Crystal structure of heteroepitaxial BaTiO₃–KNbO₃ core–shell nanocomposite particles studied by synchrotron radiation X-ray diffraction

Abstract We investigated the temperature-dependent crystal structure of a BaTiO 3 −KNbO 3 (BT−KN) nanocomposite particle in which the KN shell epitaxially covers the BT core. Synchrotron radiation X-ray diffraction experiments were performed over a temperature range of 300–800 K. Near the interface, BT and KN were found to be bonded in a pseudo-cubic crystal structure with similar lattice constants across all temperatures. As the temperature decreased, strain-gradient regions (SGRs) near the interface, caused by lattice mismatch, enlarged significantly owing to phase transitions. The largest SGRs with a tetragonal BT core and an orthorhombic KN shell were observed at 300 K. However, SGRs were minimal at 800 K, where both BT and KN possessed cubic crystal structures. Engineering interfaces such as SGRs can enhance the dielectric constant; therefore, it is crucial to consider material combinations with different crystal symmetries but similar unit cell volumes, such as BT−KN at RT.