Understanding Strain-Induced Phase Transformations in BiFeO Thin Films.

Experiments demonstrate that under large epitaxial strain a coexisting striped phase emerges in BiFeO thin films, which comprises a tetragonal-like (') and an intermediate ' polymorph. It exhibits a relatively large piezoelectric response when switching between the coexisting phase and a uniform ' phase. This strain-induced phase transformation is investigated through a synergistic combination of first-principles theory and experiments. The results show that the ' phase is energetically very close to the ' phase, but is structurally similar to the bulk rhombohedral () phase. By fully characterizing the intermediate ' polymorph, it is demonstrated that the flat energy landscape resulting in the absence of an energy barrier between the ' and ' phases fosters the above-mentioned reversible phase transformation. This ability to readily transform between the ' and ' polymorphs, which have very different octahedral rotation patterns and / ratios, is crucial to the enhanced piezoelectricity in strained BiFeO films. Additionally, a blueshift in the band gap when moving from to ' to ' is observed. These results emphasize the importance of strain engineering for tuning electromechanical responses or, creating unique energy harvesting photonic structures, in oxide thin film architectures.