Elastic Anomalies and Diffuse Phase Transitions of (1-x)(Na_0.5Bi_0.5)TiO₃-xBaTiO₃ Single Crystals Studied by Brillouin Scattering Spectroscopy

Abstract

Elastic properties of (1-x)(Na_0.5Bi_0.5)TiO₃-xBaTiO₃ (NBT-xBT: x = 0, 0.01, 0.02, 0.03, 0.05, 0.08) are measured by the Brillouin scattering method to clarify the mechanism of the diffuse phase transition. The temperature dependences of the Brillouin shift and full width at half maximum of the longitudinal acoustic mode show broad anomalies at the maximum temperature of the dielectric constant T _m in all compositions. It indicates that elastic anomalies in NBT-xBT are caused by the coupling between the polarization and the strain. We consider this coupling is the piezoelectric coupling between the localized polarization in the polar nanoregion and the elastic strain. The composition dependence of the elastic constant in NBT-xBT at room temperature (T = 293 K) shows decreasing with approaching the morphotropic phase boundary (MPB) at about x = 0.06. It seems that the softening is reflected by the instability of the strain, as well as the polarization near the MPB.