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Sökning: WFRF:(Dubrovinsky Leonid)

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  • Dzwilewski, Andrzej, et al. (författare)
  • Characterization of phases synthesized close to the boundary of C60 collapse at high temperature high pressure conditions
  • 2007
  • Ingår i: Diamond and related materials. - 0925-9635. ; 16:8, s. 1550-1556
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • Two sets of samples were synthesized at high pressure high temperature conditions in the P-T region where C-60 molecules collapse into a nearly amorphous graphite-like hard carbon phase. For the first set, heating temperature was varied at fixed pressure and preparation time. For the second set, synthesis time was varied at fixed pressure and fixed temperature. Detailed structural characterization of samples was performed using Raman spectroscopy and powder XRD. Mechanical properties of the samples have been studied by nanoindentation method. It has been found that duration of heat treatment under high pressure is an important parameter which influences the temperature of fullerene cage collapse. Both tetragonal and rhombohedral polymeric phases transform into hard carbon phase over a rather narrow temperature interval, but the tetragonal phase shows somewhat increased stability against C-60 collapse. Viscoelastic mechanical behavior during nanoindentation was observed for fullerene polymers but not for graphite-like hard carbon phase. Possible mechanism for nucleation of the hard carbon phase in polymeric C-60 networks is discussed.
  • 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
  • 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.
  • Saxena, Surendra, et al. (författare)
  • 1999
  • Ingår i: Technical Program of the 128 TMS Annual Meeting. ; s. 110
  • Konferensbidrag (refereegranskat)abstract
    • There are four well-known structural polymorphs of iron, namely alfa and delta (body centerd cubic), gamma (face centered cubic) and epsilon (hexagonal closest packed). The possible occurrence of a new iron phase was suggested by (1); the evidence was indirect, obtained through laser heating of iron above pressures of about 40 gigapascal (GPa). We found that the FCC phase (face centered cubic), when heated in its stability field, is quenchable at high pressures. Using this technique, we determined that the structure of the quenched phase at high pressures is DHCP (double hexagonal closest packed). Several experiments done using in-situ heating and x-ray have now confirmed the presence of this new phase (2). Although experimental data are sparse, it is possible to obtain a fairly quantitative thermodynamic description of all the iron phases and the double hexagonal closest packed (DHCP) beta iron. Earth’s core beginning at depths of 2900 Km (133 GPa) and continuing to the center (6730 Km, 360 GPa) consists principally of iron. Until experiments become feasible on iron above 200 GPa, extrapolation of thermodynamic data on iron is necessary for the study of the core. The assessed data on iron is based on experimental data on a) melting to 200 GPa, b) the location of the triple point HCP-DHCP-FCC at 36 GPa and 1450K and c) the location of the triple point DHCP-FCC-melt close to 60 GPa and 2800K. If no other phase transition intervenes, the melting of beta-iron at 360 GPa takes place at temperatures less than 5000K, which constrains the temperature of Earth’s center to be less than that. However there are some shock-wave data that cannot be easily reconciled with the current data obtained with the diamond-anvil cell technique unless there is yet another high P phase transition of the beta phase at about 200 GPa to increase the melting temperature by about 1000 at the center. A search for such a phase is now on.1. S. K. Saxena, Shen, G. & Lazor, P. Science, 260, 1312-1314, (1993).2. S. K. Saxena, L. S. Dubrovinsky and Haggkvist, P. Geophys. Res. Lett., 23, 2441-2444 (1996).3. O.L. Anderson, Rev. Geophys. Suppl., 429-441, (1995).
  • 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.
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