Exceptional electrostrain with minimal hysteresis and superior temperature stability under low electric field in KNN-based lead-free piezoceramics

Over the past two decades, (K_0.5Na_0.5)NbO₃ (KNN)-based lead-free piezoelectric ceramics have made significant progress. However, attaining a high electrostrain with remarkable temperature stability and minimal hysteresis under low electric fields has remained a significant challenge. To address this long-standing issue, we have employed a collaborative approach that combines defect engineering, phase engineering, and relaxation engineering. The LKNNS-6BZH ceramic, when sintered at T_sint = 1170 ℃, demonstrates an impressive electrostrain with a $d_{33}^{\ast }$ value of 0.276% and 1379 pm·V^–1 under 20 kV·cm^–1, which is comparable to or even surpasses that of other lead-free and Pb(Zr,Ti)O₃ ceramics. Importantly, the electrostrain performance of this ceramic remains stable up to a temperature of 125 ℃, with the lowest hysteresis observed at 9.73% under 40 kV·cm^–1. These excellent overall performances are attributed to the presence of defect dipoles involving ${\mathrm{V}}_{\mathrm{A}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$ and ${\mathrm{B}}_{\mathrm{N}\mathrm{b}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$, the coexistence of R–O–T multiphase, and the tuning of the trade-off between long-range ordering and local heterogeneity. This work provides a lead-free alternative for piezoelectric actuators and a paradigm for designing piezoelectric materials with outstanding comprehensive performance under low electric fields.