Tetragonal (1-x)(Bi,Na)TiO3-xBaTiO3 films exhibit enhanced piezoelectric properties due to domain switching over a wide composition range. These properties were observed over a significantly wider composition range than the morphotropic phase boundary (MPB), which typically has a limited composition range of 1-2%. The polarization axis was found to be along the in-plane direction for the tetragonal composition range x = 0.06-1.0, attributed to the tensile thermal strain from the substrate during cooling after the film formation. A "two-step increase" in remanent polarization against an applied maximum electric field was observed at the high-field region due to the domain switching, and a very high piezoelectric response (effective d33 value, denoted as d33,f) over 220 pm/V was achieved for a wide composition range of x = 0.2-0.5 with high tetragonality, exceeding previously reported values for bulk ceramics. Moreover, a transverse piezoelectric coefficient, e31,f, of 19 C/m2 measured using a cantilever structure was obtained for a composition range of at least 10 atom % (for both x = 0.2 and 0.3). This value is the highest reported for Pb-free piezoelectric thin films and is comparable to the best data for Pb-based thin films. Reversible domain switching eliminates the need for conventional MPB compositions, allowing an improvement in the piezoelectric properties over a wider composition range. This strategy could provide a guideline for the development of environmentally acceptable lead-free piezoelectric films with composition-insensitive piezoelectric performance to replace Pb-based materials with MPB composition, such as PZT.
Keywords: (Bi; Na)TiO3–BaTiO3; Pb-free; domain switching; out-of-morphotropic phase boundary composition; piezoelectric film; tetragonal structure.