Understanding Strain‐Induced Phase Transformations in BiFeO₃ Thin Films

Abstract

Experiments demonstrate that under large epitaxial strain a coexisting striped phase emerges in BiFeO 3 thin films, which comprises a tetragonal‐like ( T ′) and an intermediate S ′ polymorph. It exhibits a relatively large piezoelectric response when switching between the coexisting phase and a uniform T ′ phase. This strain‐induced phase transformation is investigated through a synergistic combination of first‐principles theory and experiments. The results show that the S ′ phase is energetically very close to the T ′ phase, but is structurally similar to the bulk rhombohedral ( R ) phase. By fully characterizing the intermediate S ′ polymorph, it is demonstrated that the flat energy landscape resulting in the absence of an energy barrier between the T ′ and S ′ phases fosters the above‐mentioned reversible phase transformation. This ability to readily transform between the S ′ and T ′ polymorphs, which have very different octahedral rotation patterns and c / a ratios, is crucial to the enhanced piezoelectricity in strained BiFeO 3 films. Additionally, a blueshift in the band gap when moving from R to S ′ to T ′ 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.