Abstract
Brillouin spectra of highly-ordered Pb(Sc1/2Ta1/2)O3 (PST-O) and highly-disordered PST (PST-D) were measured over a wide temperature range. The frequency of the longitudinal acoustic (LA) phonon mode of PST-O showed a sharp anomaly corresponding to the sharp maxima of the dielectric permittivity, whereas that of PST-D exhibits a broad anomaly. The remarkable softening of LA phonon was observed below T* = 500 K, which indicates the occurrence of a piezoelectric coupling and/or electrostrictive coupling due to the appearance of non-centrosymmetric polar nanoregions below T*. The relaxation time of the LA phonon of PST-O shows a critical slowing down for order-disorder phase transition, while that of PST-D exhibits a stretched slowing down due to random electric fields related to B-site randomness.
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