Soft acoustic mode in the ferroelectric phase transition of hexagonal barium titanate.

Abstract

A Brillouin scattering study of the hexagonal modification of barium titanate ( $h$-BaTi${\mathrm{O}}_{3}$) was done to elucidate the soft mode behavior in the ferroelectric phase transition at 74 K. Measurements were made in the temperature region $20<T<300$ K. A clear softening of the ${c}_{66}$ shear acoustic mode was found at the phase transition. The squared frequency of the ${c}_{66}$ mode exhibits a linear dependence on the temperature difference from ${T}_{c}$, both above and below ${T}_{c}$. We conclude from this that the phase transition at ${T}_{c}$ is the proper ferroelastic transition with order parameter the elastic strain ${s}_{6}$.