Large Piezoelectric Response in Lead-Free (Bi<sub>0.5</sub>Na<sub>0.5</sub>)TiO<sub>3</sub>-Based Perovskite Thin Films by Ferroelastic Domain Switching: Beyond the Morphotropic Phase Boundary Paradigm
Takao Shimizu, Miyu Hasegawa, Keisuke Ishihama, Akinori Tateyama, Wakiko Yamaoka, Risako Tsurumaru, Shintaro Yoshimura, Yusuke Sato, Hiroshi Funakubo
Abstract
Strong electromechanical coupling is observed in tetragonal Pb-free 0.7(Bi0.5Na0.5)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 (d33,f ≈ 275 pm/V) is approximately eight times higher than that for the epitaxial film on the SrTiO3 substrate (d33,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, e31,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.