Tetragonal-pseudocubic phase boundaries and octahedral order-disorder tilting transitions facilitate high piezoelectric performance in Bi_1/2Na_1/2TiO₃-based single crystals

High-performance lead-free piezoelectric single crystals are urgently needed for next-generation actuators and transducers. In this study, we reveal that a compositionally driven tetragonal-pseudocubic (T-PC) phase boundary, in conjunction with an octahedral order–disorder tilting transition, significantly enhances the piezoelectric response in Nb⁵⁺-substitution (Bi_0.48Na_0.425K_0.055Ba_0.04)(Ti_0.98Nb_0.02)O₃ (BNKBT-2Nb) single crystals. The crystal achieves an outstanding piezoelectric coefficient of d₃₃=662 ​pC/N at room temperature. In situ X-ray diffraction confirms an electric field-induced transition from the PC to T phase. Atomic-resolution HADDF-STEM analysis reveals an increase in the c/a ratio (c/a>1.01) on the local scale and ordered octahedral tilting of the a⁰a⁰c⁺ ​type driven by the poling field. The single crystals exhibit excellent piezoelectric performance over a broad temperature range, achieving a peak d₃₃ of 920 ​pC/N at approximately 92 ​°C. Furthermore, the polar states exhibit a pronounced frequency dependence near the depolarization temperature. These findings provide critical insight into the structure-property relationship and offer a promising pathway for designing advanced lead-free piezoelectric crystals for functional electromechanical applications.