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  • 1.
    Abrikosov, Igor
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Kissavos, Andreas E.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Liot, Francois
    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, 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.
    Peil, O.
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden.
    Ruban, A. V.
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden.
    Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 1, p. 014434-Article in journal (Refereed)
    Abstract [en]

    We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.

  • 2.
    Abrikosov, Igor
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Marten, Tobias
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Olsson, P.
    Uppsala University.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Electronic theory of materials properties: from fundamental understanding towards materials design2006Report (Other academic)
    Abstract [en]

           

  • 3.
    Abrikosov, Igor
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Mikhaylushkin, Arkady
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Asker, Christian
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Ab initio modeling of alloying effects at extreme conditions2008In: Second EuroMinScI Conference,2008, France: ESF , 2008Conference paper (Refereed)
    Abstract [en]

       

  • 4.
    Alling, Björn
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Khatibi, Ali
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Theoretical investigation of cubic B1-like and corundum (Cr1−xAlx)2O3 solid solutionsManuscript (preprint) (Other academic)
    Abstract [en]

    First-principles calculations are employed to investigate the stability and properties of cubic rock-salt like (Cr1−xAlx)2O3 solid solutions, stabilized by metal site vacancies as recently reported experimentally. It is demonstrated that the metal site vacancies can indeed be ordered in a way that gives rise to a favorable coordination of all O atoms in the lattice. B1-like structures with ordered and disordered metal site vacancies are studied for (Cr0.5Al0.5)2O3 and found to a have cubic lattice spacing close to the values reported experimentally, in contrast to fluorite-like and perovskite structures. The obtained B1-like structures are higher in energy than corundum solid solutions for all compositions, but with an energy offset per atom similar to other metastable systems possible to synthesize with physical vapor deposition techniques. The obtained electronic structures show that the B1-like systems are semiconducting although with smaller band gaps than the corundum structure.

  • 5.
    Alling, Björn
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Khatibi, Ali
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Theoretical investigation of cubic B1-like and corundum (Cr1−xAlx)2O3 solid solutions2013In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 31, no 3Article in journal (Refereed)
    Abstract [en]

    First-principles calculations are employed to investigate the stability and properties of cubic rock-salt-like (Cr1−xAlx)2O3 solid solutions, stabilized by metal site vacancies as recently reported experimentally. It is demonstrated that the metal site vacancies can indeed be ordered in a way that gives rise to a suitable fourfold coordination of all O atoms in the lattice. B1-like structures with ordered and disordered metal site vacancies are studied for (Cr0.5Al0.5)2O3 and found to have a cubic lattice spacing close to the values reported experimentally, in contrast to fluorite-like and perovskite structures. The obtained B1-like structures are higher in energy than corundum solid solutions for all compositions, but with an energy offset per atom similar to other metastable systems possible to synthesize with physical vapor deposition techniques. The obtained electronic structures show that the B1-like systems are semiconducting although with smaller band gaps than the corundum structure.

  • 6.
    Alling, Björn
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ruban, A. V.
    Royal Institute of Technology, Stockholm.
    Karimi, A.
    Swiss Federal Institute of Technology Lausanne (EPFL).
    Peil, O. E.
    Uppsala University.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 045123Article in journal (Refereed)
    Abstract [en]

    Wedescribe an efficient first-principles method that can be used tocalculate mixing enthalpies of transition metal nitrides with B1 structureand substitutional disorder at the metal sublattice. The technique isbased on the density functional theory. The independent sublattice modelis suggested for the treatment of disorder-induced local lattice relaxationeffects. It supplements the description of the substitutional disorder withinthe coherent potential approximation. We demonstrate the excellent accuracy ofthe method by comparison with calculations performed by means ofthe projector augumented wave method on supercells constructed as specialquasirandom structures. At the same time, the efficiency of thetechnique allows for total energy calculations on a very finemesh of concentrations which enables a reliable calculation of thesecond concentration derivative of the alloy total energy. This isa first step towards first-principles predictions of concentrations and temperatureintervals where the alloy decomposition proceeds via the spinodal mechanism.We thus calculate electronic structure, lattice parameter, and mixing enthalpiesof the quasibinary alloy c-Ti1−xAlxN. The lattice parameter follows Vegard'slaw at low fractions of AlN but deviates increasingly withincreasing Al content. We show that the asymmetry of themixing enthalpy and its second concentration derivative is associated withsubstantial variations of the electronic structure with alloy composition. Thephase diagram is constructed within the mean-field approximation.

