Origin of antipolar clusters in BiFeO3 epitaxial thin films

The microstructure of BiFeO3 (BFO) thin films is investigated using high-resolution transmission electron microscopy. Both (001)- and (101)-type domain boundaries are found in the BFO films. The antipolar clusters induced by antiparallel cation displacements are observed in the pure BFO film, and the cation displacements in the films are proved to originate from the lattice strain which can be adjusted by introduction of a buffer layer. Combining transmission electron microscopy (TEM) with fast Fourier transformation techniques, both γ-Fe2O3 and FeO phases were discovered. The γ-Fe2O3 phase stems from the decomposition of stoichiometric BFO due to the volatilization of Bi, while the FeO phase results from the decomposition of BFO with oxygen vacancies which could come from ion milling process during the TEM sample preparation. Our work sheds light on the origin of the cation displacements and provides a new idea to control the physical properties of BFO films.