A Strain-Driven Morphotropic Phase Boundary in BiFeO3

Biffed into Shape BiFeO3 is known to have a very large ferroelectric polarization. Although the bulk ground state is rhombohedral (with the electrical polarization along the [111] direction), in thin films and under epitaxial strain, the material can be tetragonally distorted (polarization along [001]). Zeches et al. (p. 977) show that under compressive strain, these films are monoclinic, a phase that is highly stable because it comprises the same symmetry as the monoclinic phase which has been reported at the so-called morphotropic phase boundaries in technologically important ferroelectrics. This work offers the possibility of obtaining large piezoelectric responses in lead-free systems. Growth of epitaxial films of BiFeO3 on various substrates may provide a route toward making lead-free ferroelectric devices. Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field–dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.