Tailoring antiferroelectric phase transitions in ultrahigh energy storage ceramics

Understanding the structure-property relationships and structural evolution under external stimuli is critical for developing high-performance lead-free antiferroelectric energy storage ceramics, while the underlying physics mechanism remains ambiguous. Here, the structural evolution, phonon behavior, structural heterogeneity, and optical transitions in (1 − x)(0.7AgNbO₃-0.3NaNbO₃)-xSr_0.7Bi_0.2TiO₃ (AN-NN-SBT) energy storage ceramics through advanced spectroscopy techniques have been studied. Doping with Sr_0.7Bi_0.2TiO₃ (SBT) induces lattice distortion via ionic radius mismatch, driving a phase transition from antiferroelectric to paraelectric states. The optical band gap (E_g) is broadened and the breakdown strength is enhanced, owing to the concomitant formation of Ti-O bonds, vacancy creation, and orbital hybridization. Furthermore, temperature-dependent Raman analyses reveal reversible phase transitions (M-O-T) governed by phonon merging, abnormal frequency shift, and soft mode suppression. Thermal modulation induces nonlinear E_g narrowing, attributed to synergistic effects of lattice thermal expansion and electron-phonon interactions. Importantly, the phase coexistence and structure evolution process with temperature were discovered by insitu Raman mapping directly. A composition-critical temperature phase diagram was established, demonstrating that SBT doping reduces the structural transition temperatures and enables the M₃/O phase boundary at room temperature in the AN-NN-SBT20 system, which in turn yields optimal energy storage performance. These findings elucidate the interplay between lattice symmetry, electronic properties, and energy storage performance, providing a strategic framework for designing high-performance lead-free energy storage ceramics.