Central peak in light scattering from the relaxor ferroelectricPbMg1/3Nb2/3O3

Abstract

A model relaxor ferroelectric ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ (PMN) in single-crystal form has been studied by Raman scattering with special attention to the low-frequency spectra in the vicinity of both the diffuse ferroelectric transition (\ensuremath{\sim}270 K) and the electric-field-induced ferroelectric transition (\ensuremath{\sim}200 K). In addition to the hard-mode features without any pronounced change, a broad central component has been observed and analyzed. This component exhibits some distinct characteristics of a relaxation mode and shows an increase in the intensity and a slow down in the relaxation times in the vicinity of both the transitions mentioned above. No usual soft mode has been found, in agreement with other studies. The central component is only a distinctive dynamic characteristic of the evolution of relaxor ferroelectrics to a ferroelectric state. The central peak activity at about 200 K is connected with the polarization fluctuations whereas the main anomaly in the central-peak behavior in the range of the diffuse transition is caused by the complex fluctuations. We postulate a competition in this range between the development of a ferroelectric state and another nonpolar phase. We emphasize an obvious correlation between the central peak dynamics and other properties, such as acoustic phonon anomalies and modified dielectric response in PMN and some other related relaxor ferroelectrics.