Identifying the Limits of Strain Engineering in NaNbO₃ Ultrathin Films

Epitaxial NaNbO₃ ultrathin films are grown on single-crystal substrates with a wide range of compressive (-6.24%) and tensile (+7.13%) strain. High-resolution X-ray diffraction reveals a coherently strained state of NaNbO₃ thin films within the strain range from -1.42 to 1.86%, beyond which the films are in relaxed states. This is accompanied by a change in the growth mode from step-flow growth in strained films to coalesced island growth in relaxed films. Piezoresponse force microscopy measurements, including domain writing and reading, reveal a single vertical ferroelectric domain for the compressive films, while the tensile films exhibit a mixture of multiple vertical domains. The first-principles calculations confirm the stability of strained NaNbO₃. Our results provide important information for the understanding and engineering of strained functional thin films.