  • 7.
    Andersson, D.A.
    et al.
    KTH.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Johansson, B.
    KTH, och Uppsala universitet.
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Skorodumova, N.V.
    Uppsala universitet.
    Modeling of CeO2, Ce2O3, and CeO2−x in the LDA+U formalism2007In: Physical review. B, Condensed matter and materials physics, Vol. 75, p. 035109-1-035109-6Article in journal (Refereed)
  • 8.
    Andersson, D.A.
    et al.
    KTH.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Skorodumova, N.V.
    Uppsala universitet.
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Johansson, B.
    KTH, och Uppsala universitet.
    Redox properties of CeO2-MO2 (M=Ti, Zr, Hf, or Th) solid solutions from first principles calculations2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, p. 31909-1-31919-3Article in journal (Refereed)
  • 9.
    Andersson, D.A.
    et al.
    KTH, Stockholm.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Skorodumova, N.V.
    Uppsala universitet.
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Johansson, B.
    KTH, Stockholm.
    Theoretical study of CeO2 doped with tetravalent ions2007In: Physical review. B, Condensed matter and materials physics, Vol. 76, p. 1741191-17411910Article in journal (Refereed)
    Abstract [en]

      

  • 10.
    Andersson, David A.
    et al.
    Royal Institute of Technology.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Skorodumova, Natalia V.
    Uppsala University.
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Johansson, Börje
    Uppsala University.
    Optimization of ionic conductivity in doped ceria2006In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 103, p. 3518-3521Article in journal (Refereed)
  • 11.
    Arapan, Sergiu
    et al.
    National Institute Mat Science, Japan; Moldavian Academic Science, Moldova.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Skorodumova, Natalia V.
    KTH Royal Institute Technology, Sweden; Uppsala University, Sweden.
    Volume-dependent electron localization in ceria2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, p. 125108-Article in journal (Refereed)
    Abstract [en]

    We have performed a numerical study of the process of electron localization in reduced ceria. Our results show that different localized charge distributions can be attained in a bulk system by varying the lattice parameter. We demonstrate that the effect of electron localization is mainly determined by lattice relaxation and an accurate account for the effects of electronic correlation is necessary to achieve localized charge distribution.

  • 12.
    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, p. 093039-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.

  • 13.
    Belonoshko, A.B.
    et al.
    Department of Material Science and Engineering The Royal Institute of Technology.
    Burakovsky, L.
    Theoretical Division Los Alamos National Laboratory.
    Chen, S.P.
    Theoretical Division The Royal Institute of Technology.
    Johansson, B.
    Department of Material Science and Engineering The Royal Institute of Technology.
    Mikhaylushkin, Arkady
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Preston, D.L.
    Physics Division Los Alamos National Laboratory.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Swift, D.C.
    Physics Division Los Alamos National Laboratory.
    Molybdenum at High Pressure and Temperature: Melting from Another Solid Phase2008In: Physical review letters / publ. by the American Physical Society, Vol. 100, no 13, p. 135701-1-135701-4Article in journal (Refereed)
    Abstract [en]

     The Gibbs free energies of bcc and fcc Mo are calculated from first principles in the quasiharmonic approximation in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K. It is found that Mo, stable in the bcc phase at low temperatures, has lower free energy in the fcc structure than in the bcc phase at elevated temperatures. Our density-functional-theory-based molecular dynamics simulations demonstrate that fcc melts at higher than bcc temperatures above 1.5 Mbar. Our calculated melting temperatures and bcc-fcc boundary are consistent with the Mo Hugoniot sound speed measurements. We find that melting occurs at temperatures significantly above the bcc-fcc boundary. This suggests an explanation of the recent diamond anvil cell experiments, which find a phase boundary in the vicinity of our extrapolated bcc-fcc boundary.

  • 14.
    Belonoshko, A.B.
    et al.
    The Royal Institute of Technology, Stockholm.
    Davis, S.
    The Rooyal Institute of Technology, Stockholm.
    Rosengren, A.
    AlbaNova University Center The Royal Institute of Technology, Stockholm.
    Ahuja, R.
    The AlbaNova University Center The oyal Institute of Technology, Stockholm.
    Johansson, B.
    Dept of Physics Uppsala University.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Burakovsky, L.
    Theoretical Division Los Alamos National Laboatoy, New Mexico, USA.
    Preston, D.L.
    Physics Division Los Alamos National Laboratory, New Mexico, USA.
    Xenon melting: Density functional theory versus diamond anvil cell experiments2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, p. 054114-1-054114-4Article in journal (Refereed)
    Abstract [en]

       

  • 15.
    Belonoshko, Anatoly B.
    et al.
    Royal Inst Technol KTH, Sweden.
    Fu, Jie
    Ningbo Univ, Peoples R China.
    Bryk, Taras
    Natl Acad Sci Ukraine, Ukraine.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Mattesini, Maurizio
    Univ Complutense Madrid, Spain; UCM, Spain.
    Low viscosity of the Earths inner core2019In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 10, article id 2483Article in journal (Refereed)
    Abstract [en]

    The Earths solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation. Here we show by first-principles molecular dynamics that the body-centered cubic phase of iron, recently demonstrated to be thermodynamically stable under the inner core conditions, is considerably less elastic than the hexagonal phase. Being a crystalline phase, the body-centered cubic phase of iron possesses the viscosity close to that of a liquid iron. The high attenuation of sound in the inner core is due to the unique diffusion characteristic of the body-centered cubic phase. The low viscosity of iron in the inner core enables the convection and resolves a number of controversies.

