Electric-field-induced local structural phenomena in relaxor ferroelectric PbSc_(0.5)Nb_(0.5)O_3 near the intermediate temperature T* studied by Raman spectroscopy

摘要

Raman spectroscopy at different temperatures and under an external electric field E was applied to PbSc_(0.5)Nb_(0.5)O_3 single crystals in order to gain further insights into the mesoscopic-scale coupling processes in perovskite-type (ABO_3) relaxor ferroelectrics. Parallel and cross-polarized Raman spectra were collected between 800–80 K with E applied along the cubic [1 0 0], [1 1 0] or [1 1 1] crystallographic directions. The analysis was focused on the field-induced changes in the temperature evolution of three low-energy phonon modes: the Pb-localized mode near 50 cm~(?1), the Pb-BO3 translation mode near 150 cm~(?1), and the B-cation-localized mode near 250 cm~(?1). The results show that competitive ferroelectric (FE) and antiferroelectric (AFE) coupling exists within the system of off-centred Pb~(2+) cations, within the system of off-centred B-site cations as well as between off-centred Pb~(2+) and B-site cations. The strong AFE-type coupling between Pb~(2+) cations along the cubic body diagonal significantly influences the coupling between the B-site cations via the Pb-BO_3 mode and results in AFE-type behaviour of the 'microscopic' T* determined from the B-cation-localized mode near 250 cm~(?1), which explains the previously reported non-trivial field dependence of the 'macroscopic' characteristic temperatures: the temperature of the dielectric-permittivity maximum T_m, T*, and the Burns temperature T_B. The comparative analysis between PbSc_(0.5)Nb_(0.5)O_3 and PbSc_(0.5)Ta_(0.5)O_3 indicates that two major displacive order parameters couple to form a relaxor state in B-site complex perovskites: the FE order associated with polar shifts of B-site cations and the AFE order associated with polar shifts of A-site cations. The latter penetrates through both polar and non-polar regions, but it is highly frustrated due to the high density of translation-symmetry faults in the chemical NaCl-type B-site order. The frustrated AFE order of off-centred A-site cations might be the key factor for the existence of a relaxor state.