effects on the desired properties. Therefore, a large range of computational tools has been developed in order to improve the capability and accuracy of first-principles methods in determining free energies. These combine electronic, vibrational and magnetic effects as well as structural defects in an integrated approach. Based on these simulation tools, one is now able to successfully predict mechanical and thermodynamic properties of metals with a hitherto not achievable accuracy. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/24/5/053202
  • Atomic forces at finite magnetic temperatures: Phonons in paramagnetic iron
    Körmann, F. and Dick, A. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    A density-functional theory (DFT) based scheme to calculate effective forces for magnetic materials at finite temperatures is proposed. The approach is based on a coarse graining procedure in the magnetic configuration space. As application we calculate phonon spectra of paramagnetic bcc and fcc iron and show good agreement with experimental data. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.125104
  • Combined ab initio, experimental, and CALPHAD approach for an improved thermodynamic evaluation of the Mg-Si system
    Schick, M. and Hallstedt, B. and Glensk, A. and Grabowski, B. and Hickel, T. and Hampl, M. and Gröbner, J. and Neugebauer, J. and Schmid-Fetzer, R.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 37 (2012)
    A new thermodynamic evaluation of the well-known Mg-Si system is presented with the aim to resolve persistent uncertainties in the Gibbs energy of its only compound, Mg 2Si. For this purpose the heat capacity and enthalpy of melting of Mg 2Si were measured by differential scanning calorimetry. Using finite temperature density functional theory and the quasiharmonic approximation, thermodynamic properties of Mg 2Si were additionally calculated up to and above its melting temperature. Using these new data, in particular the heat capacity, the Mg-Si system was evaluated thermodynamically with the CALPHAD method leading to a thermodynamic description of the system within narrow bounds. In contrast to several previous evaluations there is no problem with an inverted miscibility gap in the liquid. Although present enthalpy of melting data turned out to be inconsistent with other data in this system, the new evaluation accurately describes all other available data in this system. In particular the Gibbs energy of Mg 2Si can now be considered reliably described.© 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.calphad.2012.02.001
  • 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 (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 abstract10.1016/j.actamat.2011.11.046
  • Impact of Anxiety Parameters on Prospective and Experienced Pain Intensity in Implant Surgery
    Weisensee, W. and Scheer, M. and Muller, L. and Rothamel, D. and Kistler, F. and Bayer, G. and Johren, P. and Neugebauer, J.
    Implant Dentistry 21 (2012)
    Purpose: To investigate the relationship between anxiety and pain perception in the context of implant surgery and to examine the impact of factors such as gender and surgery duration on anxiety and pain levels. Methods: One hundred twenty-one patients undergoing implant surgery evaluated their anxiety and pain levels by completing a set of 2 questionnaires at different points in time. Results: Both anxiety and pain levels were highest on the day of surgery and showed a significant decrease when evaluated retrospectively (P < 0.01; P < 0.05). Correlations were found between preoperative anxiety and expected pain levels (r = 0.19), pain peak and duration of pain (r = 0.79), and a nearly significant correlation between preoperative anxiety and duration of pain (r = 0.18). Women showed significantly higher levels of preoperative anxiety (P < 0.05) and expected pain (P < 0.05) than men. Conclusion: The results confirm a strong negative impact of increased preoperative anxiety levels on pain perception and the recovery process. Therefore, future research should focus on individual patient's sources of dental anxiety to reduce its negative consequences. (Implant Dent 2012;21:502-506)
    view abstract10.1097/ID.0b013e3182703a44
  • Reconstructions and electronic structure of (112̄2) and (112̄2 ) semipolar AlN surfaces
    Kalesaki, E. and Lymperakis, L. and Kioseoglou, J. and Neugebauer, J. and Karakostas, T. and Komninou, P.
    Journal of Applied Physics 112 (2012)
    he energetics, atomic geometry, and electronic structure of semipolar (112̄2) and (112̄2 ) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions, metallic reconstructions are favoured on both polarity surfaces. For N rich to moderate Al rich conditions, the (112̄2) planes promote semiconducting reconstructions having 2 × 2 or c(2 × 2) periodicity. In contrast, under the particular range of the Al chemical potential the (112̄2 ) surfaces stabilize reconstructions with excess metal and it is only at the extreme N rich limit that the semiconducting c(2 × 2) N adatom structure prevails. The present study reveals that the reconstructed (112̄2) surfaces do not contain steps in contrast to (112̄2 ) where surface steps are inherent for N rich to moderate metal rich growth conditions and may result in intrinsic step-flow growth and/or growth of parasitic semipolar orientations. © 2012 American Institute of Physics.
    view abstract10.1063/1.4743007
  • Solution enthalpy of hydrogen in fourth row elements: Systematic trends derived from first principles
    Aydin, U. and Ismer, L. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Based on first-principles calculations, we identify a master curve for the solution enthalpy of H in fourth row elements including all 3d transition metals. Assuming nonmagnetic fcc crystal structures, we find two different classes of materials with either the octahedral or the tetrahedral interstitial site being preferred by hydrogen. An interaction radius for H in octahedral site of ≈0.7 Å (≈0.4Å for H in tetrahedral site) turns out to be a characteristic value for which the chemical interaction energy has an optimum for all studied elements. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.155144
  • Strong dipole coupling in nonpolar nitride quantum dots due to Coulomb effects
    Schuh, K. and Barthel, S. and Marquardt, O. and Hickel, T. and Neugebauer, J. and Czycholl, G. and Jahnke, F.
