Raman scattering in the relaxor-type ferroelectric Na1/2Bi1/2TiO3

A complex perovskite-type single crystal Na1/2Bi1/2TiO3 (NBT) with a cubic- tetragonal-trigonal sequence of phase transition, ending in a ferroelectric state, has been studied by Raman scattering. In additional to a possible s oft mode which softens far below the phase transition, a broad central comp onent has been revealed in a form which let us analyze the temperature depe ndence of its intensity. The central component has features of a relaxation mode with the highest intensity in the intertransition region, so it arise s from a critical light scattering on fluctuations of two coupled order par ameters. The correlations between the central component behaviour, acoustic anomalies and dielectric response as well as their relation to a model wit h competing interactions are discussed. It is suggested that the broad cent ral peak component in light scattering is characteristic of the relaxor-typ e ferroelectrics with a sequence of phase transformation irrespective of wh ether transitions are really observed as in NBT or whether transitions are frustrated at normal conditions as in PbMg1/3Nb2/3O3 (PMN).