Acoustic properties of betaine phosphite in the vicinity of phase transitions

A study is reported of acoustic anomalies in the vicinity of the antiferrodistortive and ferroelectric phase transitions in nominally pure betaine phosphite crystals and a crystal with 3% betaine phosphate impurity. The observed anomalies in the velocity and damping rate of longitudinal acoustic waves propagating along different crystallographic axes are interpreted within Landau theory. It is shown that the phase transitions in these crystals are characterized by a relatively weak correlation of the order parameter to strain and by a considerable contribution to the acoustic anomalies of an interaction higher in order than striction. Fiz. Tverd. Tela (St. Petersburg) 40, 1090–1096 (June 1998)