  • 16.
    Belonoshko, Anatoly B.
    et al.
    Royal Institute Technology KTH, Sweden.
    Lukinov, Timofei
    Royal Institute Technology KTH, Sweden.
    Fu, Jie
    Royal Institute Technology KTH, Sweden; Dalian University of Technology, Peoples R China.
    Zhao, Jijun
    Dalian University of Technology, Peoples R China.
    Davis, Sergio
    Chilean Comm Nucl Energy CCHEN, Chile.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Stabilization of body-centred cubic iron under inner-core conditions2017In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 10, no 4, p. 312-+Article in journal (Refereed)
    Abstract [en]

    The Earths solid core is mostly composed of iron. However, despite being central to our understanding of core properties, the stable phase of iron under inner-core conditions remains uncertain. The two leading candidates are hexagonal close-packed and body-centred cubic (bcc) crystal structures, but the dynamic and thermodynamic stability of bcc iron under inner-core conditions has been challenged. Here we demonstrate the stability of the bcc phase of iron under conditions consistent with the centre of the core using ab initio molecular dynamics simulations. We find that the bcc phase is stabilized at high temperatures by a diffusion mechanism that arises due to the dynamical instability of the phase at lower temperatures. On the basis of our simulations, we reinterpret experimental data as support for the stability of bcc iron under inner-core conditions. We suggest that the diffusion of iron atoms in solid state may explain both the anisotropy and the low shear modulus of the inner core.

  • 17.
    Bergman, Anders
    et al.
    Uppsala University.
    Burkert, Till
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Sanyal, Biplab
    Uppsala University.
    Frota-Pessôa, Sonia
    Universidade de Saõ Paulo.
    Nordström, Lars
    Uppsala University.
    Ruban, Andrei V.
    Royal Institute of Technology.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Eriksson, Olle
    Uppsala University.
    Magnetic properties of Fe /Co(001) superlattices from first- principles theory2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, p. 174409-1-174409-7Article in journal (Refereed)
  • 18.
    Burakovsky, L.
    et al.
    Los Alamos National Lab, NM 87545 USA.
    Burakovsky, N.
    Los Alamos National Lab, NM 87545 USA.
    Cawkwell, M. J.
    Los Alamos National Lab, NM 87545 USA.
    Preston, D. L.
    Los Alamos National Lab, NM 87545 USA.
    Errandonea, D.
    University of Valencia, Spain.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ab initio phase diagram of iridium2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 9, p. 094112-Article in journal (Refereed)
    Abstract [en]

    The phase diagram of iridium is investigated using the Z methodology. The Z methodology is a technique for phase diagram studies that combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. In the direct Z method, the solid phases along the melting curve are determined by comparing the solid-liquid equilibrium boundaries of candidate crystal structures. The inverse Z method involves quenching the liquid into the most stable solid phase at various temperatures and pressures to locate a solid-solid boundary. Although excellent agreement with the available experimental data (to less than or similar to 65 GPa) is found for the equation of state (EOS) of Ir, it is the third-order Birch-Murnaghan EOS with B-0 = 5 rather than the more widely accepted B-0 = 4 that describes our ab initio data to higher pressure (P). Our results suggest the existence of a random-stacking hexagonal close-packed structure of iridium at high P. We offer an explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.

  • 19.
    Burakovsky, L.
    et al.
    Los Alamos National Lab, NM 87545 USA.
    Cawkwell, M. J.
    Los Alamos National Lab, NM 87545 USA.
    Preston, D. L.
    Los Alamos National Lab, NM 87545 USA.
    Errandonea, D.
    University of Valencia, Spain.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Recent ab initio phase diagram studies: Iridium2017In: JOINT AIRAPT-25TH and EHPRG-53RD INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, 2015, IOP PUBLISHING LTD , 2017, Vol. 950, article id UNSP 042021Conference paper (Refereed)
    Abstract [en]

    The phase diagram of iridium is investigated using the Z methodology in conjunction with the VASP ab initio molecular dynamics package. The Z methodology is a novel technique for phase diagram studies which combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. We compare our results to the available experimental data on iridium. We offer explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.

  • 20.
    Burakovsky, L.
    et al.
    Los Alamos National Laboratory.
    Chen, S. P.
    Los Alamos National Laboratory.
    Preston, D. L.
    Los Alamos National Laboratory.
    Belonoshko, A. B.
    Royal Institute of Technology.
    Rosengren, A.
    Royal Institute of Technology.
    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.
    Moriarty, J. A.
    Lawrence Livermore National Laboratory.
    High-Pressure-High-Temperature Polymorphism in Ta: Resolving an Ongoing Experimental Controversy2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 104, no 25, p. 255702-Article in journal (Refereed)
    Abstract [en]

    Phase diagrams of refractory metals remain essentially unknown. Moreover, there is an ongoing controversy over the high-pressure melting temperatures of these metals: results of diamond anvil cell (DAC) and shock wave experiments differ by at least a factor of 2. From an extensive ab initio study on tantalum we discovered that the body-centered cubic phase, its physical phase at ambient conditions, transforms to another solid phase, possibly hexagonal omega phase, at high temperature. Hence the sample motion observed in DAC experiments is very likely not due to melting but internal stresses accompanying a solid-solid transformation, and thermal stresses associated with laser heating.

