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University College London, Department of Geological Sciences, London, United Kingdom
High-pressure single-crystal X-ray diffraction measurements of CaTiO 3 and CaGeO 3 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 V 0 = 223.764 + or - 0.017 Aa 3 , K (sub T,0) = 170.9 + or - 1.4 GPa, and K' = theta K/theta P = 6.6 + or - 0.3 for CaTiO 3 and V 0 = 206.490 + or - 0.017 Aa 3 , K (sub T,0) = 194.0 + or - 2.1 GPa, and K' = 6.1 + or - 0.5 for CaGeO 3 . A similar analysis of the axial compressibilities shows that the degree of anisotropic compression in both perovskites is less than 10%. In CaTiO 3 the a and b axes have similar compressibilities (K a = 168.7 + or - 2.1 GPa, K b = 168.3 + or - 1.9 GPa) whereas the c axis is the least compressible (K a = 175.3 + or - 1.5 GPa). In CaGeO 3 , the b axis (K b = 188 + or - 4 GPa) and the a axis (K a = 195 + or - 5 GPa) are more compressible than the c axis (K a = 204 + or - 3 GPa). The variations with pressure of all axes show significant curvature with increasing pressure and have K' values ranging from 5.7 + or - 0.5 to 7.0 + or - 0.4 in CaTiO 3 and 5.0 + or - 0.9 to 6.9 + or - 1.2 in CaGeO 3 . No phase transition was detected. There is evidence, however, that in CaGeO 3 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.
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