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  • 1.
    Abrikosov, Igor A.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Steneteg, Peter
    Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering.
    Hultberg, Lasse
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Yu Mosyagin, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Department of Theoretical Physics and Quantum Technologies, National Research, Technological University MISiS, Moscow, Russia.
    Lugovskoy, Andrey V.
    Department of Theoretical Physics and Quantum Technologies, National Research, Technological University MISiS, Russia.
    Barannikova, Svetlana A.
    Institute of Strength Physics and Materials Science, Siberian Branch of Russian Academy of Science, Tomsk, Russia; Department of Physics and Engineering, Tomsk State University, Tomsk, Russia.
    Finite Temperature, Magnetic, and Many-Body Effects in Ab Initio Simulations of Alloy Thermodynamics2013In: TMS2013 Supplemental Proceedings, John Wiley & Sons, 2013, 617-626 p.Chapter in book (Refereed)
    Abstract [en]

    Ab initio electronic structure theory is known as a useful tool for prediction of materials properties. However, majority of simulations still deal with calculations in the framework of density functional theory with local or semi-local functionals carried out at zero temperature. We present new methodological solution.s, which go beyond this approach and explicitly take finite temperature, magnetic, and many-body effects into account. Considering Ti-based alloys, we discuss !imitations of the quasiharmonic approximation for the treatment of lattice vibrations, and present an accurate and easily extendable method to calculate free ,energies of strongly anharmonic solids. We underline the necessity to going beyond the state-of-the-art techniques for the determination of effective cluster interactions in systems exhibiting mctal-to-insulator transition, and describe a unified cluster expansion approach developed for this class of materials. Finally, we outline a first-principles method, disordered local moments molecular dynamics, for calculations of thermodynamic properties of magnetic alloys, like Cr1-x,.AlxN, in their high-temperature paramagnetic state. Our results unambiguously demonstrate importance of finite temperature effects in theoretical calculations ofthermodynamic properties ofmaterials.

  • 2.
    Abrikosov, Igor
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. NUST MISIS, Russia.
    Ponomareva, A. V.
    NUST MISIS, Russia.
    Nikonov, A. Yu.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Zharmukhambetova, A. M.
    National Research Tomsk State University, Russia.
    Mosyagin, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. NUST MISIS, Russia.
    Lugovskoy, A. V.
    NUST MISIS, Russia.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Lind, Hans
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Dmitriev, A. I.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Barannikova, S. A.
    National Research Tomsk State University, Russia; SB RAS, Russia.
    Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys2015In: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 35, no 1, 42-48 p.Article in journal (Refereed)
    Abstract [en]

    We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti-V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti-V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti-V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression.

  • 3.
    Belonoshko, A B
    et al.
    Royal Institute of Technology.
    Derlet, P M
    Paul Scherrer Institute.
    Mikhaylushkin, Arkady
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Burakovsky, L
    Los Alamos National Laboratory.
    Swift, D C
    Los Alamos National Laboratory.
    Johansson, B
    Royal Institute of Technology.
    Quenching of bcc-Fe from high to room temperature at high-pressure conditions: a molecular dynamics simulation2009In: NEW JOURNAL OF PHYSICS, ISSN 1367-2630, Vol. 11, 093039- p.Article in journal (Refereed)
    Abstract [en]

    The new high-temperature (T), high-pressure (P), body-centered cubic (bcc) phase of iron has probably already been synthesized in recent diamond anvil cell (DAC) experiments (Mikhaylushkin et al 2007 Phys. Rev. Lett. 99 165505). These DAC experiments on iron revealed that the high-PT phase on quenching transforms into a mixture of close-packed phases. Our molecular dynamics simulation and structural analysis allow us to provide a probable interpretation of the experiments. We show that quenching of the high-PT bcc phase simulated with the embedded-atom model also leads to the formation of the mixture of close-packed phases. Therefore, the assumption of the stability of the high-PT bcc iron phase is consistent with experimental observation.

  • 4.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Thermal properties of materials from first principles2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations.

    The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems.

    Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.

    List of papers
    1. Lattice dynamics of anharmonic solids from first principles
    Open this publication in new window or tab >>Lattice dynamics of anharmonic solids from first principles
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 18, 180301- p.Article in journal (Refereed) Published
    Abstract [en]

    An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.

