S. I. Simak

Linköping University, Linköping, Östergötland, Sweden

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Publications (124)515.58 Total impact

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    ABSTRACT: 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.
    No preview · Article · Jan 2016
  • A. Ektarawong · S. I. Simak · L. Hultman · J. Birch · B. Alling
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    ABSTRACT: Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B$_{13}$C$_{2}$ 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 ordered ground state B$_{13}$C$_{2}$ is metallic, configurationally disordered B$_{13}$C$_{2}$, modeled with a superatom-special quasirandom structure method, goes through a metal to non-metal 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 B$_{11}$C$^{e}$(BBC) unit. The atomic configuration of the substitutionally disordered B$_{13}$C$_{2}$ thus tends to be dominated by a mixture between B$_{12}$(CBC) and B$_{11}$C$^{e}$(BBC). Due to splitting of valence states in B$_{11}$C$^{e}$(BBC), the electron deficiency in B$_{12}$(CBC) is gradually compensated.
    No preview · Article · Aug 2015 · Physical Review B
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    T. Lukinov · S. I. Simak · A. B. Belonoshko
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    ABSTRACT: The sound velocity of Mo along the Hugoniot adiabat is calculated from first principles using density-functional theory based molecular dynamics. These data are compared to the sound velocity as measured in recent experiments. The theoretical and experimental Hugoniot and sound velocities are in very good agreement up to pressures of 210 GPa and temperatures of 3700 K on the Hugoniot. However, above that point the experiment and theory diverge. This implies that Mo undergoes a phase transition at about the same point. Considering that the melting point of Mo is likely much higher at that pressure, the related change in the sound velocity in experiment can be ascribed to a solid-solid transition.
    Full-text · Article · Aug 2015 · Physical Review B
  • Sergiu Arapan · Sergei I. Simak · Natalia V. Skorodumova
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    ABSTRACT: 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.
    No preview · Article · Mar 2015 · Physical Review B
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    ABSTRACT: We have studied the body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp) phases of Fe alloyed with 25 at. % of Ni at Earth's core conditions using an ab initio local density approximation + dynamical mean-field theory (LDA+DMFT) approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with minimum possible cell size allowing for the proper composition. Our calculations demonstrate that the strength of electronic correlations on the Fe 3d shell is highly sensitive to the phase and local environment. In the bcc phase the 3d electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated even at extreme pressure-temperature conditions, with the local and uniform magnetic susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasi-particle lifetime {\Gamma} featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase we predict a weakly-correlated Fermi-liquid state with a temperature-independent local susceptibility and a quadratic temperature dependence of {\Gamma}. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcp-fcc-bcc sequence. Further, the inter-site magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni 3d-states.
    Preview · Article · Oct 2014 · Physical Review B
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    A. Ektarawong · S. I. Simak · L. Hultman · J. Birch · B. Alling
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    ABSTRACT: Configurationally disordered crystalline boron carbide, B$_{4}$C, 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 superatom's picture of the complex structure and combine it with a special quasi-random 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 increasing 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-3m) on average, corresponding to what is reported in the literature, is achieved after the second transition. The electronic density of states, obtained from the disordered phases indicates a sensitivity of band gap to the degree of configurational disorder in B$_{4}$C.
    Full-text · Article · Jul 2014 · Physical Review B
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    ABSTRACT: Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H⋯O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe^{3+} high-to-low spin crossover at above 45 GPa.
    Full-text · Article · Oct 2013 · Physical Review Letters
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    ABSTRACT: Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of ^{61}Ni. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09.
    No preview · Article · Oct 2013 · Physical Review Letters
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    Olle Hellman · Peter Steneteg · Igor A. Abrikosov · Sergei I. Simak
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    ABSTRACT: We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on \emph{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 details. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within \emph{ab initio} and classical molecular dynamics frameworks. In particular, we examine from first-principles the behavior of force constants upon the dynamical stabilization of body centered phase of Zr, and show that they become more localized. We also calculate phase diagram for $^4$He modeled with the Aziz \emph{et al.} potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.
    Preview · Article · Mar 2013 · Physical review. B, Condensed matter
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    ABSTRACT: We review basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: Fe-Cr and Zr-based alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of Fe-Cr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For Zr-Nb alloys, we demonstrate how microscopic parameters calculated from first-principles can be used in higher-level models.
    No preview · Article · Jan 2013 · MRS Online Proceeding Library