  • 21.
    Burakovsky, Leonid
    et al.
    Los Alamos Natl Lab, NM 87545 USA.
    Burakovsky, Naftali
    Los Alamos Natl Lab, NM 87545 USA.
    Preston, Dean
    Los Alamos Natl Lab, NM 87545 USA.
    Simak, Sergei I
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Systematics of the Third Row Transition Metal Melting: The HCP Metals Rhenium and Osmium2018In: Crystals, ISSN 2073-4352, Vol. 8, no 6, article id 243Article in journal (Refereed)
    Abstract [en]

    The melting curves of rhenium and osmium to megabar pressures are obtained from an extensive suite of ab initio quantum molecular dynamics (QMD) simulations using the Z method. In addition, for Re, we combine QMD simulations with total free energy calculations to obtain its phase diagram. Our results indicate that Re, which generally assumes a hexagonal close-packed (hcp) structure, melts from a face-centered cubic (fcc) structure in the pressure range 20-240 GPa. We conclude that the recent DAC data on Re to 50 GPa in fact encompass both the true melting curve and the low-slope hcp-fcc phase boundary above a triple point at (20 GPa, 4240 K). A linear fit to the Re diamond anvil cell (DAC) data then results in a slope that is 2.3 times smaller than that of the actual melting curve. The phase diagram of Re is topologically equivalent to that of Pt calculated by us earlier on. Regularities in the melting curves of Re, Os, and five other 3rd-row transition metals (Ta, W, Ir, Pt, Au) form the 3rd-row transition metal melting systematics. We demonstrate how this systematics can be used to estimate the currently unknown melting curve of the eighth 3rd-row transition metal Hf.

  • 22. Burkert, T
    et al.
    Eriksson, O
    James, P
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Johansson, B
    Nordstrom, L
    Calculation of uniaxial magnetic anisotropy energy of tetragonal and trigonal Fe, Co, and Ni2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 10Article in journal (Refereed)
    Abstract [en]

    The magnetic anisotropy energy (MAE) of Fe, Co, and Ni is presented for tetragonal and trigonal structures along two paths of structural distortion connecting the bcc and the fcc structure. The MAE was calculated from first principles with the full-potential linear muffin-tin orbital method and the force theorem. As is expected from symmetry considerations, the MAE increases by orders of magnitude when the cubic symmetry is broken. For tetragonal structures of Co and Ni a regular behavior of the MAE is observed, i.e., only the symmetry dictated nodes at the cubic structures appear along this path of distortion. In the case of tetragonal Fe, additional reorientations of the easy axis occur that are attributed to a topological change of the Fermi surface upon distortion. For the trigonal structures of all three elements the strain dependence of the MAE is more complicated, with additional reorientations of the easy axis and an unexpectedly large MAE for certain distortions of Ni, and a strongly nonlinear behavior for trigonal structures of Co close to fcc. Furthermore, the linear magnetoelastic coupling coefficients are calculated from the MAE at small distortions from the cubic equilibrium structure of the three elements. Two different Brillouin-zone integration techniques were used to calculate the MAE. Since the Gaussian broadening method smears out details of the Fermi surface, it results in a different MAE as compared to the tetrahedron method in some cases.

  • 23.
    Burkert, T.
    et al.
    Department of Physics, Uppsala Universitet, Box 530, 751 21 Uppsala, Sweden.
    Eriksson, O.
    Department of Physics, Uppsala Universitet, Box 530, 751 21 Uppsala, Sweden, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Ruban, A.V.
    Department of Material Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden.
    Sanyal, B.
    Department of Physics, Uppsala Universitet, Box 530, 751 21 Uppsala, Sweden.
    Nordstrom, L.
    Nordström, L., Department of Physics, Uppsala Universitet, Box 530, 751 21 Uppsala, Sweden.
    Wills, J.M.
    Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States.
    Magnetic anisotropy of L 10 FePt and Fe1-x Mnx Pt2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 13Article in journal (Refereed)
    Abstract [en]

    The uniaxial magnetic anisotropy energy (MAE) of L 10 FePt and Fe1-x Mnx Pt, x=0-0.25, was studied from first principles using two fully relativistic computational methods, the full-potential linear muffin-tin orbitals method and the exact muffin-tin orbitals method. It was found that the large MAE of 2.8 meV/f.u. is caused by a delicate interaction between the Fe and Pt atoms, where the large spin-orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states is crucial. The effect of random order on the MAE was modeled by mutual alloying of the sublattices within the coherent potential approximation (CPA), and a strong dependence of the MAE on the degree of chemical long-range order was found. The alloying of FePt with Mn was investigated with the virtual crystal approximation and the CPA as well as supercell calculations. The MAE increases up to 33% within the concentration range studied here, an effect that is attributed to band filling. Furthermore, the dependence of the MAE on the structural properties was studied. © 2005 The American Physical Society.

