Elastic and gyrotropic anomalies and acoustic activity in lead germanate

Abstract

The paper discusses elastic and gyrotropic anomalies as well as the acoustic activity of lead germanate during the phase transition from the space group C3h to C3. Expressions are derived for all the anomalies of the elastic constants, and relations among them have been established. We show that the elastic constants C11, C33, C12, and C13 suffer a dip at the transition temperature, in agreement with experimental results. Several invariants involving the order parameter, strain variables, and their derivatives are constructed and from these the contribution to the coupling energy of the gyrotropic energy has been calculated. Expressions are given for the anomalies of the various gyrotropic constants. We show that the system will exhibit anomalies in acoustic activity in the low symmetry phase, and an expression for the polarization angle has been obtained.