Temperature-Driven Structural Phase Transition in Tetragonal-Like BiFeO₃

Highly-strained BiFeO3 exhibits a "tetragonal-like, monoclinic" crystal structure found only in epitaxial films (with an out-of-plane lattice parameter exceeding the in-plane value by >20%). Previous work has shown that this phase is properly described as a MC monoclinic structure at room temperature [with a (010)pc symmetry plane, which contains the ferroelectric polarization]. Here we show detailed temperature-dependent x-ray diffraction data that evidence a structural phase transition at ~100C to a high-temperature MA phase ["tetragonal-like" but with a (1-10)pc symmetry plane]. These results indicate that the ferroelectric properties and domain structures of strained BiFeO₃ will be strongly temperature dependent.