Soft Acoustic Mode in the Ferroelectric Phase Transition of Hexagonal Barium Titanate

A Brillouin scattering study of the hexagonal modification of barium titanate ( h-BaTiO₃) 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 c₆₆ shear acoustic mode was found at the phase transition. The squared frequency of the c₆₆ mode exhibits a linear dependence on the temperature difference from Tc, both above and below Tc. We conclude from this that the phase transition at Tc is the proper ferroelastic transition with order parameter the elastic strain s₆.