  • 24.
    Bykova, E.
    et al.
    DESY, Germany; Univ Bayreuth, Germany.
    Bykov, M.
    Univ Bayreuth, Germany; Natl Univ Sci and Technol MISIS, Russia.
    Cernok, A.
    Univ Bayreuth, Germany; Open Univ, England.
    Tidholm, 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.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. CALTECH, CA 91125 USA.
    Belov, M. P.
    Natl Univ Sci and Technol MISIS, Russia.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Liermann, H. -P.
    DESY, Germany.
    Hanfland, M.
    European Synchrotron Radiat Facil, France.
    Prakapenka, V. B.
    Univ Chicago, IL 60637 USA.
    Prescher, C.
    Univ Chicago, IL 60637 USA; Univ Cologne, Germany.
    Dubrovinskaia, N.
    Univ Bayreuth, Germany.
    Dubrovinsky, L.
    Univ Bayreuth, Germany.
    Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 4789Article in journal (Refereed)
    Abstract [en]

    Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO2) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO6 octahedra, which, at odds with 3rd Paulings rule, are connected through common faces. Our results suggest that possible silicate liquids in Earths lower mantle may have complex structures making them more compressible than previously supposed.

  • 25.
    Delange, Pascal
    et al.
    University of Paris Saclay, France.
    Ayral, Thomas
    CEA, France.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ferrero, Michel
    University of Paris Saclay, France; Coll France, France.
    Parcollet, Olivier
    CEA, France.
    Biermann, Silke
    University of Paris Saclay, France; Coll France, France.
    Pourovskii, Leonid
    University of Paris Saclay, France; Coll France, France; National University of Science and Technology MISIS, Russia.
    Large effects of subtle electronic correlations on the energetics of vacancies in alpha-Fe2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, p. 100102-Article in journal (Refereed)
    Abstract [en]

    We study the effect of electronic Coulomb correlations on the vacancy formation energy in paramagnetic alpha-Fe within ab initio dynamical mean-field theory. The calculated value for the formation energy is substantially lower than in standard density-functional calculations and in excellent agreement with experiment. The reduction is caused by an enhancement of electronic correlations at the nearest neighbors of the vacancy. This effect is explained by subtle changes in the corresponding spectral function of the d electrons. The local lattice relaxations around the vacancy are substantially increased by many-body effects.

  • 26.
    Dubrovinsky, L.
    et al.
    Bayerisches Geoinstitut, Universität Bayreuth, Germany.
    Dubrovinskaia, N.
    Mineralogical Institute Heidelberg University.
    Narygina, O.
    Bayerisches Geoinstitut Universität Bayreuth, Germany.
    Kantor, I.
    Bayerisches Geoinstitut Universität Bayreuth, Germany.
    Kuznetzov, A.
    Center for Advanced Radiation Sources, University of Chicago.
    Prakapenka, V. B.
    Center for Advanced Radiation Sources University of Chicago.
    Vitos, L.
    Research Institute for Solid State Physics and Optics, Budapest, Hungary.
    Johansson, B.
    Department of Physics Uppsala University.
    Mikhaylushkin, Arkady
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Body-Centered Cubic Iron-Nickel Alloy in Earth's Core2007In: Science / American Association for the Advancement of Science, Vol. Vol. 316. no. 5833, p. 1880-1883Article in journal (Refereed)
  • 27.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    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.
    Johansson, B
    Royal Institute Technology KTH.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Configurational thermodynamics of Fe-Ni alloys at Earths core conditions2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 308, no 1-2, p. 90-96Article in journal (Refereed)
    Abstract [en]

    By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.

  • 28.
    Eklof, Daniel
    et al.
    Stockholm Univ, Sweden.
    Fischer, Andreas
    Augsburg Univ, Germany.
    Ektarawong, Annop
    Chulalongkorn Univ, Thailand; Commiss Higher Educ, Thailand.
    Jaworski, Aleksander
    Stockholm Univ, Sweden.
    Pell, Andrew J.
    Stockholm Univ, Sweden.
    Grins, Jekabs
    Stockholm Univ, Sweden.
    Simak, Sergey
    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, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Wu, Yang
    Tsinghua Univ, Peoples R China.
    Widom, Michael
    Carnegie Mellon Univ, PA 15213 USA.
    Scherer, Wolfgang
    Augsburg Univ, Germany.
    Haussermann, Ulrich
    Stockholm Univ, Sweden.
    Mysterious SiB3: Identifying the Relation between alpha- and beta-SiB32019In: ACS OMEGA, ISSN 2470-1343, Vol. 4, no 20, p. 18741-18759Article in journal (Refereed)
    Abstract [en]

    Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alpha-SiB3-x, which relates to the alpha-rhombohedral phase of boron, and as strictly ordered and stoichiometric beta-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alpha-SiB3-x and beta-SiB3 and analyze their relative thermodynamic stabilities. alpha-SiB3-x is metastable (with respect to beta-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200-1300 degrees C. At the same time, alpha-SiB3-x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable beta-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 1175-1200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of beta-SiB3 was seen at 1100 degrees C for samples pressurized to 5.5-8 GPa. beta-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of beta-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alpha-SiB3-x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alpha-SiB3-x were ascertained by a combination of single-crystal X-ray diffraction and Si-29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T amp;lt;= 1200 degrees C) and single crystal samples (obtained from Si-rich molten Si-B mixtures at T amp;gt; 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B-12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of alpha-SiB3-x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B-12 icosahedron. Accordingly, alpha-SiB3-x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alpha-SiB3-x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition beta-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alpha-SiB3-x only becomes more stable than beta-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alpha-SiB3-x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise alpha-SiB2.5 and beta-SiB3, whereas alpha-SiB3 represents a p-type conductor.

