Abstract
We report the Brillouin scattering of a disordered Pb(Sc1/2Ta1/2)O3 crystal with degree of order S=0.29. In z(x,x+y)=Z scattering geometry, the broad minimum of longitudinal acoustic mode (LA) shifts and the broad maximum of LA width were observed around 245 K corresponing to the maximum temperature Tm of low frequency dielectric constant. Relaxation times were determined by assuming a complex elastic constant from LA modes, and they were attributed for the first time to the fluctuations of polar microregions. In addition, central peaks with marked temperature dependences were observed below 400 K showing a sharp anomaly at 293 K. Concerning specific heat, no anomaly was found for the as-grown crystal, while two broad anomalies around 293 and 270 K for PST (S=0.31), and only one marked anomaly at 293 K for PST (S=0.85) were observed in the cooling process, respectively. A two-phase transition model (Fm3m-R3m, Z=2; Pm3m-R3m, Z=1) was proposed to explain the experimental results.
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