Emergent antiferroelectric-to-ferroelectric transition in NaNbO₃ membranes

NaNbO₃ is one of the most complicated perovskite systems, exhibiting antiferroelectricity at room temperature and a rich spectrum of temperature-driven phase transitions. Although NaNbO₃ has been studied for decades, previous work primarily focused on its bulk form and a significant lack of understanding still exists regarding the structure and property evolution of NaNbO₃ thin films. In this work, we study the effects of thickness on ferroic ordering and nano-mechanical properties of NaNbO₃ thin-film membranes. Using pulsed laser deposition and epitaxial lift-off techniques, we synthesize high crystalline quality, crack-free NaNbO₃ membranes with thickness ranging from 10 to 160 nm and millimeter lateral scale.  We probe an intriguing antiferroelectric-to-ferroelectric phase transition with reducing membrane thickness using a combination of characterization techniques including X-ray diffraction, piezoresponse force microscopy, and Raman spectroscopy. This emergent phase transition leads to a non-monotonic thickness dependence of Young’s modulus including a both strain gradient dominated response and a surface elasticity dominated response in the ferroelectric phase regime below 40 nm, as well as another surface dominated response in the mixed phase regime above 40 nm.