  • 29.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    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.
    Carbon-rich icosahedral boron carbides beyond B4C and their thermodynamic stabilities at high temperature and pressure from first principles2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 5, article id 054104Article in journal (Refereed)
    Abstract [en]

    We investigate the thermodynamic stability of carbon-rich icosahedral boron carbide at different compositions, ranging from B4C to B2C, using first-principles calculations. Apart fromB4C, generally addressed in the literature, B2.5C, represented by B10C2p (C-C), where C-p and (C-C) denote a carbon atom occupying the polar site of the icosahedral cluster and a diatomic carbon chain, respectively, is predicted to be thermodynamically stable under high pressures with respect to B4C as well as pure boron and carbon phases. The thermodynamic stability of B2.5C is determined by the Gibbs free energy G as a function of pressure p and temperature T, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. The stability range of B2.5C is then illustrated through the p-T phase diagrams. Depending on the temperatures, the stability range of B2.5C is predicted to be within the range between 40 and 67 GPa. At T greater than or similar to 500 K, the icosahedral C-p atoms in B2.5C configurationally disorder at the polar sites. By investigating the properties of B2.5C, e.g., elastic constants and phonon and electronic density of states, we demonstrate that B2.5C is both mechanically and dynamically stable at zero pressure, and is an electrical semiconductor. Furthermore, based on the sketched phase diagrams, a possible route for experimental synthesis of B2.5C as well as a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested.

  • 30.
    Ektarawong, Annop
    et al.
    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.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Inst Eisenforsch GmbH, Germany.
    Effect of temperature and configurational disorder on the electronic band gap of boron carbide from first principles2018In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 2, no 10, article id 104603Article in journal (Refereed)
    Abstract [en]

    The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of B4C, represented by B11CP (CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boron-rich solids. Using a first-principles approach, we explore the effect of temperature and configurational disorder on the electronic band gap of B4C. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral C-P atoms residing in the B11CP icosahedra have a minimal impact on the band gap of B4C, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of B4C is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of B-12 (CBC) and B-12 (B-4) into a matrix of B11CP (CBC) to make the composition of boron carbide approximately B4.3C, claimed to be the carbon-rich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide.

  • 31.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    First-principles prediction of stabilities and instabilities of compounds and alloys in the ternary B-As-P system2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 2, article id 024202Article in journal (Refereed)
    Abstract [en]

    We examine the thermodynamic stability of compounds and alloys in the ternary B-As-P system theoretically using first-principles calculations. We demonstrate that the icosahedral B12As2 is the only stable compound in the binary B-As system, while the zinc-blende BAs is thermodynamically unstable with respect to B12As2 and the pure arsenic phase at 0 K, and increasingly so at higher temperature, suggesting that BAs may merely exist as a metastable phase. On the contrary, in the binary B-P system, both zinc-blende BP and icosahedral B12P2 are predicted to be stable. As for the binary As-P system, As1-xPx disordered alloys are predicted at elevated temperature-for example, a disordered solid solution of up to similar to 75 at.% As in black phosphorus as well as a small solubility of similar to 1 at.% P in gray arsenic at T = 750 K, together with the presence of miscibility gaps. The calculated large solubility of As in black phosphorus explains the experimental syntheses of black-phosphorus-type As1-xPx alloys with tunable compositions, recently reported in the literature. We investigate the phase stabilities in the ternary B-As-P system and demonstrate a high tendency for a formation of alloys in the icosahedral B-12(As1-xPx)(2) structure by intermixing of As and P atoms at the diatomic chain sites. The phase diagram displays noticeable mutual solubility of the icosahedral subpnictides in each other even at room temperature as well as a closure of a pseudobinary miscibility gap around 900 K. As for pseudobinary BAs1-xPx alloys, only a tiny amount of BAs is predicted to be able to dissolve in BP to form the BAs1-xPx disordered alloys at elevated temperature. For example, less than 5% of BAs can dissolve in BP at T = 1000 K. The small solubility limit of BAs in BP is attributed to the thermodynamic instability of BAs with respect to B12As2 and As.

  • 32.
    Ektarawong, Annop
    et al.
    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.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Inst Eisenforsch GmbH, Germany.
    Structural models of increasing complexity for icosahedral boron carbide with compositions throughout the single-phase region from first principles2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174104Article in journal (Refereed)
    Abstract [en]

    We perform first-principles calculations to investigate the phase stability of boron carbide, concentrating on the recently proposed alternative structural models composed not only of the regularly studied B11Cp(CBC) and B-12(CBC), but also of B-12(CBCB) and B-12(B-4). We find that a combination of the four structural motifs can result in low-energy electron precise configurations of boron carbide. Among several considered configurations within the composition range of B10.5C and B4C, we identify in addition to the regularly studied B11Cp(CBC) at the composition of B4C two low-energy configurations, resulting in a new view of the B-C convex hull. Those are [B-12(CBC)](0.67)[B-12(B-4)](0.33) and [B-12(CBC)](0.67)[B-12(CBCB)](0.33), corresponding to compositions of B10.5C and B6.67C, respectively. As a consequence, B-12(CBC) at the composition of B6.5C, previously suggested in the literature as a stable configuration of boron carbide, is no longer part of the B-C convex hull. By inspecting the electronic density of states as well as the elastic moduli, we find that the alternative models of boron carbide can provide a reasonably good description for electronic and elastic properties of the material in comparison with the experiments, highlighting the importance of considering B-12(CBCB) and B-12(B-4), together with the previously proposed B11Cp(CBC) and B-12(CBC), as the crucial ingredients for modeling boron carbide with compositions throughout the single-phase region.