    Applied Physics Letters 100 (2012)
    Optical properties of polar and nonpolar nitride quantum dots (QDs) are determined on the basis of a microscopic theory which combines a continuum elasticity approach to the polarization potential, a tight-binding model for the electronic energies and wavefunctions, and a many-body theory for the optical properties. For nonpolar nitride quantum dots, we find that optical absorption and emission spectra exhibit a weak ground-state oscillator strength in a single-particle calculation whereas the Coulomb configuration interaction strongly enhances the ground-state transitions. This finding sheds new light on existing discrepancies between previous theoretical and experimental results for these systems, as a weak ground state transition was predicted because of the spatial separation of the corresponding electron and hole state due to intrinsic fields whereas experimentally fast optical transitions have been observed. © 2012 American Institute of Physics.
    view abstract10.1063/1.3688900
  • The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries
    Freysoldt, C. and Pfanner, G. and Neugebauer, J.
    Journal of Non-Crystalline Solids 358 (2012)
    Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder. However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnoncrysol.2011.12.090
  • The relation between ductility and stacking fault energies in Mg and Mg-Y alloys
    Sandlöbes, S. and Friák, M. and Zaefferer, S. and Dick, A. and Yi, S. and Letzig, D. and Pei, Z. and Zhu, L.-F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 60 (2012)
    The underlying mechanisms that are responsible for the improved room-temperature ductility in Mg-Y alloys compared to pure Mg are investigated by transmission electron microscopy and density functional theory. Both methods show a significant decrease in the intrinsic stacking fault I 1 energy (I 1 SFE) with the addition of Y. The influence of the SFE on the relative activation of different competing deformation mechanisms (basal, prismatic, pyramidal slip) is discussed. From this analysis we suggest a key mechanism which explains the transition from primary basal slip in hexagonal close-packed Mg to basal plus pyramidal slip in solid solution Mg-Y alloys. This mechanism is characterized by enhanced nucleation of 〈c + a〉 dislocations where the intrinsic stacking fault I 1 (ISF 1) acts as heterogeneous source for 〈c + a〉 dislocations. Possible electronic and geometric reasons for the modification of the SFE by substitutional Y atoms are identified and discussed. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.02.006
  • 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 (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 abstract10.3390/ma5101853
  • Trends in the elastic response of binary early transition metal nitrides
    Holec, D. and Friák, M. and Neugebauer, J. and Mayrhofer, P.H.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Motivated by an increasing demand for coherent data that can be used for selecting materials with properties tailored for specific application requirements, we studied elastic response of nine binary early transition metal nitrides (ScN, TiN, VN, YN, ZrN, NbN, LaN, HfN, and TaN) and AlN. In particular, single-crystal elastic constants, Young's modulus in different crystallographic directions, polycrystalline values of shear and Young's moduli, and the elastic anisotropy factor were calculated. Additionally, we provide estimates of the third order elastic constants for the ten binary nitrides. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.064101
  • Vacancy formation energies in fcc metals: Influence of exchange-correlation functionals and correction schemes
    Nazarov, R. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    The performance of various exchange-correlation functionals (LDA, PBE, PW91, and AM05) in predicting vacancy formation energies has been evaluated for 12 fcc metals. A careful analysis of the results shows that differences between the theoretical result and experiment are mainly related to the way the various exchange-correlation functionals describe the internal surface of the vacancy. Based on this insight we propose a modified version of the correction scheme of Mattsson, Wixom, and Armiento. Applying this approach to our results yields a perfect alignment of vacancy formation energies for all exchange-correlation functionals. These corrected values are also in very good agreement with the vacancy formation energies obtained in experiment. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.144118
  • A density functional theory based estimation of the anharmonic contributions to the free energy of a polypeptide helix
    Ismer, L. and Ireta, J. and Neugebauer, J.
    Journal of Chemical Physics 135 (2011)
    We have employed density functional theory to determine the temperature dependence of the intrinsic stability of an infinite poly-L-alanine helix. The most relevant helix types, i.e., the - and the 310 - helix, and several unfolded conformations, which serve as reference for the stability analysis, have been included. For the calculation of the free energies for the various chain conformations we have explicitly included both, harmonic and anharmonic contributions. The latter have been calculated by means of a thermodynamic integration approach employing stochastic Langevin molecular dynamics, which is shown to provide a dramatic increase in the computational efficiency as compared to commonly employed deterministic molecular dynamics schemes. Employing this approach we demonstrate that the anharmonic part of the free energy amounts to the order of 0.1-0.4 kcal/mol per peptide unit for all analysed conformations. Although small, the anharmonic contribution stabilizes the helical conformations with respect to the fully extended structure. © 2011 American Institute of Physics.
    view abstract10.1063/1.3629451
  • A flexible, plane-wave-based formulation of continuum elasticity and multiband k·p models
    Marquardt, O. and Schulz, S. and O'Reilly, E.P. and Freysoldt, C. and Boeck, S. and Hickel, T. and Neugebauer, J.
    Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD (2011)
    We present a highly flexible, plane-wave based formulation of continuum elasticity and multiband k·p-formalism to study the elastic and electronic properties of semiconductor nanostructures. This approach has been implemented in the framework of the density functional theory (DFT) software library S/Phi/nX [1] and allows the investigation of arbitrary-shaped nanostructures such as quantum wells, wires and dots consisting of various materials. Moreover, our approach grants the flexibility to employ user-generated k·p Hamiltonians suited to the requirements of the study regarding accuracy and computational costs. © 2011 IEEE.
