Room-temperature ferroelectricity in NaNbO₃ membrane

Sodium niobate (NaNbO₃), traditionally recognized as an antiferroelectric material, has garnered significant attention for its potential in energy storage, electromechanical devices, and electrocaloric applications. Central to unlocking its full potential is precise control over its ferroic orders, particularly the transition between antiferroelectric and ferroelectric phases. In this study, we propose a topochemical conversion strategy for fabricating pure ferroelectric NaNbO₃ membranes with a P2₁ma orthorhombic phase regardless of membrane thickness. Using Scanning transmission electron microscopy, we identify two distinct regions with large ( > 10°) or small ( < 1°) octahedral tilts in the ferroelectric NaNbO₃ membranes. Polarization analyses in these regions reveal that the ferroelectricity originates from the out-of-plane displacement of Nb⁵⁺ cations. Additionally, the pure ferroelectric NaNbO₃ membranes demonstrate thickness-dependent piezoelectric enhancement, achieving a d₃₃ value of 93.7 pm/V at 30 nm thickness, as well as pronounced resistance switching behaviors. This study introduces an innovative molten salt environment-controlled synthesis strategy, enabling efficient production of high-quality pure ferroelectric NaNbO₃ membranes, and advancing their integration into next-generation functional devices.