High-temperature structural transformations in the relaxor ferroelectricsPbSc0.5Ta0.5O3andPb0.78Ba0.22Sc0.5Ta0.5O3

Abstract

Temperature-driven structural transformations in Pb-based perovskite-type relaxors are studied by using polarized Raman spectroscopy, high-resolution powder, and synchrotron single-crystal x-ray diffraction applied to PbSc0.5 Ta0.5 O3 (PST) and Pb0.78 Ba0.22 Sc0.5 Ta0.5 O3 (PBST). The two compounds were chosen as model systems because PST is a relaxor that exhibits ferroelectric long-range order on cooling, whereas PBST shows canonical relaxor behavior. The temperature evolution of phonon anomalies and the pseudocubic unit-cell parameter for both PST and PBST reveals the existence of a characteristic temperature T between the Burns temperature TB and the temperature of the dielectric-permittivity maximum Tm. T is associated with the coupling of initially nucleated small polar clusters and their aggregation into larger polar clusters. The temperature range between TB and T is characterized by a coupling between adjacent off-centered BO6 octahedra to form initial polar clusters, while the range between T and Tm is characterized by a coupling between off-centered B cations from adjacent polar clusters. Off-centered Pb atoms exist even above the Burns temperature and their length of coherence governs the coupling between polar regions comprising B -cation off-center shifts and, consequently, directs the formation of the ferroelectric state to a normal, long-range ordered or nonergodic relaxor state.