PbMg1/3Nb2/3O3 as a model object for light scattering experiments

Abstract

A classical relaxor material, PbMg1/3Nb2/3O3 (PMN), has a long history of investigations by Raman and Brillouin spectroscopies. Earlier results on Raman scattering have emphasized a predominant role of disorder and, on the whole, confirmed a model of compositional fluctuations, first suggested by Smolensky and his colleagues. The extensive microstructural studies of PMN in recent years require re-consideration of many conclusions. In the present paper, we initiated analysis of rather complex Raman scattering in order to reach a definite agreement with the recent direct microstructural studies. Different unambiguous features were analyzed in light of polarization measurements, a breakdown in the Raman selection rules, assignment of modes consistent with the Fm3m space group, the mode behavior for some ion substitutions, the key role of the high temperature transformation of the spectra in explaining the origin of Raman active modes, and some other aspects. The behavior of optical phonons is predominantly determined by nanoscale ordering into clusters. However, the evolution to a ferroelectric state affects optical phonons very little. Brillouin scattering gives valuable information about the transition dynamics. The hypersonic damping exhibits a pronounced anomaly at 210K. in the range of a frustrated ferroelectric transition where an anomalous response of other characteristics appears only in an external electric field. Finally, a careful analysis of the Brillouin and Raman spectra of PMN leads to detection of a central peak in light scattering. Quasi-Rayleigh scattering reveals the most important dynamic features in the unique evolution of PMN to a ferroelectric state. Comparison of PMN and some other closely related materials, such as PbSc1/2Ta1/2O3 (PST) and Na1/2Bi1/2TiO3 (NBT) gives evidence of a broad pretransitional range in relaxors with competing interactions between a ferroelectric ordering and another structural transition that develops at higher temperatures.