Structural diversity of the (Na1−xKx)0.5Bi0.5TiO3 perovskite at the morphotropic phase boundary

In-situ-heating transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses were performed on the (Na1−xKx)0.5Bi0.5TiO3 perovskite solid solution at the morphotropic phase boundary (MPB) in order to evaluate the structural contribution to the enhanced electromechanical properties of piezoelectrics. Prior to heating in the TEM, the MPB sample exhibits a lamellar domain structure and an in-phase tilting of the oxygen octahedrons, both of which are characteristic of a tetragonal structure. We found that after heating the lamellar domains do not reappear; instead, a tweed-like, nanodomain structure is established with the in-phase tilt system to a large extent preserved. The XRD measurements of the MPB samples confirmed that the original structure of the MPB material is pseudocubic. When exposed to various external fields (mechanical, electrical, or temperature), the material adjusts its crystal form and orientation to the type and direction of the field, leading to the formation of a rhombohedral crystal structure, a tetragonal one or the coexistence of both structures. These analyses reveal that the source of the enhanced electromechanical responses of piezomaterials at their MPBs is their “easily adaptable” relaxor-type character with the field-induced structural conformism.