  The nanostructural evolution of the strain-induced structural phase
transition in BiFeO3 is examined. Using high-resolution X-ray diffraction and
scanning-probe microscopy-based studies we have uniquely identified and
examined the numerous phases present at these phase boundaries and have
discovered an intermediate monoclinic phase in addition to the previously
observed rhombohedral- and tetragonal-like phases. Further analysis has
determined that the so-called mixed-phase regions of these films are not
mixtures of rhombohedral- and tetragonal-like phases, but intimate mixtures of
highly-distorted monoclinic phases with no evidence for the presence of the
rhombohedral-like parent phase. Finally, we propose a mechanism for the
enhanced electromechanical response in these films including how these phases
interact at the nanoscale to produce large surface strains.
