Coexisting Phases in NaNbO₃ Thin Films Influenced by Epitaxial Strain and Size Effects

Antiferroelectrics are a promising class of materials for applications in capacitive energy storage and multi-state memory, but comprehensive control of their functional properties requires further research. In thin films, epitaxial strain and size effects are important tuning knobs but difficult to probe simultaneously due to low critical thicknesses of common lead-based antiferroelectrics. Antiferroelectric NaNbO₃ enables opportunities for studying size effects under strain, but electrical properties of ultra-thin films have not been thoroughly investigated due to materials challenges. Here, high-quality, epitaxial, coherently-strained NaNbO₃ films are synthesized from 35- to 250- nm thickness, revealing a transition from a fully ferroelectric state to coexisting ferroelectric and antiferroelectric phases with increasing thickness. The electrical performance of this phase coexistence is analyzed through positive-up negative-down and first-order reversal curve measurements. Further increasing thickness leads to a fully ferroelectric state due to a strain relief mechanism that suppresses the antiferroelectricity. The potential of engineering competing ferroic orders in NaNbO₃ for multiple applications is evaluated, reporting significantly enhanced recoverable energy density (20.6 J cm^-3 at 35 nm) and energy efficiency (90% at 150 nm) relative to pure bulk NaNbO₃ as well as strong retention and fatigue performance with multiple accessible polarization states in the intermediate thickness films.