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.

Electrostrain in lead-free relaxor piezoceramics is governed by the interplay between phase heterogeneity and defect-mediated polarization dynamics. Here, a BNT-SBT ceramic system is employed to elucidate how thermal post-processing modulates electromechanical behavior without compositional modification or external conditioning. The treatment induces a redistribution of rhombohedral (R3c) and tetragonal (P4bm) phase fractions, accompanied by reconfiguration of defect-related environments. Electron paramagnetic resonance reveals the presence of oxygen-vacancy−related defect states, while XPS analysis indicates a redistribution of near-surface oxygen chemical states. Despite the mixed-phase structure, polarization measurements exhibit diffuse relaxor characteristics, suggesting that long-range domain switching plays a limited role, whereas the reversible strain response is predominantly governed by electrostrictive polarization dynamics, facilitated by oxygen-vacancy–related defect states. A significant enhancement in electrostrictive response is confirmed by the increased electrostrictive coefficient (Q₃₃) and the linear S–P² relationship. Consequently, the maximum strain increases from 0.25% to 0.51%, with a normalized strain coefficient of 729 pm⋅V⁻¹. The results indicate that the enhanced strain originates from an electrostriction-dominated response enabled by processing-induced defect-phase coupling and local structural heterogeneity. This work provides a processing-driven pathway for tuning electromechanical properties in BNT-based relaxor ceramics and offers insight into strain generation mechanisms beyond conventional domain-switching models.