ELASTICITY OF CATIO3, SRTIO3 AND BATIO3 PEROVSKITES UP TO 3.0 GPA - THE EFFECT OF CRYSTALLOGRAPHIC STRUCTURE

Elasticity of CaTiO3, SrTiO3 and BaTiO3 perovskites has been experimen tally investigated as a function of pressure up to 3.0 GPa in a liquid -medium piston cylinder apparatus using a high precision ultrasonic in terferometric technique. Specimens used are hot-pressed fine-grained ( 3-10 mum) polycrystalline aggregates with low porosity (< 1.5%). Compr essional and shear wave velocities and their pressure derivatives have been measured. The results are compared with previous studies on othe r perovskites and the role of structural transitions is examined. We f ind that the role of Ti-O6 PolYhedral tilting (such as observed in CaT iO3) is small in the sense that a single well-defined general trend ex ists in perovskites with a wide range of tilting angles, although ther e is suggestion that cubic perovskites have slightly higher bulk modul us than orthorhombic perovskites. In contrast, cation-anion displaceme nt that changes crystal symmetry from cubic to tetragonal in BaTiO3 ha s very large effects on elasticity. This distortion significantly redu ces the bulk modulus (but not much the shear modulus) and results in a n unusually large pressure derivative of bulk modulus (dK/dP approxima tely 10). A large change in elasticity in BaTiO3 associated with the s tructural transition (without a significant volume change) is a clear example of the breakdown of the Birch's law between densities and elas tic wave velocities.