  • 33.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    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.
    Thermodynamic stability and properties of boron subnitrides from first principles2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064206Article, review/survey (Refereed)
    Abstract [en]

    We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different alpha-rhombohedral-boron-like boron subnitrides, with the compositions of B6N, B13N2, and B38N6, proposed in the literature. We find that, out of these subnitrides with the structural units of B-12(N-N), B-12(NBN), and [B-12(N-N)](0.33)[B-12(NBN)](0.67), respectively, only B38N6, represented by [B-12(N-N)](0.33)[B-12(NBN)](0.67), is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and a-tetragonalboron- like boron subnitride B50N2. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B-12(N-N)](0.33)[B-12(NBN)](0.67) is bothmechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B-12(N-N)](0.33)[B-12(NBN)](0.67) are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of a-rhombohedral-boron-like boron subnitride.

  • 34.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Configurational order-disorder induced metal-nonmetal transition in B13C2 studied with first-principles superatom-special quasirandom structure method2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 1, article id 014202Article in journal (Refereed)
    Abstract [en]

    Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B13C2 has been a controversial topic in the field of icosahedral boron-rich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while the ordered ground state B13C2 is metallic, the configurationally disordered B13C2, modeled with a superatom-special quasirandom structure method, goes through a metal to nonmetal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chain-end carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a B-12 icosahedron bonded to it, giving rise to a B11Ce(BBC) unit. The atomic configuration of the substitutionally disordered B13C2 thus tends to be dominated by a mixture between B-12(CBC) and B11Ce(BBC). Due to splitting of valence states in B11Ce(BBC), the electron deficiency in B-12(CBC) is gradually compensated.

  • 35.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 2, article id 024204Article in journal (Refereed)
    Abstract [en]

    Configurationally disordered crystalline boron carbide, with the composition B4C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatoms picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R (3) over barm) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B4C phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B4C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in B4C.

  • 36.
    Ektarawong, Annop
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Wang, Fei
    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.
    Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 13, article id 134503Article in journal (Refereed)
    Abstract [en]

    The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)1−x(B13C2)x, as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)1−x(B13C2)x is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational  disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young’s and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)1−x(B13C2)x system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)1−x(B13C2)x. The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

  • 37.
    Fashandi, Hossein
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Dahlqvist, Martin
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Palisaitis, Justinas
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film 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.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Rosén, Johanna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Andersson, Mike
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for high-temperature-stable Ohmic contacts to SiC2017In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 16, no 8, p. 814-818Article in journal (Refereed)
    Abstract [en]

    The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 degrees C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

  • 38.
    Filippov, Stanislav
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Stockholm Univ, Sweden.
    Grinderslev, Jakob B.
    Aarhus Univ, Denmark.
    Andersson, Mikael S.
    Chalmers Univ Technol, Sweden.
    Armstrong, Jeff
    Rutherford Appleton Lab, England.
    Karlsson, Maths
    Chalmers Univ Technol, Sweden.
    Jensen, Torben R.
    Aarhus Univ, Denmark.
    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.
    Haussermann, Ulrich
    Stockholm Univ, Sweden.
    Analysis of Dihydrogen Bonding in Ammonium Borohydride2019In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 47, p. 28631-28639Article in journal (Refereed)
    Abstract [en]

    The structural and vibrational properties of ammonium borohydride, NH4BH4, have been examined by first-principles density functional theory (DFT) calculations and inelastic neutron scattering (INS). The H disordered crystal structure of NH4BH4 is composed of the tetrahedral complex ions NH4+ and BH4-, which are arranged as in the fcc NaCl structure and linked by intermolecular dihydrogen bonding. Upon cooling, the INS spectra revealed a structural transition between 45 and 40 K. The reversible transition occurs upon heating between 46 and 49 K. In the low-temperature form reorientational dynamics are frozen. The libration modes for BH4- and NH4+ are near 300 and 200 cm(-1), respectively. Upon entering the fcc high-temperature form, NH4+ ions attain fast reorientational dynamics, as indicated in the disappearance of the NH4+ libration band, whereas BH4- ions become significantly mobile only at temperatures above 100 K. The vibrational behavior of BH4- ions in NH4BH4 compares well to the heavier alkali metal borohydrides, NaBH4-CsBH4. DFT calculations revealed a nondirectional nature of the dihydrogen bonding in NH4BH4 with only weak tendency for long-range order. Different rotational configurations of complex ions appear quasi-degenerate, which is reminiscent of glasses.

