Reversible electric-field-induced phase transition in Ca-modified NaNbO₃ perovskites for energy storage applications

Sodium niobate (NaNbO₃) is a potential material for lead-free dielectric ceramic capacitors for energy storage applications because of its antipolar ordering. In principle, a reversible phase transition between antiferroelectric (AFE) and ferroelectric (FE) phases can be induced by an application of electric field (E) and provides a large recoverable energy density. However, an irreversible phase transition from the AFE to the FE phase usually takes place and an AFE-derived polarization feature, a double polarization (P)-E hysteresis loop, does not appear. In this study, we investigate the impact of chemically induced hydrostatic pressure (p_chem) on the phase stability and polarization characteristics of NaNbO₃-based ceramics. We reveal that the cell volume of Ca-modified NaNbO₃ [(Ca_xNa_1-2xV_x)NbO₃], where V is A-site vacancy, decreases with increasing x by a positive p_chem. Structural analysis using micro-X-ray diffraction measurements shows that a reversible AFE-FE phase transition leads to a double P-E hysteresis loop for the sample with x = 0.10. DFT calculations support that a positive p_chem stabilizes the AFE phase even after the electrical poling and provides the reversible phase transition. Our study demonstrates that an application of positive p_chem is effective in delivering the double P-E loop in the NaNbO₃ system for energy storage applications.