    Place, publisher, year, edition, pages
    American Physical Society, 2011
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-72810 (URN)10.1103/PhysRevB.84.180301 (DOI)000297099800001 ()
    Note
    Funding Agencies|Swedish Research Council (VR)||Available from: 2011-12-09 Created: 2011-12-08 Last updated: 2017-12-08
    2. Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures
    Open this publication in new window or tab >>Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures
    Show others...
    2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, 125406-1-125406-7 p.Article in journal (Refereed) Published
    Abstract [en]

    We study the structural properties of ultrathin AgxPd1−x films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principles calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.

    Place, publisher, year, edition, pages
    APS, 2008
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-44362 (URN)10.1103/PhysRevB.77.125406 (DOI)76396 (Local ID)76396 (Archive number)76396 (OAI)
    Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13Bibliographically approved
    3. Charge Redistribution Mechanisms of Ceria Reduction
    Open this publication in new window or tab >>Charge Redistribution Mechanisms of Ceria Reduction
    2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 13, 135504- p.Article in journal (Refereed) Published
    Abstract [en]

    Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.

    Place, publisher, year, edition, pages
    American Physical Society, 2012
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-76952 (URN)10.1103/PhysRevLett.108.135504 (DOI)000302173000017 ()
    Note
    Funding Agencies|Swedish Research Council, (VR)||Swedish Government||Available from: 2012-05-02 Created: 2012-04-27 Last updated: 2017-12-07
    4. Tuning ionic conductivity in ceria by volume optimization
    Open this publication in new window or tab >>Tuning ionic conductivity in ceria by volume optimization
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Effect of volume change on the ionic conductivity in ceria has been studied in the framework of the density functional theory. We show that properly controlling external conditions one can treat the lattice constant of ceria as an adjustable parameter and change the topology of the energy landscape for the oxygen ion diffusion. We reveal the existence of the narrow range of lattice parameters, which optimize the ionic conductivity in ceria.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-78752 (URN)
    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2012-06-20Bibliographically approved
    5. Temperature dependent effective potential method for accurate free energy calculation of solids
    Open this publication in new window or tab >>Temperature dependent effective potential method for accurate free energy calculation of solids
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-78753 (URN)
    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2012-06-20Bibliographically approved
    6. Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
    Open this publication in new window or tab >>Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
    Show others...
    2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 9Article in journal (Refereed) Published
    Abstract [en]

    Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

    Place, publisher, year, edition, pages
    American Physical Society, 2013
    National Category
    Natural Sciences Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-78754 (URN)10.1103/PhysRevB.87.094114 (DOI)000316791600001 ()
    Note

    Funding Agencies|Swedish Research Council|621-2008-5535621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs|10-0026|project Designed Multicomponent Coatings (MultiFilms)||Erasmus Mundus doctoral program DocMase||Ministry of Education and Science of the Russian Federation within the framework of Program Research and Pedagogical Personnel for Innovative Russia|14.B37.21.089010.09.2012|

    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2017-12-07Bibliographically approved
  • 5.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Temperature-dependent effective third-order interatomic force constants from first principles2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 14Article in journal (Refereed)
    Abstract [en]

    The temperature-dependent effective potential (TDEP) method is generalized beyond pair interactions. The second- and third-order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the mode Grüneisen parameters for Si. We show that the extension of TDEP to a higher order allows for an efficient calculation of the phonon life time, in Si as well as in ε-FeSi; a system that exhibits anomalous softening with temperature.

  • 6.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Lattice dynamics of anharmonic solids from first principles2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 18, 180301- p.Article in journal (Refereed)
    Abstract [en]

    An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.

  • 7.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Broido, David A.
    Boston Coll, MA 02467 USA.
    Phonon thermal transport in Bi2Te3 from first principles2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 13, 134309- p.Article in journal (Refereed)
    Abstract [en]

    We present first-principles calculations of the thermal and thermal transport properties of Bi2Te3 that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as Bi2Te3. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, kappa(l), calculated using the AIMD approach nicely matches measured values, showing better agreement than the kappa(l) obtained using temperature-independent IFCs. A significant contribution to kappa(l) from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.

  • 8.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Skorodumova, N V
    Royal Institute Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Charge Redistribution Mechanisms of Ceria Reduction2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 13, 135504- p.Article in journal (Refereed)
    Abstract [en]

    Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.

