A Soft Acoustic Mode in the Ferroelastic Phase Transition of LaNbO₄

Abstract

An anisotropic softening of the acoustic mode in the ferroelastic phase transition of LaNbO 4 has been observed by the Brillouin scattering experiment. The transverse mode propagating along a direction deviated from the crystal axis by 23° in the a b -plane shows a complete softening at the transition temperature T 0 . The direction is parallel or normal to the ferroelastic domain boundary as predicted by the phenomenological analysis of the phase transition from 4/ m to 2/ m symmetry. Thc temperature dependence of the elastic susceptibility is well described by the classical indices γ=γ'=1 in both phases, suggesting a typical proper ferroelastic transition.