Acoustic and dielectric properties of betaine phosphite and a phenomenological model with coupled order parameters

Abstract

An ultrasonic study of betaine phosphite and betaine phosphite with 3%of betaine phosphate in the region of antiferrodistortive and ferroelectric phase transitions has been performed. Acoustic and dielectric properties are explained in the framework of a Landau-type thermodynamic potential with coupled structural and polar order parameters. The model describes the temperature dependence of the dielectric constant in the antiferrodistortive phase. It is shown that the interaction between the order parameters manifests itselfs in a change of the slope of the velocity temperature dependence at the ferroelectric phase transition. The change of the slope is clearly observed for the longitudinal waves propagating along polar axis, when long-range dipolar forces suppress the main anomaly due to striction. The noticeable value of such an anomaly at the ferroelectric phase transition can be due only to a strong biquadratic coupling between the strain and the structural order parameter, which is the case for betaine phosphite. The value of the anomaly calculated from the model is very close to the experimental value.