Tetragonal-pseudocubic phase boundaries and octahedral order-disorder tilting transitions facilitate high piezoelectric performance in Bi1/2Na1/2TiO3-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 Nb 5+ -substitution (Bi 0.48 Na 0.425 K 0.055 Ba 0.04 )(Ti 0.98 Nb 0.02 )O 3 (BNKBT-2Nb) single crystals. The crystal achieves an outstanding piezoelectric coefficient of d 33 =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 a 0 a 0 c + type driven by the poling field. The single crystals exhibit excellent piezoelectric performance over a broad temperature range, achieving a peak d 33 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. This study demonstrates that the presence of a tetragonal–pseudocubic phase boundary, combined with octahedral order-disorder tilting, enhances the piezoelectric response of BNT-based single crystals, achieving a high d 33 of 662 pC/N at room temperature. Under an applied poling field, the tetragonal structure and ordered octahedral tilting are reestablished. Near the depolarization temperature, the polar state exhibits frequency-dependent tunability. • This work proposes a multiscale cooperative control strategy that combines a compositionally driven T–PC phase boundary with an engineered transition between ordered and disordered octahedral tilts, achieved by incorporating Nb 5+ on the B site. • A record-high room-temperature d 33 value of 662 pC/N (representing a 184% enhancement), was obtained and sets a new benchmark. • The strain response exhibits a strong frequency dependence in the vicinity of the depolarization temperature.