Explicating the irreversible electric-field-assisted ferroelectric phase transition in the otherwise antiferroelectric sodium niobate for energy storage systems

To meet the increasing demand for environment-friendly, high-performance energy devices, sodium niobate (NaNbO 3 ) is considered one of the most promising lead-free antiferroelectric (AFE) oxide perovskites for green energy storage applications. However, as disclosed by recent experimental reports, under an external electric field, the room-temperature AFE P phase of NaNbO 3 has been demonstrated to undergo an irreversible phase transition to the ferroelectric (FE) Q phase. This puzzle challenges our current atomic-scale understanding of this field-induced AFE-to-FE transition, and thus hinders the widespread use of NaNbO 3 in lead-free AFE energy storage devices. To unravel this puzzle, we perform first-principles density-functional theory calculations to establish phase stability maps of the NaNbO 3 polymorphs determined from group-subgroup relations. For the first time, we identify two new key intermediates (P′ and Q′) via the symmetry-adapted phonon mode analysis based on high-symmetry cubic phase and minimum energy pathway transition state searches, that facilitate de novo phase transition pathways for the switching of polarization with significantly lowered energy barriers. By means of a phenomenological Landau-Devonshire model, we predict and explain why these new intermediates can rationalize the persistent lack of a double polarization-electric field hysteresis for NaNbO 3 under an applied field. This sets the design platform for future precise engineering of NaNbO 3 at the atomic-scale for lead-free AFE energy storage applications. By means of a first-principles-based Landau-Devonshire model, we predict and explain why newly discovered intermediates can rationalize the persistent lack of a double polarization-electric field hysteresis for NaNbO 3 under an applied field.