S. I. Simak

Linköping University, Linköping, Östergötland, Sweden

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Publications (96)389.46 Total impact

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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.
  • Physical Review B 07/2014; 90(2). DOI:10.1103/PhysRevB.90.024204 · 3.66 Impact Factor
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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.
    Physical Review Letters 10/2013; 111(15):157601. DOI:10.1103/PhysRevLett.111.157601 · 7.73 Impact Factor
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    ABSTRACT: First-principles calculations are employed to investigate the stability and properties of cubic rock-salt-like (Cr1-xAlx)(2)O-3 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)(2)O-3 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. (C) 2013 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 05/2013; 31(3):030602. DOI:10.1116/1.4795392 · 2.14 Impact Factor
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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.
    Physical review. B, Condensed matter 12/2012; 86(22). DOI:10.1103/PhysRevB.86.224107 · 3.66 Impact Factor
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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.
    Physical review. B, Condensed matter 04/2012; 87(11). DOI:10.1103/PhysRevB.87.115130 · 3.66 Impact Factor
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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.
    Physical Review Letters 03/2012; 108(13):135504. DOI:10.1103/PhysRevLett.108.135504 · 7.73 Impact Factor
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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.
    Physical review. B, Condensed matter 09/2011; 84(9). DOI:10.1103/PhysRevB.84.094422 · 3.66 Impact Factor
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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.
    Earth and Planetary Science Letters 08/2011; 308(1):90-96. DOI:10.1016/j.epsl.2011.05.035 · 4.72 Impact Factor
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    ABSTRACT: The peculiar bonding situation in γ boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B(12) clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B(12) dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
    Physical Review Letters 05/2011; 106(21):215502. DOI:10.1103/PhysRevLett.106.215502 · 7.73 Impact Factor
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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.
    Physical review. B, Condensed matter 04/2011; 83(13). DOI:10.1103/PhysRevB.83.132106 · 3.66 Impact Factor
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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.
    Physical review. B, Condensed matter 03/2011; 84(18). DOI:10.1103/PhysRevB.84.180301 · 3.66 Impact Factor
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    ABSTRACT: The substitution of oxygen for carbon in Ti2AlC M(n+1)AXn (MAX) phase, forming Ti2AlC1−xOx, has recently been reported. In this paper we simulate the effect of oxygen incorporation on mechanical and electronic properties using ab initio calculations. While the mechanical properties are not sensitive to the change in composition, the electronic properties can be tuned by varying the oxygen concentration. As the concentration increases, the conduction changes from in plane, typical of MAX phases, to conduction also in the c-direction. The conduction along c passes from insulating to n-type and then finally to p-type. These findings reveal an anisotropic semiconducting material.
    Applied Physics Letters 08/2010; 97(7):073103-073103-3. DOI:10.1063/1.3472280 · 3.52 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 06/2010; 104(25):255702. DOI:10.1103/PhysRevLett.104.255702 · 7.73 Impact Factor
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    ABSTRACT: 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.
    New Journal of Physics 09/2009; 11(9):093039. DOI:10.1088/1367-2630/11/9/093039 · 3.67 Impact Factor
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    ABSTRACT: Oganov et al report a discovery of an ionic high-pressure /hitherto unknown phase of boron/. We show that this phase has been known since 1965, and it is a covalent material. Comment: This manuscript was submitted to Nature (as Brief Communication Arising) on March 1, 2009; rejected by Nature with a decision on publishing an Addendum by Oganov et al (doi:10.1038/nature08164; Nature 460, 292 (2009))
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    ABSTRACT: Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy (up to six hydrogen atoms) of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.
    Physical review. B, Condensed matter 07/2009; 80(2). DOI:10.1103/PhysRevB.80.024101 · 3.66 Impact Factor
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    ABSTRACT: An orthorhombic (space group Pnnm) boron phase was synthesized at pressures above 9 GPa and high temperature, and it was demonstrated to be stable at least up to 30 GPa. The structure, determined by single-crystal x-ray diffraction, consists of B12 icosahedra and B2 dumbbells. The charge density distribution obtained from experimental data and ab initio calculations suggests covalent chemical bonding in this phase. Strong covalent interatomic interactions explain the low compressibility value (bulk modulus is K300=227 GPa) and high hardness of high-pressure boron (Vickers hardness HV=58 GPa), after diamond the second hardest elemental material.
    Physical Review Letters 06/2009; 102(18):185501. DOI:10.1103/PhysRevLett.102.185501 · 7.73 Impact Factor
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    ABSTRACT: Mechanical and thermodynamic stability of the isoelectronic ternary inverse perovskites Sc3EN (E=B,Al,Ga,In) has been studied from first principles. We confirm stability of recently synthesized cubic phases Sc3AlN and Sc3InN, and predict the stability of cubic Sc3GaN and a triclinic phase aP20-Sc3BN. Substantial phonon softening in Sc3AlN and Sc3GaN is observed indicating a possibility that structural defects could form readily. In accord, our experiments show that magnetron sputter deposited films contain regions with high density of nonperiodic stacking faults along the ⟨111⟩ growth direction. We suggest that defect-free crystals may exhibit anomalies in the carrier properties, promising for electronic applications.
    Physical review. B, Condensed matter 04/2009; 79(13). DOI:10.1103/PhysRevB.79.134107 · 3.66 Impact Factor
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    ABSTRACT: The influence of the tetragonal and orthorhombic axial distortions on the body-centered cubic (bcc) phase of Fe at extreme conditions has been studied by means of first-principles calculations. We unambigiously demonstrate that the energy minimum corresponding to the body-centered tetragonal (bct) (c/a≈0.9) structure, previously found in Fe upon the axial tetragonal distortion of the bcc phase along the Bain’s path under compression at zero temperature, is an artifact of the structural constraint. When the bcc structure is examined using the orthorhombic distortion involving the tetragonal distortion as a particular case, the bct (c/a≈0.9) structural framework represents a saddle point between two mirrored face-centered cubic minima rather than a local minimum. Therefore we conclude that there is no ground to emphasize on possible thermal stabilization of the bct structure with a particular c/a ratio apart from the whole family of structures obtained by tetragonal, orthorhombic, or another type of axial distortions.
    Physical review. B, Condensed matter 04/2009; 79(13). DOI:10.1103/PhysRevB.79.132106 · 3.66 Impact Factor

Publication Stats

3k Citations
389.46 Total Impact Points

Institutions

  • 2005–2013
    • Linköping University
      • Department of Physics, Chemistry and Biology (IFM)
      Linköping, Östergötland, Sweden
  • 1997–2005
    • Uppsala University
      • Department of Physics and Astronomy
      Uppsala, Uppsala, Sweden
  • 2000–2003
    • Chalmers University of Technology
      • Department of Applied Physics
      Goeteborg, Västra Götaland, Sweden
    • Stockholm University
      • Department of Organic Chemistry
      Tukholma, Stockholm, Sweden
  • 2001
    • University of Pennsylvania
      • Laboratory for Research on the Structure of Matter
      Philadelphia, Pennsylvania, United States
  • 1996
    • Technical University of Denmark
      • Department of Physics
      Lyngby, Capital Region, Denmark