    view abstract10.1109/NUSOD.2011.6041165
  • Ab initio based determination of thermodynamic properties of cementite including vibronic, magnetic, and electronic excitations
    Dick, A. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The thermodynamic properties of cementite have been evaluated with a hybrid approach, which describes the vibrational and electronic excitations based on density functional theory calculations, while the magnetic free energy is evaluated using the spin quantum Monte Carlo scheme. Our ansatz allows us to calculate the heat capacity and free energy of cementite with a high accuracy resulting in a free-energy difference of less than 10 meV/atom at 1500 K when compared with experiment. For the formation energy of cementite we observe, however, that the accuracy of density functional theory within the Perdew-Burke-Ernzerhof exchange-correlation functional is not sufficient to provide quantitative agreement with experiment. We show that the main limit in the accuracy of this exchange-correlation functional is the T=0 K potential energy surface. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.125101
  • Ab initio study of electron paramagnetic resonance hyperfine structure of the silicon dangling bond: Role of the local environment
    Pfanner, G. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The hyperfine coupling of the electron spin to the nuclear spins nearby contains information on the local environment such as the chemical composition, distances, or bond angles. A correct interpretation of this information requires a comparison to a theoretical model. In this work, we employ spin-density-functional theory and ab initio pseudopotentials to study how the hyperfine couplings of a Si dangling bond change under systematic variations of the local environment. For our network models, which take the effect of the extended host into account (supercell approach), we show that the hyperfine tensor of the undercoordinated silicon atom is governed by the interplay between s-p hybridization and the degree of localization of the defect state. © 2011 American Physical society.
    view abstract10.1103/PhysRevB.83.144110
  • Ab initio study of pressure stabilized NiTi allotropes: Pressure-induced transformations and hysteresis loops
    Holec, D. and Friák, M. and Dlouhý, A. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Changes in stoichiometric NiTi allotropes induced by hydrostatic pressure have been studied employing density functional theory. By modeling the pressure-induced transitions in a way that imitates quasistatic pressure changes, we show that the experimentally observed B19′ phase is (in its bulk form) unstable with respect to another monoclinic phase, B19″. The lower symmetry of the B19″ phase leads to unique atomic trajectories of Ti and Ni atoms (that do not share a single crystallographic plane) during the pressure-induced phase transition. This uniqueness of atomic trajectories is considered a necessary condition for the shape memory ability. The forward and reverse pressure-induced transition B19′ B19″ exhibits a hysteresis that is shown to originate from hitherto unexpected complexity of the Born-Oppenheimer energy surface. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.224119
  • Advanced Calculations for Defects in Materials: Electronic Structure Methods
    Alkauskas, A. and Deàk, P. and Neugebauer, J. and Pasquarello, A. and Van de Walle, C.G.
    Advanced Calculations for Defects in Materials: Electronic Structure Methods (2011)
    This book investigates the possible ways of improvement by applying more sophisticated electronic structure methods as well as corrections and alternatives to the supercell model. In particular, the merits of hybrid and screened functionals, as well as of the +U methods are assessed in comparison to various perturbative and Quantum Monte Carlo many body theories. The inclusion of excitonic effects is also discussed by way of solving the Bethe-Salpeter equation or by using time-dependent DFT, based on GW or hybrid functional calculations. Particular attention is paid to overcome the side effects connected to finite size modeling. The editors are well known authorities in this field, and very knowledgeable of past developments as well as current advances. In turn, they have selected respected scientists as chapter authors to provide an expert view of the latest advances. The result is a clear overview of the connections and boundaries between these methods, as well as the broad criteria determining the choice between them for a given problem. Readers will find various correction schemes for the supercell model, a description of alternatives by applying embedding techniques, as well as algorithmic improvements allowing the treatment of an ever larger number of atoms at a high level of sophistication. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/9783527638529
  • Advanced Calculations for Defects in Solids - Electronic Structure Methods
    Alkauskas, A. and Deák, P. and Neugebauer, J. and Pasquarello, A. and Van de Walle, C.G.
    Physica Status Solidi (B) Basic Research 248 (2011)
    view abstract10.1002/pssb.201140804
  • Anisotropic mechanical behavior of ultrafine eutectic TiFe cast under non-equilibrium conditions
    Schlieter, A. and Kühn, U. and Eckert, J. and Löser, W. and Gemming, T. and Friák, M. and Neugebauer, J.
    Intermetallics 19 (2011)
    The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2010.10.012
  • Combined multifrequency EPR and DFT study of dangling bonds in a-Si:H
    Fehr, M. and Schnegg, A. and Rech, B. and Lips, K. and Astakhov, O. and Finger, F. and Pfanner, G. and Freysoldt, C. and Neugebauer, J. and Bittl, R. and Teutloff, C.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy using S-, X-, Q-, and W-band frequencies (3.6, 9.7, 34, and 94 GHz, respectively) was employed to study paramagnetic coordination defects in undoped hydrogenated amorphous silicon (a-Si:H). The improved spectral resolution at high magnetic field reveals a rhombic splitting of the g tensor with the following principal values: g x=2.0079, g y=2.0061, and g z=2.0034, and shows pronounced g strain, i.e., the principal values are widely distributed. The multifrequency approach furthermore yields precise 29Si hyperfine data. Density functional theory (DFT) calculations on 26 computer-generated a-Si:H dangling-bond models yielded g values close to the experimental data but deviating hyperfine interaction values. We show that paramagnetic coordination defects in a-Si:H are more delocalized than computer-generated dangling-bond defects and discuss models to explain this discrepancy. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.245203
  • Construction and performance of fully numerical optimum atomic basis sets
    Lange, B. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We propose a method for constructing small atomic basis sets that optimally mimic the Kohn-Sham wave functions of an underlying plane-wave calculation. The key quantity of the optimization procedure is the spillage: the relative amount of the occupied Hilbert-space norm lost in the projection onto the atomic basis. To ensure full flexibility in the radial shape, we represent the basis functions by radial momentum-space spline functions. With our approach we reach spillages in the order of 10-4 when using minimal basis sets. Band structure calculations on top of a self-consistent linear-combination-of-atomic- orbitals (LCAO) run reproduce the occupied part of the band structure within a few tens meV. However, minimal basis sets are not flexible enough for obtaining material properties such as the lattice constant and the bulk modulus but require basis sets with f orbitals. Basis sets generated for Si, GaN, Al, and NaCl reach a spillage of ∼10-5 and reproduce lattice constants and bulk moduli within 0.03% and 5%, respectively. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.085101
  • 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 abstract10.1063/1.3655905
  • Determining the elasticity of materials employing quantum-mechanical approaches from the electronic ground state to the limits of materials stability
    Friák, M. and Hickel, T. and Körmann, F. and Udyansky, A. and Dick, A. and Von Pezold, J. and Ma, D. and Kim, O. and Counts, W.A. and Šob, M. and Gebhardt, T. and Music, D. and Schneider, J. and Raabe, D. and Neugebauer, J.