  • 9.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Skorodumova, N. V.
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Tuning ionic conductivity in ceria by volume optimizationManuscript (preprint) (Other academic)
    Abstract [en]

    Effect of volume change on the ionic conductivity in ceria has been studied in the framework of the density functional theory. We show that properly controlling external conditions one can treat the lattice constant of ceria as an adjustable parameter and change the topology of the energy landscape for the oxygen ion diffusion. We reveal the existence of the narrow range of lattice parameters, which optimize the ionic conductivity in ceria.

  • 10.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Steneteg, Peter
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Temperature dependent effective potential method for accurate free energy calculation of solidsManuscript (preprint) (Other academic)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.

  • 11.
    Hellman, Olle
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Steneteg, Peter
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Temperature dependent effective potential method for accurate free energy calculations of solids2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 10Article in journal (Refereed)
    Abstract [en]

    We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for 4He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.

  • 12.
    Isaeva, L.
    et al.
    Uppsala University, Sweden.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. CALTECH, CA 91125 USA.
    Lashley, J. C.
    Los Alamos National Lab, NM 87545 USA.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Eriksson, O.
    Uppsala University, Sweden.
    Dynamic stabilization of cubic AuZn2015In: MATERIALS TODAY-PROCEEDINGS, ELSEVIER SCIENCE BV , 2015, Vol. 2, 569-572 p.Conference paper (Refereed)
    Abstract [en]

    A recently developed temperature-dependent effective potential method is employed to study the martensitic phase transformation in AuZn. This method is based on ab initio molecular dynamics and allows to obtain finite-temperature lattice vibrational properties. We show that the transversal acoustic TA(2)[110] mode associated with the phase transition is stabilized at 300 K. Temperature evolution of single-phonon dynamic structure factor at the wave vector q= 1/3[1,1,0], associated with phonon softening and Fermi surface nesting, was also studied. (C) 2015 The Authors. Published by Elsevier Ltd.

  • 13.
    Kerdsongpanya, Sit
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, USA.
    Sun, Bo
    Department of Mechanical Engineering, National University of Singapore, Block EA, Singapore..
    Koh, Yee Kan
    Department of Mechanical Engineering, National University of Singapore, Block EA, Singapore..
    Van Nong, Ngo
    Dept. of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Denmark.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Simak, Sergei I.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Phonon Thermal Conductivity of Scandium Nitride for Thermoelectric Applications from First-Principles CalculationsManuscript (preprint) (Other academic)
    Abstract [en]

    The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since most technologies either require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity with the effect of microstructure. This is based on ab initio description that includes the temperature dependence of the interatomic force constants, and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions with the experimental data by Time Domain Thermoreflectance (TDTR). Our results show a trend of reduction in lattice thermal conductivity with decreasing grain size, with good agreement between the theoretical model and experimental data. There results suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN. More importantly, we provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on first-principles calculations.

  • 14.
    Li, C.W.
    et al.
    Oak Ridge National Lab, TN 37831 USA .
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ma, J.
    Oak Ridge National Lab, TN 37831 USA .
    May, A.F.
    Oak Ridge National Lab, TN 37831 USA .
    Cao, H.B.
    Oak Ridge National Lab, TN 37831 USA .
    Chen, X.
    Oak Ridge National Lab, TN 37831 USA .
    Christianson, A.D.
    Oak Ridge National Lab, TN 37831 USA .
    Ehlers, G.
    Oak Ridge National Lab, TN 37831 USA .
    Singh, D.J.
    Oak Ridge National Lab, TN 37831 USA .
    Sales, B.C.
    Oak Ridge National Lab, TN 37831 USA .
    Delaire, O.
    Oak Ridge National Lab, TN 37831 USA .
    Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 17, 175501- p.Article in journal (Refereed)
    Abstract [en]

    The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.

  • 15.
    Marten, Tobias
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Ruban, Andrei V.
    Department of Materials Science and Engineering Royal Institute of Technology, Stockholm.
    Olovsson, Weine
    Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
    Kramer, Charlotte
    Department of Physics and Nanotechnology Aalborg University, Aalborg, Denmark.
    Godowski, Jan P.
    Institute of Experimental Physics, University of Wroclaw, 50-204 Wroclaw, Poland.
    Bech, Lone
    Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark.
    Li, Zheshen
    Institute of Storage Ring Facilities, University of Aarhus, DK-8000 Aarhus, Denmark.
    Onsgaard, Jens
    Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Double-segregation effect in AgxPd1−x /Ru(0001) thin film nanostructures2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, 125406-1-125406-7 p.Article in journal (Refereed)
    Abstract [en]

