Emerging phase transitions in lead-free ferroelectric heterostructures and membranes

Enhanced susceptibilities in ferroelectrics often emerge near phase boundaries between competing ground states. While chemically induced phase boundaries have enabled ultrahigh electrical and electromechanical responses in lead-based ferroelectrics, achieving precise chemical control in lead-free alternatives, such as (K,Na)NbO₃ (KNN) thin films, remains challenging due to the high volatility of alkali metals. In this presentation, I will discuss our recent advances on sodium niobate (NaNbO₃), a key end-member of the KNN family, known for its rich structural polymorphism and numerous phase transitions. Through a combined experimental and theoretical approach, we uncover strain-induced, morphotropic phase boundary-like polymorphic nanodomain structures in chemically simple, lead-free epitaxial NaNbO₃ thin films. The competition between these low-symmetry phases facilitates field-induced polarization rotation and phase transitions, enabling multi-state polarization switching and significant enhancements in dielectric susceptibility and tunability across a broad frequency spectrum [1]. Moreover, we explore the intrinsic size dependence of antiferroelectricity in freestanding NaNbO₃ membranes by eliminating substrate-induced effects. We reveal a surface-driven antiferroelectric-to-ferroelectric transition as membrane thickness decreases, leading to a mixed-phase state with coexisting ferroelectric and antiferroelectric domains [2]. These findings open new avenues for engineering lead-free oxide thin films with tailored functionalities for next-generation electronic and energy applications.