Breaking the strain–symmetry trade-off via electrostriction-mediated reversible phase transition in B-site-engineered BNKT-based ceramics

Achieving large electrostrain together with a symmetric bipolar response remains challenging in lead-free piezoceramics, as mechanisms that generate large strain often involve irreversible polarization processes that limit strain reversibility. Here, B-site Zr engineering in Bi_0.495La_0.005Na_0.400K_0.100Ti_1−xZr_xO₃ (x = 0.000–0.025) enables a large electrostrain (~0.52%) together with a nearly symmetric bipolar strain–electric field (S–E) response. The optimized composition (x = 0.015) exhibits a large normalized strain coefficient (d^*₃₃ ≈ 867 pm/V), while x = 0.025 shows an enhanced electrostrictive coefficient (~0.055 m⁴/C²), indicating strengthened electrostriction-dominated behavior. The enhanced electromechanical response originates from Zr-induced lattice softening and R3c–P4bm phase coexistence, which flatten the free-energy landscape and promote reversible field-driven polarization dynamics. The reduced remanent polarization and coercive field suppress irreversible domain-wall motion, thereby favoring electrostriction-governed strain generation. These results demonstrate that coupling lattice softening with phase coexistence provides an effective design pathway for achieving large, nearly symmetric bipolar strain through electrostriction-dominated mechanisms in lead-free piezoceramics.