The broadband gigahertz dynamics of a relaxor ferroelectric crystal 0.70Pb(Sc 1/2 Nb 1/2 )O 3 -0.30PbTiO 3 with a composition close to the starting point of the morphotropic phase boundary is studied by Brillouin scattering. The significant anomaly of the longitudinal acoustic (LA) mode is observed in the vicinity of the Curie temperature, T C ¼ 500 K. Upon cooling, it starts below the Burns temperature, T B ¼ 670 K, due to the interaction between the LA mode and dynamic polar nanoregions (PNRs). The broad central peak (CP) clearly appears below T B , and its intensity becomes a maximum at T C . The relaxation time, s CP , determined from the CP width, shows the typical critical slowing down of the order-disorder phase transition in the vicinity of T C . The size of a dynamic PNR significantly increases below the intermediate temperature, T* ¼ 562 K.

Elastic properties of (1-x)(Na 0.5 Bi 0.5 )TiO 3 -xBaTiO 3 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.

No abstract