    Steel Research International 82 (2011)
    Quantum-mechanical (so-called ab initio) calculations have achieved considerable reliability in predicting physical and chemical properties and phenomena. Due to their reliability they are becoming increasingly useful when designing new alloys or revealing the origin of phenomena in existing materials, also because these calculations are able to accurately predict basic material properties without experimental input. Due to the universal validity of fundamental quantum mechanics, not only ground-state properties, but also materials responses to external parameters can reliably be determined. The focus of the present paper is on ab initio approaches to the elasticity of materials. First, the methodology to determine single-crystalline elastic constants and polycrystalline moduli of ordered compounds as well as disordered alloys is introduced. In a second part, the methodology is applied on α-Fe, with a main focus on (i) investigating the influence of magnetism on its elasticity and phase stability and (ii) simulating extreme loading conditions that go up to the theoretical tensile strength limits and beyond. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000264
  • Electronic structure of 1/6〈202̄3〉 partial dislocations in wurtzite GaN
    Kioseoglou, J. and Kalesaki, E. and Lymperakis, L. and Neugebauer, J. and Komninou, Ph. and Karakostas, Th.
    Journal of Applied Physics 109 (2011)
    The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {112̄0} (a-plane) and {11̄00} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6〈202̄3〉 PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found. © 2011 American Institute of Physics.
    view abstract10.1063/1.3569856
  • Electrostatic Interactions between Charged Defects in Supercells
    Freysoldt, C. and Neugebauer, J. and Van de Walle, C.G.
    Advanced Calculations for Defects in Materials: Electronic Structure Methods (2011)
    view abstract10.1002/9783527638529.ch14
  • Electrostatic interactions between charged defects in supercells
    Freysoldt, C. and Neugebauer, J. and Van de Walle, C.G.
    Physica Status Solidi (B) Basic Research 248 (2011)
    Most theoretical calculations for point defects employ the supercell approach. The supercell consists of a few dozen or 100 atoms of the bulk material with a single defect, and is subject to periodic boundary conditions. However, the large density and periodic arrangement of the defects introduce artifacts. They need to be corrected for to extrapolate to the isolated-defect limit. This is particularly important for electrostatic interactions between charged defects, which decay only very slowly (asymptotically like L-1) with increasing supercell lattice constant L. In this paper, we summarize the underlying electrostatics in condensed matter. A novel defect scheme is derived from this analysis. It overcomes limitations of previous schemes with respect to applicability, systematic improvement, and formal justification. Good performance is demonstrated for vacancies in diamond and GaAs. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201046289
  • Erratum: Hydrogen-enhanced local plasticity at dilute bulk H concentrations: The role of H-H interactions and the formation of local hydrides (Acta Materialia (2011) 59 (2969-2980))
    Pezold, J.V. and Lymperakis, L. and Neugebauer, J.
    Acta Materialia 59 (2011)
    view abstract10.1016/j.actamat.2011.05.031
  • 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 (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 abstract10.1016/j.actamat.2011.01.044
  • First-principles study on the interaction of H interstitials with grain boundaries in α- and γ-Fe
    Du, Y.A. and Ismer, L. and Rogal, J. and Hickel, T. and Neugebauer, J. and Drautz, R.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The presence of hydrogen may weaken the bonding of iron atoms at grain boundaries, leading to intergranular embrittlement and thus failure of the bulk material. In this paper, we study the interaction of hydrogen interstitials with close-packed and open grain boundary structures in α- and γ-Fe using density-functional theory. We find that hydrogen accommodation within the grain boundaries strongly depends on the local coordination of the available interstitial sites. Within the open grain boundary structures larger interstitial sites are available, enhancing the solubility as compared to that in the respective bulk phases. The mobility of hydrogen within the investigated grain boundaries is low compared to diffusion in perfect single-crystalline bulk. The grain boundaries do not provide fast diffusion channels for hydrogen, but act as hydrogen traps. Hydrogen that is accumulated within the grain boundaries can lead to a lowering of the critical strain required to fracture the material. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.144121
  • Formation Energies of Point Defects at Finite Temperatures
    Grabowski, B. and Hickel, T. and Neugebauer, J.
    Advanced Calculations for Defects in Materials: Electronic Structure Methods (2011)
    view abstract10.1002/9783527638529.ch15
  • Formation energies of point defects at finite temperatures
    Grabowski, B. and Hickel, T. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 248 (2011)
    The dominant entropy contribution affecting defect concentrations is configurational entropy. Other contributions such as harmonic and anharmonic lattice vibrations are second order effects and computationally expensive to calculate. Therefore, such contributions have been rarely considered in defect investigations. However, to achieve the next accuracy level in defect calculations and thus significantly improve the agreement with experiment, the inclusion of these contributions is critical. In this paper, we present the methods needed to compute highly accurate free energies of point defects from first principles. We demonstrate how to include all relevant free energy contributions up to the melting point. The focus will be on nonmagnetic metals and point defects in the dilute limit. We consider all relevant excitation mechanisms: electronic excitations and ionic vibrations both in the quasiharmonic approximation and explicitly including anharmonicity (i.e., phonon-phonon interaction). Since computing such interactions requires to sample large parts of the phase space, straightforward ab initio based simulations (such as molecular dynamics) are in most cases out of reach even on supercomputers. To overcome this difficulty, a recently developed hierarchical scheme will be presented which allows to coarse grain the configuration space and thus to efficiently calculate anharmonic contributions to defect formation. We discuss the performance and accuracy of the developed methodology for the example of vacancies in aluminum. An important insight is that the entropy of vacancy formation is significantly affected by anharmonicity. We further show that the inclusion of all the aforementioned excitation mechanisms is critical to guarantee an accurate description of thermodynamic properties up to the melting point. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201046302
  • Growth process, characterization, and modeling of electronic properties of coupled InAsSbP nanostructures
    Marquardt, O. and Hickel, T. and Neugebauer, J. and Gambaryan, K.M. and Aroutiounian, V.M.