    We study the structural properties of ultrathin AgxPd1−x films on top of a Ru(0001) substrate. Effective interatomic interactions, obtained from first-principles calculations, have been used in Monte Carlo simulations to derive the distribution of the alloy components in a four-monolayer (4-ML) Ag-Pd film. Though Ag-Pd alloys show complete solubility in the bulk, the thin film geometry leads to a pronounced segregation between Ag and Pd atoms with a strong preference of Ag atoms toward the surface and Pd atoms toward the interface. The theoretical prediction of this double-segregation effect is strongly supported by photoelectron spectroscopy experiments carried out for 4-ML thin films. We also show, in an additional experiment, that even in the case where initially 1 ML Ag is buried under 6 ML Pd, the whole Ag ML segregates to the surface.

  • 16.
    Mei, A. B.
    et al.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. CALTECH, CA 91125 USA.
    Wireklint, N.
    Chalmers, Sweden.
    Schlepuetz, C. M.
    Argonne National Lab, IL 60439 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Rockett, A.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Dynamic and structural stability of cubic vanadium nitride2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 5, 054101- p.Article in journal (Refereed)
    Abstract [en]

    Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

  • 17.
    Miranda, Alonso L.
    et al.
    CINVESTAV, Mexico; University of Liege, Belgium.
    Xu, Bin
    University of Liege, Belgium; University of Arkansas, AR 72701 USA; University of Arkansas, AR 72701 USA.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Romero, Aldo H.
    CINVESTAV, Mexico; W Virginia University, WV 26506 USA.
    Verstraete, Matthieu J.
    University of Liege, Belgium.
    Ab initio calculation of the thermal conductivity of indium antimonide2014In: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 29, no 12, 124002- p.Article in journal (Refereed)
    Abstract [en]

    A theoretical study based on the density functional theory and the temperature-dependent effective potential method is performed to analyze the changes in the phonon band structure as a function of temperature for indium antimonide. In particular, we show changes in the thermal expansion coefficient and the thermal resistivity that agree rather well with experimental measurements. From the theoretical side, we show a weak dependence with respect to the chosen thermostat used to obtain the inter-atomic force constants, which strengthens our conclusions.

  • 18.
    Nilsson, Johan O.
    et al.
    KTH Royal Institute Technology, Sweden.
    Yu Vekilova, Olga
    KTH Royal Institute Technology, Sweden.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Klarbring, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Skorodumova, Natalia V.
    KTH Royal Institute Technology, Sweden; Uppsala University, Sweden.
    Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 2, 024102- p.Article in journal (Refereed)
    Abstract [en]

    A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.

  • 19.
    Palumbo, M.
    et al.
    Ruhr University of Bochum, Germany .
    Burton, B.
    NIST, MD 20899 USA .
    Costa e Silva, A.
    EEIMVR UFF, Brazil .
    Fultz, B.
    CALTECH, CA 91125 USA .
    Grabowski, B.
    Max Planck Institute Eisenforsch GmbH, Germany .
    Grimvall, G.
    KTH Royal Institute Technology, Sweden .
    Hallstedt, B.
    Rhein Westfal TH Aachen, Germany .
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Lindahl, B.
    KTH Royal Institute Technology, Sweden .
    Schneider, A.
    Vallourec Deutschland GmbH, Germany .
    Turchi, P. E. A.
    Lawrence Livermore National Lab, CA USA .
    Xiong, W.
    University of Wisconsin, WI 53706 USA .
    Thermodynamic modelling of crystalline unary phases2014In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 251, no 1, 14-32 p.Article in journal (Refereed)
    Abstract [en]

    Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy G(T) of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first-principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to G(T), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first-principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to G(T) are treated on the basis of the density of states N(E) for metals, with emphasis on effects at high T. Further, we consider G(T) below 300K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, G(T) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams. (C) 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

  • 20.
    Romero, A. H.
    et al.
    W Virginia University, WV 26506 USA; Max Planck Institute Mikrostrukturphys, Germany.
    Gross, E. K. U.
    Max Planck Institute Mikrostrukturphys, Germany.
    Verstraete, M. J.
    University of Liege, Belgium; University of Liege, Belgium.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. CALTECH, CA 91125 USA.
    Thermal conductivity in PbTe from first principles2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 21, 214310- p.Article in journal (Refereed)
    Abstract [en]

