Successive phase transitions of hexagonal barium titanate: soft acoustic mode and coupled acoustic-soft optic mode

Abstract

The successive phase transitions at T 0 (222 K) and T C (74 K) of hexagonal BaTiO 3 have been studied by Brillouin scattering. An intense central peak was found near T 0 and was identified as a result of scattering from the coupled soft optic and acoustic mode. The coupled mode model was employed in order to explain the observed spectral changes. The ω 0 2Γ 0 ratio of the soft optic mode frequency ω 0 and the damping constant Γ 0, and the square of bilinear coupling constant A show linear dependence on temperature difference T − T 0, indicating the mean field behavior. The clear softening of c 66 shear acoustic mode was found near T C. The square of frequency of c 66 mode depends linearly on T − T C around the transition temperature. The results conclusively show that the phase transition at T C is the proper ferroelastic transition with the order parameter strain s 6.