    Journal of Applied Physics 110 (2011)
    Quaternary III-V InAsSbP quantum dots (QDs) have been grown in the form of cooperative InAsSb/InAsP structures using a modified version of the liquid phase epitaxy. High resolution scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectrometry were used to investigate these so-called nano-camomiles, mainly consisting of a central InAsSb QD surrounded by six InAsP-QDs, that shall be referred to as leaves in the following. The observed QDs average density ranges from 0.8 to 2 × 109 cm -2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the leaves is equal to (6-10) × 109 cm-2 with dimensions of approx. 5 to 40 nm in width and depth. To achieve a first basic understanding of the electronic properties, we have modeled these novel nanostructures using second-order continuum elasticity theory and an eight-band k p model to calculate the electronic structure. Our calculations found a clear localization of hole states in the central InAsSb dot. The localization of electron states, however, was found to be weak and might thus be easily influenced by external electric fields or strain. © 2011 American Institute of Physics.
    view abstract10.1063/1.3624621
  • In situ scanning tunneling microscopy study of selective dissolution of Au3Cu and Cu3Au (0 0 1)
    Renner, F.U. and Eckstein, G.A. and Lymperakis, L. and Dakkouri-Baldauf, A. and Rohwerder, M. and Neugebauer, J. and Stratmann, M.
    Electrochimica Acta 56 (2011)
    We present an electrochemical study of Au3Cu (0 0 1) single crystal surfaces in 0.1 mol dm-3 H2SO4 and 0.1 mol dm-3 H2SO4 + 0.1 mmol dm-3 HCl, and of Cu3Au (0 0 1) in 0.1 mol dm-3 H2SO 4. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential. © 2010 Elsevier Ltd All rights reserved.
    view abstract10.1016/j.electacta.2010.09.061
  • Methodological challenges in combining quantum-mechanical and continuum approaches for materials science applications
    Friák, M. and Hickel, T. and Grabowski, B. and Lymperakis, L. and Udyansky, A. and Dick, A. and Ma, D. and Roters, F. and Zhu, L.-F. and Schlieter, A. and Kühn, U. and Ebrahimi, Z. and Lebensohn, R.A. and Holec, D. and Eckert, J. and Emmerich, H. and Raabe, D. and Neugebauer, J.
    European Physical Journal Plus 126 (2011)
    Multi-methodological approaches combining quantum-mechanical and/or atomistic simulations with continuum methods have become increasingly important when addressing multi-scale phenomena in computational materials science. A crucial aspect when applying these strategies is to carefully check, and if possible to control, a variety of intrinsic errors and their propagation through a particular multimethodological scheme. The first part of our paper critically reviews a few selected sources of errors frequently occurring in quantum-mechanical approaches to materials science and their multi-scale propagation when describing properties of multi-component and multi-phase polycrystalline metallic alloys. Our analysis is illustrated in particular on the determination of i) thermodynamic materials properties at finite temperatures and ii) integral elastic responses. The second part addresses methodological challenges emerging at interfaces between electronic structure and/or atomistic modeling on the one side and selected continuum methods, such as crystal elasticity and crystal plasticity finite element method (CEFEM and CPFEM), new fast Fourier transforms (FFT) approach, and phase-field modeling, on the other side. © Società Italiana di Fisica / Springer-Verlag 2011.
    view abstract10.1140/epjp/i2011-11101-2
  • Orientational ordering of interstitial atoms and martensite formation in dilute Fe-based solid solutions
    Udyansky, A. and Von Pezold, J. and Dick, A. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    We study the thermodynamic stability of dilute C, N, O, B, and C+N interstitial distributions in bcc iron, combining parameter-free density functional theory (DFT) in the generalized gradient approximation and microscopic elasticity theory. This scheme allows us to fully capture the long-range elastic impurity-impurity interactions using moderately sized DFT calculations. Employing this approach we compute temperature-concentration phase diagrams including the effects of external pressure, and provide direct insight into the formation mechanisms of martensite. For all investigated impurities, except for B, tetragonal states are predicted to be preferred even at low impurity concentrations. The preference is shown to originate from a thermodynamically driven orientational ordering of the interstitials. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.184112
  • Quasiparticle band offsets of semiconductor heterojunctions from a generalized marker method
    Mitra, C. and Lange, B. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We propose an approach to reliably calculate band offsets at heterointerfaces. It is based on standard density-functional theory, but overcomes the band-gap problem by including quasiparticle effects at the level of GW theory. Quasiparticle corrections are extracted from a heterojunction superlattice by translating the experimental concept of marker levels into a theoretical approach. The proposed scheme allows one to exploit the robust prediction of relative band positions within GW and therefore does not rely on the transferability of absolute GW corrections for the respective bulk materials. For zinc-blende GaN/AlN (001), we obtain a natural band offset of 0.55 eV, compared to 0.39 eV at the local-density approximation level. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.193304
  • Robustness and optimal use of design principles of arthropod exoskeletons studied by ab initio-based multiscale simulations
    Nikolov, S. and Fabritius, H. and Petrov, M. and Friák, M. and Lymperakis, L. and Sachs, C. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 4 (2011)
    Recently, we proposed a hierarchical model for the elastic properties of mineralized lobster cuticle using (i) ab initio calculations for the chitin properties and (ii) hierarchical homogenization performed in a bottom-up order through all length scales. It has been found that the cuticle possesses nearly extremal, excellent mechanical properties in terms of stiffness that strongly depend on the overall mineral content and the specific microstructure of the mineral-protein matrix. In this study, we investigated how the overall cuticle properties changed when there are significant variations in the properties of the constituents (chitin, amorphous calcium carbonate (ACC), proteins), and the volume fractions of key structural elements such as chitin-protein fibers. It was found that the cuticle performance is very robust with respect to variations in the elastic properties of chitin and fiber proteins at a lower hierarchy level. At higher structural levels, variations of design parameters such as the volume fraction of the chitin-protein fibers have a significant influence on the cuticle performance. Furthermore, we observed that among the possible variations in the cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition due to the smart hierarchical organization of the cuticle design. © 2010 Elsevier Ltd.
