Central peak in light scattering from the relaxor ferroelectric PbMg1/3Nb2/3O3

A model relaxer ferroelectric PbMg1/3Nb2/3O3 (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 (similar to 270 K) and the electric-field-induced ferroelectric transition (similar to 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 relaxer 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 relaxer ferroelectrics.