Reversible phase transition induced large piezoelectric response in Sm-doped BiFe O3 with a composition near the morphotropic phase boundary

Materials with the morphotropic phase boundary (MPB) exhibit an ultrahigh mechanical response to electrical inputs, which has been widely used in applications such as sensors and actuators. Recently, the rare-earth element doped BiFeO3 (BFO) was found to possess a MPB between a rhombohedral polar phase and an orthorhombic antipolar phase with enhanced piezoelectric response, enabling it to be an attractive alternative to toxic Pb-based piezoelectric materials. Despite theoretical and experimental efforts, the phase transition behavior under electric fields has not been directly confirmed, leaving a gap in the understanding of the origin of enhanced piezoelectricity. Here, we have demonstrated an irreversible electric-field induced phase transition from the antipolar phase to the polar phase in Sm-doped BFO with the pre-MPB composition, and a reversible phase transition between the polar phase and the antipolar/nonpolar phase in Sm-doped BFO with the MPB composition. In situ transmission electron microscopy technique combined with thermodynamic calculation based on the Ginzburg-Landau-Devonshire theory indicates that the electric-field induced reversible phase transition leads to enhanced piezoelectric response and double P-E hysteresis loops. These results provide us a deep insight into the mechanism of exotic electromechanical response in the rare-earth element doped BFO system with the composition near the MPB.