Giant strains in (Bi_0.5Na_0.5)TiO₃ based ceramics are usually attributed to electric field induced nonpolar to polar phase transition. Whether it is an ergodic relaxor R3c/P4mm ferroelectric (FE) to long-range ordered FE phase transformation or a reversible P4bm antiferroelectric (AFE) to FE phase transition is still unclear. Herein, lead-free (0.88-x)(Bi_0.5Na_0.5)TiO₃-0.12BaTiO₃-xNaNbO₃ ceramics exhibit a composition-modulated FE tetragonal P4mm to relaxor AFE tetragonal P4bm phase transition, in which double hysteresis loop, sprout-shaped S-E curves, near-zero quasi-static d₃₃ together with a large volume change suggest the AFE characteristics of P4bm phase. An interesting finding is that the reversibility of field-induced AFE P4bm phase to FE P4mm phase transition strongly depends on the NN content, from being completely irreversible at x = 0.01–0.02, to partially reversible at x = 0.03–0.05, and finally to completely reversible at x = 0.06–0.08. It is indicated that the variation of reversibility should be attributed to the change of relative free energy caused by decreasing the FE to AFE phase transition temperature with increasing the NN content.

Compared with antiferroelectric (AFE) orthorhombic R phases, AFE orthorhombic P phases in NaNbO₃ (NN) ceramics have been rarely investigated, particularly in the field of energy-storage capacitors. The main bottleneck is closely related to the contradiction between difficultly-achieved stable relaxor AFE P phase and easily induced P-R phase transition during modifying chemical compositions. Herein, we report a novel lead-free AFE ceramic of (1-x)NN-x(Bi_0.5K_0.5)ZrO₃ ((1-x)NN-xBKZ) with a pure AFE P phase structure, which exhibits excellent energy-storage characteristics, such as an ultrahigh recoverable energy density (W_rec) ~4.4 J/cm³ at x = 0.11, a large powder density P_D ~104 MW/cm³ and a fast discharge rate t_0.9–45 ns. The analysis of polarization-field response, Raman spectrum and transmission electron microscopy demonstrates that the giant amplification of W_rec by ≥ 177 % should be mainly ascribed to the simultaneously and effectively enhanced AFE P-phase stability and its relaxor characteristics, resulting in a diffused reversible electric field-induced AFE P-ferroelectric phase transition with concurrently increased driving electric fields. Different from most (1-x)NN-xABO₃ systems, it was found that the reduced polarizability of B-site cations dominates the enhanced AFE P-phase stability in (1-x)NN-xBKZ ceramics, but the almost unchanged tolerance factor tends to cause the AFE R phase to be induced at a relatively high x value.