| 1. |
- Arulraj, A, et al.
(författare)
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Shear strain in Nd0.5Ca0.5MnO3 at high pressures
- 2005
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Ingår i: Physical Review Letters. - 1079-7114. ; 94:16, s. 29-31
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure x-ray powder diffraction has been measured on the half doped rare earth manganite Nd0.5Ca0.5MnO3 up to a pressure of 15 GPa. We report the presence of a quantifiable amount of shear distortion of the MnO6 octahedra in Nd0.5Ca0.5MnO3 at high pressures. The lattice strain of Nd0.5Ca0.5MnO3 is minimal at a crossover pressure of p*similar to 7 GPa, with the same lattice strain above and below this pressure achieved by shear and Jahn-Teller-type distortions, respectively. The increase in shear strain with increasing pressure provides a mechanism for the insulating behavior of manganites at high pressures that has not been considered before.
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| 2. |
- Dinnebier, RE, et al.
(författare)
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Bulk moduli and high-pressure crystal structures of minium, Pb3O4, determined by X-ray powder diffraction
- 2003
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Ingår i: American Mineralogist. - 0003-004X. ; 88:7, s. 996-1002
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Tidskriftsartikel (refereegranskat)abstract
- We report the pressure dependence of the crystal structure of lead tetroxide (P less than or equal to 41.05 GPa, T 298 K) using high-resolution angle-dispersive X-ray powder diffraction. Pb3O4 shows two reversible phase transitions in the measured pressure range. The crystal structures of the modifications identified have in common frameworks of Pb+4O6 octahedra and irregular Pb+2O4+1 respectively Pb2+O6+1 polyhedra. At ambient conditions, Pb3O4 crystallizes in space group P4(2)/mbc (phase 1). Between 0.11 and 0.3 GPa it exhibits a displacive second order phase transition to a structure with space group Pbam (phase II). A second displacive phase transition occurs between 5.54 and 6.6 GPa to another structure with space group Pbam (phase 111) but halved c dimension. A non-linear compression behavior over the entire pressure range is observed, which can be described by two Vinet relations in the ranges from 0.28 to 5.54 GPa and from 6.6 to 41.05 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to be K-0 = 21(2) GPa for phase 11 and K-0 = 91(3) GPa for phase III. The crystal structures of all phases were refined from X-ray diffraction powder data collected at several pressures between 0.06 and 41.05 GPa. Except for their cell dimensions, phases I and 11 were found to be isostructural to the corresponding phases at low temperatures, whereas phase III can be derived from the Sr2PbO4 aristotype. With increasing pressure, the lone pair which is localized at Pb2+ adopts increasingly pure s-character, which is reflected by the similar coordination polyhedra of Ph2+ in Pb3O4 (phase 111) and of Sr2+ in Sr2PbO4.
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| 3. |
- Wunschel, M, et al.
(författare)
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High-pressure phase transitions in tetrakis(trimethylsilyl)silane Si[Si(CH3)(3)](4)
- 2003
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Ingår i: High Pressure Research. - : Informa UK Limited. - 1477-2299 .- 0895-7959. ; 23:4, s. 425-437
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Tidskriftsartikel (refereegranskat)abstract
- The compound tetrakis(trimethylsilyl)silane S-i[Si(CH3,)(3)](4) (TSi) has been studied at room temperature for pressures up to 2 1.5(1) GPa, using X-ray powder diffraction with synchrotron radiation. Three different phases are reported in the investigated pressure range. The c.c.p.-type structure is stable up to a pressure of 0.19(1) GPa (Fm (3) over barm, a = 13.52110)Angstrom, V = 2471.95(1)Angstrom(3), Z = 4 at 0.02(1)GPa). It shows a severe orientational disorder of the molecules. At pressures 0.19(1)-0.71(1)GPa a monoclinic phase (HP1) is found with space group P2(1)/n and Z=8 (a = 17.746(1)Angstrom, b = 16.098(1)Angstrom, c= 16.871(1)Angstrom, gamma = 111.437(5)degrees and V = 4486.3(8) Angstrom(3) at 0.24(1)GPa). Another monoclinic phase (HP2) exists at pressures 0.52(1)-8.67(5)GPa. This phase has a smaller unit cell than the HP1 phase with Z(.)=2 (a =8.3778(8)Angstrom, b=9.1050(7)Angstrom, c= 13.3024(8)Angstrom, gamma= 111.63(1)degrees and V = 943.22(6) Angstrom(3) at 1.73(2)GPa). The pressure dependencies of the unit-cell volumes are successfully described by Mumaghan or Vinet-type equations of state. The structures at different pressures and temperatures of TSi and related compounds are described as distorted superstructures of the c.c.p. arrangement. The occurrence of different superstructures is rationalized as the result of an optimized packing of ordered molecules of various shapes.
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