Abstract
ABSTRACTRaman scattering was used to study two model relaxor ferroelectrics, PbMg1/3Nb2/3O3 (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 spite of a different stoichiometric ratio the Raman spectra of both materials are consistent with the Fm3m space symmetry which implies the existence of similar 1:1 ordered clusters at least in nanoscale regions. The spectra show some anomalous features in the temperature range preceding a ferroelectric state, namely a broad central peak appears in PMN and a complex structure develops from the initially singlet line in PST. Those phenomena are considered as the dynamic features in course of evolution of the relaxors to a ferroelectric state. The preceding 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. Comparing all available data, one can assume the determinant role of heterophase fluctuations in that process. The fluctuations in a special preceding phase are caused by a competition between two phases, namely between the ferroelectric phase and an additional nonpolar phase.
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