Structure analysis of bismuth sodium titanate-based A-site relaxor ferroelectrics by electron diffraction

This study investigates the microstructure, elemental distribution, diffuse phase transition (DPT) and crystalline phases of BNBK 85.4/2.6/12 lead-free ferroelectric ceramics, a high-strain bismuth sodium titanate-based relaxor system with complex A-site compositions. TEM electron diffraction studies indicate that the ceramics are close to a compositionally driven morphotropic phase boundary between a conventional rhombohedral (R) structure and a nanoscale tetragonal (T) structure. The ceramics exhibit a core–shell grain microstructure – the shell region shows a cubic average symmetry with the presence of tetragonal distortions; while the core region is of a mixture of T and R phases. Analysis of the corresponding superlattice reflections and dark-field images reveals an R phase-dominant core with the presence of R-twin domains. The high resolution TEM image of the shell region reveals the existence of T-phase nano-domains. The DPT behavior is mainly attributed to the heterogeneous distribution of cations of high valences and concentrations across the core–shell grains.