    We investigate the harmonic and anharmonic contributions to the phonon spectrum of lead telluride and perform a complete characterization of how thermal properties of PbTe evolve as temperature increases. We analyze the thermal resistivitys variationwith temperature and clarify misconceptions about existing experimental literature. The resistivity initially increases sublinearly because of phase space effects and ultra strong anharmonic renormalizations of specific bands. This effect is the strongest factor in the favorable thermoelectric properties of PbTe, and it explains its limitations at higher T. This quantitative prediction opens the prospect of phonon phase space engineering to tailor the lifetimes of crucial heat carrying phonons by considering different structure or nanostructure geometries. We analyze the available scattering volume between TO and LA phonons as a function of temperature and correlate its changes to features in the thermal conductivity.

  • 21.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia; Tomsk State University, Russia.
    Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 9, 094305- p.Article in journal (Refereed)
    Abstract [en]

    We revisit the color-diffusion algorithm [Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k(NE) increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k(NE)(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study.

  • 22.
    Shulumba, Nina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, The Institute of Technology.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Mozafari, Elham
    Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
    Steneteg, Peter
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab initio molecular dynamics2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 17, 174108- p.Article in journal (Refereed)
    Abstract [en]

    We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics with the temperature-dependent effective potential method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.

  • 23.
    Shulumba, Nina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Raza, Zamaan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Influence of vibrational free energy on the phase stability of alloys from first principles2015Manuscript (preprint) (Other academic)
    Abstract [en]

    We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and pressure for substitutional alloys from first principles. Taking the example of the technologically important hard coating alloy Ti1-xAlxN as an example, we investigate the effect on the vibrational free energy of substituting Ti for other group IV elements. By constructing the phase diagrams for these three alloys, we show why Zr1-xAlxN and Hf1-xAlxN are so difficult to experimentally synthesise in a metastable solid solution: both have solubility regions that span only a small low-AlN concentration range at temperatures above 1500 K. Moreover, Hf1-xAlxN is dynamically unstable at low temperatures and across most of the concentration range. We also show the chemical and thermal expansion effects dominate mass disorder in the Gibbs free energy of mixing.

  • 24.
    Shulumba, Nina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Raza, Zamaan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany.
    Barrirero, Jennifer
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. Functional Materials, Saarland University, Campus D3 3, Saarbrücken, Germany.
    Mücklich, Frank
    Functional Materials, Saarland University, Campus D3 3, Saarbrücken, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Anharmonicity changes the solid solubility of an alloy at high temperatures2015Manuscript (preprint) (Other academic)
    Abstract [en]

    We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1−xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstrate that the anharmonic contribution and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1−xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 K to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.

  • 25.
    Shulumba, Nina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Rogström, Lina
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Raza, Zamaan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia / LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Temperature-dependent elastic properties of Ti1−xAlxN alloys2015In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 107, no 23Article in journal (Refereed)
    Abstract [en]

    Ti1−xAlxN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C11 decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.

  • 26.
    Shulumba, Nina
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. University of Saarland, Germany.
    Raza, Zamaan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Materials Modeling and Development Laboratory, NUST “MISIS”, Moscow, Russia; LACOMAS Laboratory, Tomsk State University, Tomsk, Russia.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, 104305Article in journal (Refereed)
    Abstract [en]

    Wurtzite aluminium nitride is a technologically important wide band gap semiconductor with an unusually high thermal conductivity, used in optical applications and as a heatsink substrate. Many of its properties depend on an accurate description of its lattice dynamics, which have thus far only been captured in the quasiharmonic approximation. In this work, we demonstrate that anharmonicity has a considerable impact on its phase stability and transport properties, since anharmonicity is much stronger in the rocksalt phase. We compute a pressure-temperature phase diagram of AlN, demonstrating that the rocksalt phase is stabilised by increasing temperature, with respect to the wurtzite phase. We demonstrate that including anharmonicity, we can recover the thermal conductivity of the wurtzite phase (320 Wm−1K−1 under ambient conditions), and compute the hitherto unknown thermal conductivity of the rocksalt phase (96 Wm−1K−1). We also show that the electronic band gap decreases with temperature. These findings provide further evidence that anharmonic effects cannot be ignored in high temperature applications.

  • 27.
    Steneteg, Peter
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Vekilova, Olga
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Shulumba, Nina
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 9Article in journal (Refereed)
    Abstract [en]

    Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

1 - 27 of 27
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