Breaking the strain–symmetry trade-off via electrostriction-mediated reversible phase transition in B-site-engineered BNKT-based ceramics
Lead-free piezoceramics face a persistent strain–symmetry trade-off: large electrostrain typically arises from irreversible polarization processes that degrade bipolar strain reversibility. This study investigates B-site Zr engineering in Bi0.495La0.005Na0.400K0.100Ti1−xZrxO3 (x = 0.000–0.025) to decouple strain magnitude from symmetry. The optimized composition (x = 0.015) achieves a large electrostrain of ~0.52% with a nearly symmetric bipolar S–E response and a normalized strain coefficient d*33 ≈ 867 pm/V. At x = 0.025, the electrostrictive coefficient reaches ~0.055 m4/C2, indicating strengthened electrostriction-dominated behavior. Structural analysis attributes the enhanced electromechanical response to Zr-induced lattice softening and R3c–P4bm phase coexistence, which flatten the free-energy landscape and promote reversible field-driven polarization dynamics. Reduced remanent polarization and coercive field suppress irreversible domain-wall motion, favoring electrostriction-governed strain generation. The electromechanically optimized composition also exhibits noncytotoxic behavior and preliminary surface mineral deposition after immersion in simulated body fluid (SBF), suggesting biofunctional potential. These findings establish B-site lattice engineering as an effective strategy for achieving large, nearly symmetric bipolar strain through electrostriction-dominated mechanisms in lead-free piezoceramics, with implications for actuator and biomedical applications.