Pressure-induced polar–nonpolar–polar phase conversion in Na0.5Bi0.5TiO3

Na0.5Bi0.5TiO3 (NBT) is a promising lead-free ferroelectric material and has a unique A-site substitution structure with rhombohedral (R3c) symmetry at ambient conditions. Combining in situ polarization–electric field (P–E) hysteresis loops, Raman spectroscopy, x-ray diffraction measurements, and ab initio calculations, here we report a pressure-induced polar–nonpolar–polar phase crossover in NBT with a two-stage structural transition from R3c to P21/m and then to Pmn21 symmetry. Electrical resistance and absorption spectroscopy measurements demonstrate the abnormal changes around 5 and 13 GPa, corresponding to the two-stage phase transitions. Detailed charge density difference analyses reveal that the structural transition causes a synergistic change of Ti cation off-center displacement and octahedron tilting angle, leading to the ferroelectric–paraelectric–ferroelectric transition under pressure. These results establish a phase transition sequence of NBT with intriguing ferroelectric and electronic evolution, which helps to resolve the controversial structural conversion process in NBT and expand our understanding of lead-free ferroelectric under extreme conditions.