Local structure heterogeneity in unique tetragonal BaTiO₃-based relaxor featuring ultrahigh electrostrictive effect

Relaxor ferroelectrics are characterized by nanoscale structural heterogeneity and show superior electrical properties. However, the complicated local structures in lead-free counterparts are not well deciphered, impeding the understanding of the corresponding relationships between structure and properties. Herein, the temperature-dependent local structures of the unique tetragonal BaTiO₃-(Bi_0.5Li_0.5)TiO₃ relaxor, which presents an ultrahigh electrostrictive effect, were studied using neutron total scattering. Significant tetragonal distortion at the 1-10 Å local scale persisted between 100 and 500 K, despite the long-range structure having undergone a tetragonal to cubic phase transition. Interestingly, nanoscale Li-Bi clusters were indicated to present in the Ba matrix, with these clusters showing large polar displacements even at temperatures above T_m. A gradual disorder in the local polarization direction with increasing temperature was observed, accompanied by a nearly constant magnitude of the spontaneous polarization. This phenomenon was found to lead to a vanishing of macroscopic polarization during the diffuse phase transition. These findings have contributed to a better understanding of the role of local chemical heterogeneity in the relaxor behavior, and have provided a structural foundation for designing relaxors by utilizing short-range chemical ordering.