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Compression of CaTiO3 and CaGeO3 perovskites

Abstract High-pressure single-crystal X-ray diffraction measurements of CaTiO3 and CaGeO3 perovskite have been carried out to 9.7 and 8.6 GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation-of-state to the P-V data yields values of V0 = 223.764 ± 0.017 Å3, KT,0 = 170.9 ± 1.4 GPa, and K′ = ∂K/∂P = 6.6 ± 0.3 for CaTiO3 and V0 = 206.490 ± 0.017 Å3, KT,0 = 194.0 ± 2.1 GPa, and K′ = 6.1 ± 0.5 for CaGeO3. A similar analysis of the axial compressibilities shows that the degree of anisotropic compression in both perovskites is less than 10%. In CaTiO3 the a and b axes have similar compressibilities (Ka= 168.7 ±2.1 GPa, Kb= 168.3 ±1.9 GPa) whereas the c axis is the least compressible (Kc = 175.3 ±1.5 GPa). In CaGeO3, the b axis (Kb = 188 ± 4 GPa) and the a axis (Ka = 195 ± 5 GPa) are more compressible than the c axis (Kc = 204 ± 3 GPa). The variations with pressure of all axes show significant curvature with increasing pressure and have K′ values ranging from 5.7 ± 0.5 to 7.0 ± 0.4 in CaTiO3 and 5.0 ± 0.9 to 6.9 ± 1.2 in CaGeO3. No phase transition was detected. There is evidence, however, that in CaGeO3 the tetragonal to orthorhombic spontaneous strain decreases slightly with pressure which may indicate that a phase transition occurs at a pressure above 10 GPa. Elasticity trends of Ca-perovskites relating bulk modulus and molar volume are independent of both the degree of distortion from cubic symmetry and the symmetry of the structure.