Structural and piezoelectric properties of hydrothermally grown Bi_0.5(Na,K)_0.5TiO₃ films consisting of single-domain grains

We report the piezoelectric properties of Bi_0.5(Na,K)_0.5TiO₃ (BNKT) thin films synthesized using a hydrothermal method. Their structural and piezoelectric properties were investigated using high-resolution X-ray diffraction (HR-XRD) and piezoelectric force microscopy (PFM). The HR-XRD analysis revealed that the film exhibited epitaxial-like growth with a 90° in-plane rotation of the film and substrate, indicating a rhombohedral structure. Local polarization switching indicated that the as-grown BNKT epitaxial film processed a ferroelectric reversal property owing to the driving voltage. By identifying the PFM responses of the out-of-plane and in-plane signals, the film was found to exhibit multiple diagonal polarization directions originating from its crystalline structure. Consequently, PFM analysis suggests that a single grain in the hydrothermally grown BNKT epitaxial-like film can be considered a single-domain with its own polarization. We propose that the generation of a single grain with a single-domain is related to the hydrothermal growth mechanism. This single-domain property in hydrothermally grown films can be applied in eco-friendly applications because of its lead-free, low-cost, piezoelectric, and self-poling properties in as-grown film.