Topology and temperature dependence of the diffuse X-ray scattering in Na _0.5Bi _0.5TiO ₃ ferroelectric single crystals

Abstract

The high-resolution temperature-dependent diffuse X-ray scattering from an Na _0.5Bi _0.5TiO ₃ perovskite-based single crystal is measured on the approach to the polymorphic phase transition. The previously unseen topological features of the diffuse scattering and their temperature evolution are reported.

Abstract

The results of high-resolution measurements of the diffuse X-ray scattering produced by a perovskite-based Na _0.5Bi _0.5TiO ₃ ferroelectric single crystal between 40 and 620 K are reported. The study was designed as an attempt to resolve numerous controversies regarding the average structure of Na _0.5Bi _0.5TiO ₃, such as the mechanism of the phase transitions between the tetragonal, P4 bm, and rhombohedral | monoclinic, R3 c |  Cc, space groups and the correlation between structural changes and macroscopic physical properties. The starting point was to search for any transformations of structural disorder in the temperature range of thermal depoling (420–480 K), where the average structure is known to remain unchanged. The intensity distribution around the {032} pseudocubic reflection was collected using a PILATUS 100K detector at the I16 beamline of the Diamond Light Source (UK). The data revealed previously unknown features of the diffuse scattering, including a system of dual asymmetric L-shaped diffuse scattering streaks. The topology, temperature dependence, and relationship between Bragg and diffuse intensities suggest the presence of complex microstructure in the low-temperature R3 c |  Cc phase. This microstructure may be formed by the persistence of the higher-temperature P4 bm phase, built into a lower-temperature R3 c |  Cc matrix, accompanied by the related long-range strain fields. Finally, it is shown that a correlation between the temperature dependence of the X-ray scattering features and the temperature regime of thermal depoling is present.