Large Piezoelectric Response in Lead-Free (BiNa)TiO-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(BiNa)TiO-0.3BaTiO 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 SrTiO substrate exhibits out-of-plane polarization. - curve analysis reveals that the obtained piezoelectric coefficient for the film on the Si substrate ( ≈ 275 pm/V) is approximately eight times higher than that for the epitaxial film on the SrTiO substrate ( ≈ 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, , of ∼19 C/m 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.