TRANSITION DYNAMICS IN RELAXOR FERROELECTRICS

Raman scattering was used to study two model relaxer ferroelectrics, n amely PbMg1/3 Nb2/3O3(PMN) with the 1:2 stoichiometric composition of Mg2+ and Nb5+ ions in the oxygen octahedrons and PbSc1/2Ta1/2O3 (PST) with the 1:1 stoichiometric composition of Sc3+ and Ta5+ ions. In spit e of a different stoichiometric ratio the Raman spectra of both materi als are consistent with the Fm3m space symmetry which implies the exis tence of similar 1:1 ordered clusters at least in nanoscale regions. T he spectra show some anomalous features in the temperature range prece ding a ferroelectric state, for example, a complex structure develops from the initially singlet line in PST and a broad central peak appear s in PMN. Those phenomena are considered as the dynamics features in c ourse of evolution of the relaxors to a ferroelectric state. The prece ding phase is characterized by a breakdown in the selection rules for Raman scattering, so some points in the Brillouin zone can contribute to the light scattering spectra. The results obtained in PST and PMN w ere compared with the behavior of a related relaxer Na1/2Bi1/2TiO3 (NB T) With a cubic-tetragonal-rhombohedral sequence of ''preceding'' phas es. The important similarity was found in the behavior of PST and NET (double hysteresis loops) and in the behavior of PMN and NET (acoustic anomalies, central peaks). The comparison of these three relaxer ferr oelectrics gives evidence of competing interactions between the ferroe lectric phase and some upper non-polar phase.