Ferroelectric–relaxor phase evolution and enhanced electromechanical strain response in LaAlO₃-modified Bi_0.5Na_0.5TiO₃–Bi_0.5K_0.5TiO₃ lead-free ceramics

LaAlO₃-modified 0.78Bi_0.5Na_0.5TiO₃–0.22Bi_0.5K_0.5TiO₃ (BNKT–xLA, 0 ≤ x ≤ 0.03) piezoelectric ceramics for actuator have been prepared and investigated in this study. With increase in LA substitution, the average long-range crystal structure of the materials gradually develops from rhombohedral–tetragonal mixed ferroelectric phases to a single pseudocubic symmetry. Electrical measurements indicate that the dominant ferroelectric order in unmodified BNKT is severely destroyed and transforms into an ergodic relaxor state with the formation of nanoscale domains, which is further verified by high-resolution transmission electron microscope images. Meanwhile, a great enhanced electric field-induced strain response of 0.36% under 60 kV/cm with a relatively low strain hysteresis is realized at the critical composition x = 0.01. By analyzing the origin of large strain, we believe that the reversible transition of a relaxor state characterized by multiple coexistent nano-domains into an electric field-induced long-range ferroelectric state contributes most to the attainment of these characteristics.