    view abstract10.1016/j.jmbbm.2010.09.015
  • Role of spin quantization in determining the thermodynamic properties of magnetic transition metals
    Körmann, F. and Dick, A. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    We propose a combined ab initio-spin quantum Monte Carlo (QMC) approach to compute thermodynamic properties of magnetic materials by first principles. The key to the proposed approach is a mapping of the magnetic long-range system onto an effective, nearest-neighbor quantum Heisenberg model, for which the QMC approach provides a numerically exact solution. The performance of the proposed method is demonstrated for the transition metals Fe, Co, and Ni by computing magnetization shapes, specific heat capacities, and free energies. Spin-quantization effects are found to be critical, even close to T C.© 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.165114
  • Strain-induced effects on the electronic structure and N K-edge ELNES of wurtzite A1N and AlxGa1-xN
    Petrov, M. and Holec, D. and Lymperakis, L. and Neugebauer, J. and Humphreys, C.J.
    Journal of Physics: Conference Series 326 (2011)
    Analysis of the electron energy loss near edge structure (ELNES) provides an experimental tool to probe the density of unoccupied states. Here we present a first principles study on the projected density of states (PDOS) of AlN, GaN, and AlxGa1-xN alloys in order to investigate the impact of strain on the N K-edge ELNES. Uni-axial and bi-axial strain, volume conserving, and bi-axial stress deformation modes are calculated for the whole compositional range from AlN to GaN. Our results show that only the strain along the c-axis has a pronounced impact on the PDOS. Furthermore, we find that bi-axial stress in the basal plane, which is present in pseudomorphic polar heteroepitaxial layers, does not significantly influence the N K-edge spectra. However, strain-induced changes may appear for different deformation modes and/or specimen geometries.
    view abstract10.1088/1742-6596/326/1/012016
  • Temperature-driven phase transitions from first principles including all relevant excitations: The fcc-to-bcc transition in Ca
    Grabowski, B. and Söderlind, P. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The temperature-driven fcc-to-bcc phase transition in calcium is examined by a fully ab initio-based integrated technique including all relevant finite-temperature excitation mechanisms. The approach is based on density-functional-theory calculations with a controlled numerical stability of below 0.5 meV/atom for the electronic, quasiharmonic, and structural excitations and better than 1 meV/atom for the explicitly anharmonic contribution. The latter is achieved by successfully utilizing the recently developed hierarchical upsampled thermodynamic integration using Langevin dynamics method. This approach gives direct access to a numerically highly precise volume- and temperature-dependent free-energy surface and derived properties. It enables us to assign the remaining deviations from experiment to inherent errors of the presently available exchange-correlation functionals. Performing the full analysis with both of the conventional functionals, local density approximation and generalized gradient approximation, we demonstrate that-when considered on an absolute scale-thermodynamic properties are dictated by a strikingly similar free energy vs volume curve. Further, we show that, despite an error in the T=0 K energy difference between the two phases (∼6 meV in the present case), an excellent agreement of the temperature dependence of the Gibbs energy difference with experimentally derived data is feasible. This makes it possible, for instance, to unveil unreliable and possibly erroneous experimental input used in popular thermodynamic databases as we explicitly demonstrate for the isobaric heat capacity of calcium. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.214107
  • The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/23/24/246003
  • 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 (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 abstract10.1016/j.cpc.2010.09.016
  • The relation between shear banding, microstructure and mechanical properties in Mg and Mg-Y alloys
    Sandlöbes, S. and Schestakow, I. and Yi, S. and Zaefferer, S. and Chen, J. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science Forum 690 (2011)
    The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth. © (2011) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/MSF.690.202
  • Theoretical modeling of growth processes, extended defects, and electronic properties of III-nitride semiconductor nanostructures
    Lymperakis, L. and Abu-Farsakh, H. and Marquardt, O. and Hickel, T. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 248 (2011)
    Ab initio based simulations have been proven in the past to be and still are a valuable and indispensable tool in the field of III-nitride semiconductors. They have been successfully used to explain, describe and guide growth and characterization experiments and to address a large variety of material problems at different length scales. In the present report we review on five selected topics which span different length scales, various method developments, and diverse material properties that have been theoretically addressed within the research group "Physics of nitride-based, nanostructured, light emitting devices." Schematic representation of theoretical modeling in synergy with experiment. Left: Ab initio calculated potential energy surface for adatom diffusion on the side facets of a GaN nanowire [theory from L. Lymperakis et al., Phys. Rev. B 79, 241308 (2009), SEM image from T. Aschenbrenner et al., Nanotechnology 20, 075604 (2009)]. Right: Electrostatic potential in a QD calculated by Poisson solver [theory from O. Marquardt et al., J. Appl. Phys. 106, 083707 (2009), HRTEM image from A. Pretorius et al., J. Cryst. Growth 310, 748 (2008)]. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201046511
  • Ab Initio guided design of bcc ternary Mg-Li-X (X=Ca, Al, Si, Zn, Cu) alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 (2010)
    Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fundamental physical properties (like formation energies and elastic constants) of a set of bcc Mg-Li and Mg-Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/p) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg-Li system, an Ashby map containing Y/r versus G/B shows that it is not possible to increase Y/r without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg-Li-X ternaries (X=Ca, Al, Si, Cu, Zn) based on stoichiometric Mg-Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900308
  • Ab initio study of the anomalous volume-composition dependence in Fe-Al alloys
    Friák, M. and Neugebauer, J.
