Li ⁺ and Nb ⁵⁺ co-doped BNT-based ceramic with ultrahigh electrostrain under a moderate electric field

Lead-free piezoceramics have been an important research topic over the past two decades aiming to replace Pb-based piezoceramics for sensor and actuator applications. In this study, LiNbO₃ (LN) doping was employed in (1-x)Bi1/2(Na0.8K0.2)1/2Ti0.99Nb0.01O₃-xLN ceramics near its morphotropic phase boundary (MPB). The composition with x = 0.02 exhibits a remarkable unipolar strain (Suni) response, achieving a high Suni of 0.57% and a large piezoelectric strain coefficient (d₃₃*) of 952 pm/V under a moderate electric field of 60 kV/cm. This enhancement originates from local disorder induced by co-substitutions at the A and B sites, which facilitates a transition from a non-ergodic relaxor (NER) to an ergodic relaxor (ER) state. Comprehensive analysis indicates that the overall performance in electrostrain surpasses most reported lead-free ceramics. By investigating the state of polar nanoregions, including thermal evolution, and the electric-field-induced phase transition, the mechanisms underlying the macroscopic strain response to the LN content have been revealed, providing valuable guidance for designing high-performance lead-free relaxor ferroelectrics for sensor and actuator applications.