T* of Relaxor Ferroelectric (1 − x) Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃ Single Crystals Revisited Using Brillouin Spectroscopy

The relaxor characteristics of Pb[(Zn_1/3Nb_2/3)_1–xTi_x]O₃ (PZN-xPT) single crystals (with x = 0 and x = 0.08) are investigated using temperature-dependent Brillouin scattering spectroscopy. The longitudinal acoustic (LA) mode shows typical relaxor behaviors for PZN, whereas its mode frequency exhibits a discontinuous change at the phase transition temperature (T_C) for PZN-8%PT. The transverse acoustic (TA) mode, which is forbidden in an ideal cubic phase at the backscattering geometry, appears in both samples at a certain temperature above either the dielectric maximum temperature (T_m) or T_C. This breakdown of the Brillouin selection rule in the cubic phase indicates that the noncubic polar regions become appreciable at this temperature where the interaction between the acoustic waves and the polar nanoregions (PNRs) is strong enough to enable the appearance of the TA mode. The intermediate characteristic temperature T* is determined in terms of the minimum of the temperature derivative of the half width of the LA mode, and the values are nearly the same as those determined via the acoustic emission signal measurements. These comparisons reveal the universal feature of the local phase transformation from dynamic to quasistatic PNRs at T*, giving rise to the appearance of the TA mode.