    Intermetallics 18 (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 abstract10.1016/j.intermet.2010.03.014
  • Ab initio study of the solubility and kinetics of hydrogen in austenitic high Mn steels
    Ismer, L. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    Chemical trends for the solubility and diffusivity of hydrogen in austenitic high Mn steels have been studied employing density-functional theory. Considering the dilute limit of hydrogen, we observe strong volumetric effects of substitutional Mn and interstitial carbon on the energetics of a hydrogen atom within the lattice. This volume dependence yields a significant increase both in the solubility and the mobility of the H impurity when comparing Fe1-x Mnx Cy with pure Fe. By means of kinetic Monte Carlo calculations, we also show that H impurities can use Mn percolation chains as efficient diffusion channels. These trends may explain why Mn-rich steels are often observed to be more prone to hydrogen embrittlement than conventional austenitic steels. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.094111
  • Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite
    Elstnerová, P. and Friák, M. and Fabritius, H.O. and Lymperakis, L. and Hickel, T. and Petrov, M. and Nikolov, S. and Raabe, D. and Ziegler, A. and Hild, S. and Neugebauer, J.
    Acta Biomaterialia 6 (2010)
    Arthropoda, which represent nearly 80% of all known animal species, are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional biocomposite based on chitin and proteins. Some groups, such as Crustacea, reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle (Becker et al., Dalton Trans. (2005) 1814). Motivated by these facts, we present a theoretical parameter-free quantum-mechanical study of the phase stability and structural and elastic properties of Mg-substituted calcite crystals. The Mg-substitutions were chosen as examples of states that occur in complex chemical environments typical for biological systems in which calcite crystals contain impurities, the role of which is still the topic of debate. Density functional theory calculations of bulk (Ca,Mg)CO3 were performed employing 30-atom supercells within the generalized gradient approximation as implemented in the Vienna Ab-initio Simulation Package. Based on the calculated thermodynamic results, low concentrations of Mg atoms are predicted to be stable in calcite crystals in agreement with experimental findings. Examining the structural characteristics, Mg additions nearly linearly reduce the volume of substituted crystals. The predicted elastic bulk modulus results reveal that the Mg substitution nearly linearly stiffens the calcite crystals. Due to the quite large size-mismatch of Mg and Ca atoms, Mg substitution results in local distortions such as off-planar tilting of the CO32- group. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2010.07.015
  • 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 (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 abstract10.1016/j.intermet.2010.02.025
  • First-principles study of the thermodynamics of hydrogen-vacancy interaction in fcc iron
    Nazarov, R. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The interaction of vacancies and hydrogen in an fcc iron bulk crystal was studied combining thermodynamic concepts with ab initio calculations and considering various magnetic structures. We show that up to six H atoms can be trapped by a monovacancy. All of the studied point defects (single vacancy, H in interstitial positions, and H-vacancy complexes) cause an anisotropic elastic field in antiferromagnetic fcc iron and significantly change the local and total magnetization of the system. The proposed thermodynamical model allows the determination of the equilibrium vacancy concentration and the concentration of dissolved hydrogen for a given temperature and H chemical potential in the reservoir. For H-rich conditions a dramatic increase in the vacancy concentration in the crystal is found. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.224104
  • Generation and performance of special quasirandom structures for studying the elastic properties of random alloys: Application to Al-Ti
    Von Pezold, J. and Dick, A. and Friák, M. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    The performance of special-quasirandom structures (SQSs) for the description of elastic properties of random alloys was evaluated. A set of system-independent 32-atom-fcc SQS spanning the entire concentration range was generated and used to determine C11, C12, and C44 of binary random substitutional AlTi alloys. The elastic properties of these alloys could be described using the set of SQS with an accuracy comparable to the accuracy achievable by statistical sampling of the configurational space of 3×3×3 (108 atom, C44) and 4×4×4 (256 atom, C11 and C12) fcc supercells, irrespective of the impurity concentration. The smaller system size makes the proposed SQS ideal candidates for the ab initio determination of the elastic constants of random substitutional alloys. The set of optimized SQS is provided. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.094203
  • Hydrogen adsorption on polar ZnO(0001)-Zn: Extending equilibrium surface phase diagrams to kinetically stabilized structures
    Valtiner, M. and Todorova, M. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Combining density-functional-theory calculations and atomistic thermodynamics we study hydrogen adsorption on the Zn-terminated polar ZnO(0001) surface. We extend the concept of equilibrium surface phase diagrams to include kinetically stabilized surface reconstructions by constructing metastable phase diagrams and going beyond the thermodynamic limit. Under these conditions we find a monolayer H coverage to be extremely favorable on triangular-shaped reconstructions due to a simultaneous protonation of step-edge oxygen atoms and surface terminating Zn atoms. Experimental situations, which realize hydrogen chemical potentials outside the stability range of the H2 molecule are discussed. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.165418
  • L21-ordered Fe-Al-Ti alloys
    Krein, R. and Friak, M. and Neugebauer, J. and Palm, M. and Heilmaier, M.
