Broadband gigahertz dynamics of relaxor ferroelectric Pb(Sc _1/2Nb_1/2)O₃-xPbTiO₃ single crystal probed by Brillouin scattering

The broadband gigahertz dynamics of a relaxor ferroelectric crystal 0.70Pb(Sc_1/2Nb_1/2)O₃-0.30PbTiO₃ 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 = 500 K. Upon cooling, it starts below the Burns temperature, T_B = 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, τ_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* = 562 K.