Transition dynamics in relaxor ferroelectrics

Abstract

Raman scattering was used to study two model relaxor ferroelectrics, namely PbMg_1/3 Nb_2/3O₃(PMN) with the 1:2 stoichiometric composition of Mg²⁺ and Nb⁵⁺ ions in the oxygen octahedrons and PbSc_1/2Ta_1/2O₃ (PST) with the 1:1 stoichiometric composition of Sc³⁺ and Ta^5- 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 dusters at least in nanoscale regions. The spectra show some anomalous features in the temperature range preceding a ferroelectric state, for example, a complex structure develops from the initially singlet line in PST and a broad central peak appears in PMN. Those phenomena are considered as the dynamics 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. The results obtained in PST and PMN were compared with the behavior of a related relaxor Na_1/2Bi_1/2TiO₃ (NBT) with a cubic-tetragonal-rhombohedral sequence of “preceding” phases. The important similarity was found in the behavior of PST and NBT (double hysteresis loops) and in the behavior of PMN and NBT (acoustic anomalies, central peaks). The comparison of these three relaxor ferroelectrics gives evidence of competing interactions between the ferroelectric phase and some upper non-polar phase.

Key Words:

• Relaxor ferroelectrics
• phase transition dynamics
• Raman scattering