    Intermetallics 18 (2010)
    Fe-Al-Ti alloys with the ordered L21-structure (Heusler phase) belong to the few Fe-Al-based alloys which show comparably high-strength at high temperatures, e.g. at 800 °C. However, like many other high-temperature materials based on intermetallics they show limited ductility even at high temperatures. In order to further explore the possibilities in increasing their strength and ductility, alloys with four different microstructures, i.e. single-phase L21, L21 with incoherent precipitates of TiB2 or Laves phase, and coherent L21 + A2, were produced. Also, the influence of alloying with Cr and B has been investigated. The Young's modulus of Fe-25Al-20Ti-4Cr (at.%) in dependence of temperature up to 900 °C has been determined and results of the compressive flow stress, creep strength and brittle-toductile transition temperatures (BDTT) are summarised and compared to those of binary Fe3Al (D03), Fe-Al-Ti-based alloys, and some commercial alloys. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2009.12.036
  • Native and hydrogen-containing point defects in Mg3 N 2: A density functional theory study
    Lange, B. and Freysoldt, C. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    The formation energy and solubility of hydrogen in magnesium nitride bulk (antibixbyite Mg3 N2) have been studied employing density functional theory in the generalized gradient approximation. The effect of doping and the presence of native defects and complex formation have been taken into account. Our results show that magnesium nitride is a nearly defect-free insulator with insignificant hydrogen-storage capacity. Based on this insight we derive a model that highlights the role of the formation and presence of the parasitic Mg3 N2 inclusions in the activation of p -doped GaN in optoelectronic devices. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.224109
  • Phytochrome as Molecular Machine: Revealing Chromophore Action during the Pfr -> Pr Photoconversion by Magic-Angle Spinning NMR Spectroscopy
    Rohmer, T. and Lang, C. and Bongards, C. and Gupta, K. B. S. S. and Neugebauer, J. and Hughes, J. and Gartner, W. and Matysik, J.
    Journal of the American Chemical Society 132 (2010)
    The cyanobacterial phytochrome Cph1 can be photoconverted between two thermally stable states, Pr and Pfr. The photochemically induced Pfr -> Pr back-reaction has been followed at low temperature by magic-angle spinning (MAS) NMR spectroscopy, allowing two intermediates, Lumi-F and Meta-F, to be trapped. Employing uniformly (13)C- and (15)N-labeled open-chain tetrapyrrole chromophores, all four states Pfr, Lumi-F, Meta-F, and Pr-have been structurally characterized. In the first step, the double bond. photoisomerization forming Lumi-F occurs. The second step, the transformation to Meta-F, is driven by the release of the mechanical tension. This process leads to the break of the hydrogen bond of the ring D nitrogen to Asp-207 and triggers signaling. The third step is protonically driven allowing the hydrogen-bonding interaction of the ring D nitrogen to be restored. Compared to the forward reaction, the order of events is changed, probably caused by the different properties of the hydrogen bonding partners of N24, leading to the directionality of the photocycle.
    view abstract10.1021/ja9108616
  • Plane-wave implementation of the real-space k ṡ p formalism and continuum elasticity theory
    Marquardt, O. and Boeck, S. and Freysoldt, C. and Hickel, T. and Neugebauer, J.
    Computer Physics Communications 181 (2010)
    In this work we demonstrate how second-order continuum elasticity theory and an eight-band k ṡ p model can be implemented in an existing density functional theory (DFT) plane-wave code. The plane-wave formulation of these two formalisms allows for an accurate and efficient description of elastic and electronic properties of semiconductor nanostructures such as quantum dots, wires, and films. Gradient operators that are computationally expensive in a real-space formulation can be calculated much more efficiently in reciprocal space. The accuracy can be directly controlled by the plane-wave cutoff. Furthermore, minimization schemes typically available in plane-wave DFT codes can be applied straightforwardly with only a few modifications to a plane-wave formulation of these continuum models. As an example, the elastic and electronic properties of a III-nitride quantum dot system are calculated. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cpc.2009.12.009
  • Rescaled Monte Carlo approach for magnetic systems: Ab initio thermodynamics of bcc iron
    Körmann, F. and Dick, A. and Hickel, T. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    A combined ab initio approach to calculate the thermodynamic properties of bcc iron including vibrational, electronic, and magnetic free-energy contributions is derived. Special emphasis is placed on the magnetic contribution that is obtained using the frozen-magnon approach combined with Monte Carlo (MC) calculations. The importance of spin quantum-mechanical effects has been studied for magnetically nonfrustrated model systems by comparing classical and quantum MC. Based on this analysis, we propose a rescaling scheme which allows an approximate ad hoc inclusion of the quantum effects into the classical MC simulations. The rescaled MC scheme is found to be robust with respect to the specific magnetic configuration and the lattice type and is therefore expected to yield an approximate yet reliable description of the magnetic contribution for cases where quantum MC calculations are not practical. Applying the method to bcc iron and combining the magnetic, vibronic and electronic contributions, we find an excellent agreement with experimental data for the heat capacity and free energy, both, below and above the Curie temperature. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.134425
  • Revealing the design principles of high-performance biological composites using Ab initio and multiscale simulations: The example of lobster cuticle
    Nikolov, S. and Petrov, M. and Lymperakis, L. and Friák, M. and Sachs, C. and Fabritius, H.-O. and Raabe, D. and Neugebauer, J.
    Advanced Materials 22 (2010)
    Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantummechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness. (Figure Presented) © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,.
    view abstract10.1002/adma.200902019
  • 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 (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 abstract10.1016/j.calphad.2010.01.004
  • Using Ab initio calculations in designing bcc MgLi-X alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 (2010)
    Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201000225
  • ab initio calculations

  • defects

  • electronic structure

  • epitaxial growth

  • grain boundaries

  • modelling and simulation

  • molecular dynamics

  • solidification

  • surfaces

  • thermodynamics

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