Large Piezoelectric Response in Lead-Free (Bi_0.5Na_0.5)TiO₃‑Based Perovskite Thin Films by Ferroelastic Domain Switching: Beyond the Morphotropic Phase Boundary Paradigm

Strong electromechanical coupling is observed in tetragonal Pb-free 0.7­(Bi_0.5Na_0.5)­TiO₃-0.3BaTiO₃ films, which is far from the morphotropic phase boundary, prepared by pulsed laser deposition on a Si substrate. The tensile strain induced during cooling causes in-plane polarization in an oriented film on a Si substrate, while an epitaxial film grown on a SrTiO₃ substrate exhibits out-of-plane polarization. S–E curve analysis reveals that the obtained piezoelectric coefficient for the film on the Si substrate (d_33,f ≈ 275 pm/V) is approximately eight times higher than that for the epitaxial film on the SrTiO₃ substrate (d_33,f ≈ 34 pm/V). In situ X-ray diffraction analysis confirms the occurrence of domain switching under an electric field from in-plane to out-of-plane polarization. An effective piezoelectric stress coefficient, e_31,eff, of ∼19 C/m² is obtained from a Si cantilever sample, which is the highest among the reported values for Pb-free piezoelectric films and is comparable to those for Pb-based films. The significant piezoelectric response produced by domain switching in the Pb-free materials with the composition far from the morphotropic phase boundary will expand future applications due to their both outstanding properties and environmental sustainability.