Atomic structure and microstructures of supertetragonal multiferroic BiFeO3 thin films

We revisit the atomic structure and microstructure of the so-called supertetragonal phases of highly strained epitaxial BiFeO3 thin films. Quantitative atomic resolution scanning transmission electron microscopy is used to directly image the atomic positions. A crystallographic phase suggested by electron diffraction and predicted by ab initio calculations is evidenced. Microtwins are reported in thickest films. Electron energy loss spectroscopy is further employed to reveal subtle electronic structure features, which, interpreted in a framework of antiferrodistortive distortions coupling with the substrate, point towards a phase closer to the P4mm purely tetragonal phase.