Perovskites, phase transitions

Murakami, M., Hirose, K., Kawamura, K., Sata, N., Ohishi, Y. (2004). Post-perovskite phase transition in MgSiOj. Science 304,855-858. 

Xiao P, Cheng J-G, Zhou J-S, Goodenough JB, Henkelman G (2013) Mechanism of the Calr03 post-perovskite phase transition under pressure. Phys Rev B 88 144102... 

Spinel to perovskite phase transitions in MgSiO and Mg2SiO ... 

The results of the distorted ion calculations for the enthalpies relevant to this phase transition are shown inFigure 5a. At zero pressure, spinel is the most stable phase. At 24.5 GPa, the two-oxide assemblage (MgO periclass + Si02 stishovite) becomes stable, and remains stable to 31.5 GPa, at which point the perovskite phase becomes stable. The spinel to perovskite phase transition occurs at 26.0 GPa for the Mg2Si04 stoichiom-... 

The effects of covalency on the quartz to stishovite phase transition in silica and the spinel to perovskite phase transition in magnesium silicates can be compared. Both of these phase transitions are from a phase based on tetra-coordinated silicon ions to a phase based on hexa-coordinated silicon ions. For the phase transition in silica, the inclusion of covalent effects reduces the transition pressure by over 15 GPa stishovite is stabilized relative to quartz because it has more bonds, and the bonds are significantly covalent in both structures. In contrast, for the magnesium silicate phase transition, the inclusion of covalent effects increases the transition pressure by 20 GPa although there are more bonds in perovskite than in spinel, spinel is stabilized more because the bonds are more covalent in spinel than in perovskite. 

Uchino K, Cross LE, Newnham RE, Nomura S (1980) Electrostrictive effects in non-polar perovskites. Phase Transitions 1 333... 

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

F can he suppressed hy the high site symmetry of the central atom In many perovskite-like structures of the ABO3 type the lone pair of the B-cat-ion leads not to a structural distortion. In CsPbF3 under ambient conditions no lone-pair activity observed [27], but upon cooling a phase transition is observed that leads to less symmetrical surrounding of Pb by fluoride [28]. 

Alonso, J. A., Martinez-Lopez, M. J., Casais, M. T., Martinez, J. L., Demaseau, G., Largeteau, A., Garcia-Munoz, J. L., Munoz, A., Femandez-Diaz, M. T., High-pressure preparation, crystal structure, magnetic properties, and phase transitions in GdNi03 and DyNiOs perovskites, Chem. Mater. 11, 2463-2469 (1999). 

Phase transitions. Examples BaTiO (> 120°C, cubic perovskite type) -y BaTiOj (< 120°C, tetragonal), cf. Fig. 19.5, p. 230 CaCl2 (> 217°C, rutile type) CaCl2 (< 217°C), cf. Fig. 4.1, p. 33. For second-order phase transitions it is mandatory that there is a group-subgroup relation between the involved space groups (Section 18.4). 

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