Coupled phase-boundary engineering and defect control for ultrahigh electrostrain with low hysteresis in lead-free BNBT-based ceramics

The development of lead-free ferroelectric ceramics that combine high strain with low hysteresis is critical for precision actuators and displacement sensors, yet conventional (Bi_0.5Na_0.5)_0.93Ba_0.07TiO₃ (BNBT) ceramics are limited by high strain hysteresis (>30%), elevated driving fields (>4 kV/mm), and poor thermal stability. Here, a Zr–Nb–vacancy co-doping strategy at Ti sites is employed to construct (Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1-x (Zr_0.375Nb_0.5□_0.125) _x O₃ (BNBT- x ZN, x = 0–0.04), enabling synergistic control of dual-phase structure and defect concentration. At x = 0.01, the ceramic exhibits coexisting rhombohedral ( R 3 c ) and tetragonal ( P 4 bm ) phases with phase fractions of 51.6% and 48.4%, respectively. This composition achieves a unipolar strain ( S _uni) of 0.70% and an effective piezoelectric coefficient (d₃₃^*) of 1168 pm/V at 70 °C, while maintaining exceptional thermal stability. S _max remains above 0.45% across 30–150 °C and hysteresis is reduced to 8%. The enhanced electromechanical performance is attributed to a cooperative mechanism in which moderate defect concentrations reduce domain wall pinning and dual-phase interfaces lower polarization reversal energy barriers, enabling efficient domain switching. This study establishes a rational design approach for achieving ultrahigh strain and low hysteresis in lead-free ferroelectric ceramics, providing a pathway for their application in precision actuators and sensors.