Influence of electric field on local phase transformations in relaxor ferroelectrics PbSc_(0.5)Ta_(0.5_O_3 and Pb_(0.78)Ba_(0.22_Sc_(0.5)Ta_(0.5)O_3

摘要

In situ temperature-dependent Raman-scattering experiments under an external dc electric field E have been performed on PbSc_(0.5)Ta_(0.5)O_3 (PST) and Pb_(0.78)Ba_(0.22)Sc_(0.5)Ta_(0.5)O_3 (PST-Ba) in order to give further insights into the atomistic mechanism of polar coupling in perovskite-type (ABO3) relaxor ferroelectrics near the intermediate characteristic temperature T~*. The polarized Raman spectra collected under an electric field E applied along the cubic [100], [110], or [111] crystallographic direction were analyzed in terms of hard-mode spectroscopy. The T~*(E) dependence was derived from the behavior of the phonon mode near 230 cm~(-1) localized in off-centered B-site cations. In the case of stoichiometric PST, the increase of T~* with E is much stronger when the field is applied along [111] than when E is along [100] or [110]. This indicates that the actual polar B-cation shifts are along the cubic body diagonals, implying a rhombohedral structure of the polar nanoregions. The T~*(E)-dependence reveals that the local structural distortions associated with locally coupled polar displacements of B-site cations reach saturation near 0.5 kV/cm. When Pb~(2+) is partially replaced by Ba~(2+), the strong increase of T~* with E occurs if the electric field is applied along the [110] direction. This indicates that the substitution disorder on the A-site lowers the symmetry of the polar nanoregions to orthorhombic or monoclinic. The T~*(E) dependence determined from the B-cation localized mode shows saturation near 2.0 kV/cm, indicating that the zero-field structural state of PST-Ba exhibits less coupled polar shifts of B-site cations as compared to that of PST. According to the E-dependence of the Raman scattering near 55 cm~(-1), for both compounds the overall response of the Pb system to the external electric field in the vicinity of T~*(E) resembles antiferroelectric behavior, which along with the fact that the coupling between the B-site cations is ferroelectric, suggests that the polar nanoregions in Pb-based relaxors are ferrielectric in nature.