Brillouin scattering study of the anisotropic acoustic anomaly in ferroelectric and ferroelastic phase transitions

Abstract

A brief review is given for the Brillouin scattering studies of the dependence of the acoustic anomaly on the direction of sound propagation in structural phase transitions in ferroelectric and ferroelastic crystals. Near the ferroelectric phase transition temperature, the polarization fluctuation shows a characteristic anisotropic anomaly due to electrostatic and elastic long range forces. This anisotropy is observed directly or indirectly as the anisotropic acoustic anomaly by the Brillouin scattering. Near the ferroelastic phase transition, the acoustic mode softens only along a characteristic direction in the crystallographic system. Brillouin scattering has an advantage to observe directly the dependence of the sound wave frequency on the propagation direction.