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    ABSTRACT: The lattice stability trends of the primary candidate for Earth's core material, the Fe-Ni alloy, were examined from first principles. We employed the exact muffin-tin orbital method (EMTO) combined with the coherent potential approximation (CPA) for the treatment of alloying effects. It was revealed that high pressure reverses the trend in the relative stabilities of the body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) phases observed at ambient conditions. In the low pressure region the increase of Ni concentration in the Fe-Ni alloy enhances the bcc phase destabilization relative to the more close-packed fcc and hcp phases. However, at 300 GPa (Earth's core pressure), the effect of Ni addition is opposite. The reverse of the trend is associated with the suppression of the ferromagnetism of Fe when going from ambient pressures to pressure conditions corresponding to those of Earth's core. The first-principles results are explained in the framework of the canonical band model.
    No preview · Article · Dec 2012 · Physical review. B, Condensed matter
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    ABSTRACT: We employ state-of-the-art ab initio simulations within the dynamical mean-field theory to study three likely phases of iron (hexogonal close-packed, hcp, face centered cubic, fcc, and body centered cubic, bcc) at the Earth's core conditions. We demonstrate that the correction to the electronic free energy due to correlations can be significant for the relative stability of the phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi liquid behaviour, and where a Curie-Weiss behaviour of the uniform susceptbility hints at a local magnetic moment still existing at 5800 K and 300 GPa. We predict that all three structures have sufficiently high magnetic susceptibility to stabilize the geodynamo.
    Full-text · Article · Apr 2012 · Physical review. B, Condensed matter
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    O Hellman · N V Skorodumova · S I Simak
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    ABSTRACT: 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.
    Preview · Article · Mar 2012 · Physical Review Letters
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    ABSTRACT: Herein, we use first principles calculations to study the energy of the (112̅ 1) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal close-packed metals. The (112̅ 1) twin boundary is unique in that it is composed of—and can form by the glide of—basal dislocations nucleating at every c lattice parameter. The effect of the number of atoms between boundaries on the boundary energy, and the resulting lattice strains of the relaxed structures are quantified. It is shown that the energies obtained converge within 32–64 atoms/supercell. The structures with a higher second-order elastic constant term, c44, also have higher boundary energies. It is further shown that the critical resolved shear stresses of the basal dislocations at 0 K, which make up the (112̅ 1) twin, are so low as to be below the threshold of the first principles calculations.
    No preview · Article · Nov 2011 · Physical Review B
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    ABSTRACT: The effect of hydrostatic pressure on the phase stability of Fe-Cr alloys has been studied using ab initio methods. We show that while pressure decreases the tendency toward the phase separation in the paramagnetic state of bcc alloys, in the ferromagnetic state it reduces the alloy stability at low Cr concentration and vice versa, makes the solid solution more stable at higher concentrations. This behavior of the phase stability can be predicted from the deviation of the lattice parameter from Vegard’s law in bcc Fe-Cr alloys. On the atomic level, the pressure effect can be explained by the suppression of the local magnetic moments on Cr atoms, which gives rise to a decrease of the Fe-Cr magnetic exchange interaction at the first coordination shell and, as a result, to the observed variation of the ordering tendency between the Fe and Cr atoms.
    Preview · Article · Sep 2011 · Physical review. B, Condensed matter
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    Full-text · Conference Paper · Aug 2011
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    ABSTRACT: 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 Earth's 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 120–360GPa and 1000–6600K. Our calculations show that alloying of Fe with Ni below 3000K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000K the effect is reversed, and at conditions corresponding to those of the Earth's inner core, Ni acts as an agent to stabilise the hcp phase.
    No preview · Article · Aug 2011 · Earth and Planetary Science Letters
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    ABSTRACT: An accurate equation of state (EOS) is determined for the high-pressure orthorhombic phase of boron, B28, 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 B28. 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 B28, giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.
    No preview · Article · Apr 2011 · Physical review. B, Condensed matter
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    Olle Hellman · I. A. Abrikosov · S. I. Simak
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    ABSTRACT: 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.
    Preview · Article · Mar 2011 · Physical review. B, Condensed matter
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Oct 2010 · ChemInform

Publication Stats

4k Citations
515.58 Total Impact Points


  • 2005-2015
    • Linköping University
      • Department of Physics, Chemistry and Biology (IFM)
      Linköping, Östergötland, Sweden
  • 1998-2010
    • Stockholm University
      • Department of Organic Chemistry
      Tukholma, Stockholm, Sweden
  • 1996-2005
    • Uppsala University
      • Department of Physics and Astronomy
      Uppsala, Uppsala, Sweden
  • 1999-2003
    • Chalmers University of Technology
      • Department of Applied Physics
      Goeteborg, Västra Götaland, Sweden
  • 2002
    • University of Gothenburg
      Goeteborg, Västra Götaland, Sweden
  • 2001
    • University of Pennsylvania
      • Laboratory for Research on the Structure of Matter
      Philadelphia, Pennsylvania, United States