Abstract
Abstract The successive phase transitions at T 0 (222 K) and T c (74 K) of hexagonal BaTiO3 have been studied by Brillouin scattering. The anomalous central peak was found near T 0, and drastic changes in spectral shape and intensity were observed below T 0. The coupled-mode model for a soft optic and acoustic mode was employed to fit the experimental data in order to explain the observed spectral changes. The ω2 0/Γ0 ratio of soft optic mode frequency ω0 and damping constant Γ0 and the square of bilinear coupling constant show linear dependence on T - T 0, indicating the mean-field behavior. The complete softening of c66 shear acoustic mode was found near T c . The square of the frequency of c66 shear acoustic mode depends linearly on the temperature T –- T c . The results suggest that the phase transition at T c is the proper ferroelastic transition with the order parameter strain s6.
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