  • 39.
    Filippov, Stanislav
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Stockholm Univ, Sweden.
    Klarbring, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Haussermann, Ulrich
    Stockholm Univ, Sweden.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Temperature-induced phase transition and Li self-diffusion in Li2C2: A first-principles study2019In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 3, no 2, article id 023602Article in journal (Refereed)
    Abstract [en]

    Lithium carbide, Li2C2, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from the ground-state orthorhombic (Immm) to the high-temperature cubic (Fm (3) over barm) crystal structure is not well understood and here we elucidate it with help of first-principles calculations. We show that stabilization of the cubic phase is a result of a temperature-induced disorientation of the C-C dumbbells and their further thermal rotations. Due to these rotations rather large deviatoric stress, which is associated with the dumbbell alignment along one of the crystallographic axes, averages out making the cubic structure mechanically stable. At high temperature we observe a type-II superionic transition to a state of high Li self-diffusion involving collective ionic motion mediated by the formation of Frenkel pairs.

  • 40.
    Grigoriev, A.
    et al.
    Dept of Physics Uppsala University.
    Skorodumova, N.V.
    Dept ofh Physics Uppsala University.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Wendin, G.
    Dept of Microtechnology and Nanoscience-MC2 Chalmers University, Göteborg.
    Johansson, B.
    Dept of Physics Uppsala University.
    Ahuja, R.
    Dept of Physics Uppsala University.
    Electron Transport in Stretched Monoatomic Gold Wires2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, p. 236807-1-236807-4Article in journal (Refereed)
  • 41.
    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, p. 180301-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.

  • 42.
    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, p. 135504-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.

  • 43.
    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.

  • 44.
    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.

  • 45.
    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.

  • 46.
    Isaev, E.I.
    et al.
    Moscow Technological University.
    Ahuja, R.
    Uppsala University.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Lichtenstein, A.I.
    Universität Hamburg.
    Vekilov, Yu. Kh.
    Moscow Technological University.
    Johansson, B.
    Uppsala University.
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics.
    Anomalously enhanced superconductivity and ab initio lattice dynamics in transition metal carbides and nitrides2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 72, no 6, p. 4515-Article in journal (Refereed)
    Abstract [en]

    The lattice dynamics for the monocarbides and mononitrides of the early transition metals has been studied using first-principles density functional perturbation theory. It is shown that the superconductivity of transition metal carbides is directly related to anomalies in the phonon spectra which in their turn are connected to the number of valence electrons. The calculated electron-phonon interaction constants are in excellent agreement with experimental data. Superconductivity is considerably enhanced for substitutional M Cx N1-x alloys. We also predict that perfect VC in the sodium chloride structure (B1) is a superconductor with a transition temperature Tc =11.5 K. The experimental failure to sythesize some transition metal carbides and nitrides within the B1 structure is connected to their dynamical instability. © 2005 The American Physical Society.

  • 47.
    Isaev, E.I.
    et al.
    Moscow State Institute of Steel and Alloys Technological University, Moscow.
    Simak, Sergey
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Ahuja, R.
    Uppsala universitet.
    Vekilov, Yu.Kh.
    Moscow State Institute of Steel and Alloys Technological University, Moscow.
    Katsnelson, M.I.
    Radboud University of Nujmegen, The Netherlands.
    Lichtenstein, A.I.
    Universitaet, Hamburg.
    Johansson, B.
    KTH, och Uppsala universitet.
    Phonon related properties of transition metals, their carbides, and nitrides: A first-principles study2007In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, p. 123519-1-12351918Article in journal (Refereed)
    Abstract [en]

      

  • 48.
    Isaev, Eyvas
    et al.
    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.
    Mikhaylushkin, Arkady
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Vekilov, Yu. Kh.
    Moscow State Institute Steel and Alloys.
    Zarechnaya, E. Yu.
    Harvard University.
    Dubrovinsky, L.
    University of Bayreuth.
    Dubrovinskaia, N.
    University of Heidelberg.
    Merlini, M.
    University of Milan.
    Hanfland, M.
    ESRF.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Impact of lattice vibrations on equation of state of the hardest boron phase2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 13, p. 132106-Article in journal (Refereed)
    Abstract [en]

    An accurate equation of state (EOS) is determined for the high-pressure orthorhombic phase of boron, B(28), experimentally as well as from ab initio calculations. The unique feature of our experiment is that it is carried out on the single crystal of B(28). In theory, we take into consideration the lattice vibrations, often neglected in first-principles simulations. We show that the phonon contribution has a profound effect on the EOS of B(28), giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.

  • 49.
    Kerdsongpanya, Sit
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Rensselaer Polytech Institute, NY 12180 USA.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. CALTECH, CA 91125 USA.
    Sun, Bo
    National University of Singapore, Singapore.
    Kan Koh, Yee
    National University of Singapore, Singapore.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Van Nong, Ngo
    Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Roskilde, Denmark.
    Simak, Sergei I.
    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, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 19, article id 195417Article in journal (Refereed)
    Abstract [en]

    The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since many modern technologies require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity, which takes into account the effect of microstructure. It 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 to the experimental data by time-domain thermoreflectance. Our experimental results show a trend of reduction in lattice thermal conductivity with decreasing domain size predicted by the theoretical model. These results suggest a possibility to control thermal conductivity by microstructural tailoring and provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on ab initio calculations.

  • 50.
    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.

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