Compression of CaTiO3 and CaGeO3 perovskites

High-pressure single-crystal X-ray diffraction measurements of CaTiO3 and C aGeO3 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 +/- 0.017 Angstrom(3), K-T,K -0 = 170.9 +/- 1.4 GPa, and K' = partial derivative K/partial derivative P = 6.6 +/- 0.3 for CaTiO3 and V-0 = 206.490 +/- 0.017 Angstrom(3), K-T,K-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 compressio n in both perovskites is less than 10%. In CaTiO3 the a and b axes have sim ilar compressibilities (K-a = 168.7 +/- 2.1 GPa, K-b = 168.3 +/- 1.9 GPa) w hereas the c axis is the least compressible (K-c = 175.3 +/- 1.5 GPa). In C aGeO3, the b axis (K-b = 188 +/- 4 GPa) and the a axis (K-a = 195 +/- 5 GPa ) are more compressible than the c axis (K-c = 204 +/- 3 GPa). The variatio ns with pressure of all axes show significant curvature with increasing pre ssure 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 b oth the degree of distortion from cubic symmetry and the symmetry of the st ructure.