Specific features of hypersonic damping in relaxor ferroelectrics

A comparative analysis of the hypersonic damping behavior in a wide temperature range has been carried out in a relaxor ferroelectric PbMg1/3Nb2/3O3-(PMN) and in a related crystal Na1/2Bi1/2TiO3- (NBT). The damping of longitudinal acoustic phonons was obtained from Brillouin scattering data. The main very broad maxima in hypersonic damping are found to be shifted to higher temperatures with respect to the corresponding main dielectric anomalies. Additional sharp peaks in the hypersonic damping are connected with an inner structural rearrangement in both crystals. This inner evolutionary process contributes to the hypersonic damping regardless of whether there is a real phase transition like in NBT or such a phase transition is frustrated like in PMN. Comparing NBT with PMN we suggest that the dynamics of these materials is determined by two coupled order parameters.