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Sökning: WFRF:(Dubrovinsky Leonid) > Linköpings universitet

  • Resultat 1-6 av 6
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  • Mondal, Swastik, et al. (författare)
  • Electron-Deficient and Polycenter Bonds in the High-Pressure gamma-B-28 Phase of Boron
  • 2011
  • Ingår i: PHYSICAL REVIEW LETTERS. - American Physical Society. - 0031-9007. ; 106:21, s. 215502
  • Tidskriftsartikel (refereegranskat)abstract
    • The peculiar bonding situation in gamma 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-2 dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
  • Raza, Zamaan, et al. (författare)
  • First-principles calculations of properties of orthorhombic iron carbide Fe7C3 at the Earths core conditions
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - American Physical Society. - 1098-0121. ; 91:21, s. 214112
  • Tidskriftsartikel (refereegranskat)abstract
    • A recently discovered phase of orthorhombic iron carbide o-Fe7C3 [Prescher et al., Nat. Geosci. 8, 220 (2015)] is assessed as a potentially important phase for interpretation of the properties of the Earths core. In this paper, we carry out first-principles calculations on o-Fe7C3, finding properties to be in broad agreement with recent experiments, including a high Poissons ratio (0.38). Our enthalpy calculations suggest that o-Fe7C3 is more stable than Eckstrom-Adcock hexagonal iron carbide (h-Fe7C3) below approximately 100 GPa. However, at 150 GPa, the two phases are essentially degenerate in terms of Gibbs free energy, and further increasing the pressure towards Earths core conditions stabilizes h-Fe7C3 with respect to the orthorhombic phase. Increasing the temperature tends to stabilize the hexagonal phase at 360 GPa, but this trend may change beyond the limit of the quasiharmonic approximation.
  • Sergueev, I., et al. (författare)
  • Hyperfine Splitting and Room-Temperature Ferromagnetism of Ni at Multimegabar Pressure
  • 2013
  • Ingår i: Physical Review Letters. - AMER PHYSICAL SOC. - 0031-9007. ; 111:15
  • Tidskriftsartikel (refereegranskat)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 Mossbauer transition of Ni-61. 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-Mossbauer factor increases from 33 me V at ambient pressure to 60 me V at 100 GPa. The change of this energy over volume compression is well described by a Gruneisen parameter of 2.09.
  • Steneteg, Peter, et al. (författare)
  • Missing-atom structure of diamond Sigma 5 (001) twist grain boundary
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - American Physical Society. - 1098-0121. ; 84:14, s. 144112
  • Tidskriftsartikel (refereegranskat)abstract
    • We carried out a combined experimental and theoretical study of grain boundaries in polycrystalline diamond, aimed at achieving the conditions in which grain boundaries are equilibrated. Raman spectra of compacted at high-pressure and high-temperature diamond powders allow us to identify signals from sp(2)-bonded atoms, in addition to a strong peak at 1332 cm(-1), corresponding to sp(3)-bonded carbon. To verify our interpretation of the experiment, Sigma 5 (001) twist grain boundaries of polycrystalline diamond were studied by means of molecular dynamics simulations using the technique proposed by von Alfthan et al. [Phys. Rev. Lett. 96, 055505 (2006)]. We find that grain-boundary (GB) configurations, from which one atom is removed, have significantly lower energy compared to those obtained with conventional techniques. These calculated GBs are highly ordered, a few monolayers thick, in agreement with experimental observations, and are primarily sp(2) bonded. This paper underlines the importance of varying the number of atoms within GBs in molecular dynamics simulations to correctly predict the GB ground-state structure.
  • Vekilova, Olga, et al. (författare)
  • Effect of non-uniform stresses on bcc Fe and Fe-Ni alloys at Earth's core conditions
  • ????
  • Annan publikation (övrigt vetenskapligt)abstract
    • Elucidation of Earth's core composition and structure is needed for the understanding of various phenomena, from geodynamics to geomagnetism. Geochemical data suggest that solid Fe alloyed with Ni is Earth's core main constituent, however, the exact content and crystal structure remain unclear. We report the results of ab initio molecular dynamics study of the mechanical stability of the body-centered cubic (bcc) phase of Fe and Fe0.89Ni0.ll. A striking saw-toothed effect in the dependence of stresses on finite distortions is revealed. The calculated region of mechanical stability of the bcc iron under non- hydrostatic stress at Earth's core conditions is narrow, and about 5 GPa stresses should destabilize it. While it is unlikely that stresses in Earth's core are sufficient to mechanically destabilize bcc iron, experimental controversies and puzzling stabilization of diverse phases may be well explained by the observed saw-toothed stress effect. It might be common for different materials close to dynamical instability and has crucial importance for determination of equilibrium phase boundaries.
  • Xu, Weiming M., et al. (författare)
  • Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide
  • 2013
  • Ingår i: Physical Review Letters. - 0031-9007. ; 111:17
  • Tidskriftsartikel (refereegranskat)abstract
    • Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H center dot center dot center dot 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, Mossbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe3+ high-to-low spin crossover at above 45 GPa.
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