Microstructure and temperature stability of highly strained tetragonal-like BiFeO3 thin films

A highly strained tetragonal (T)-like BiFeO₃ (BFO) phase interspersed between two rhombohedral (R)-like phase regions in BFO films grown on LaAlO₃ substrates was confirmed by transmission electron microscopy (TEM). The typical width of the T-like phase was about 5-35 nm and the areal fraction of the T-like phase was estimated to be 35%. A lattice misalignment of 3.8° between adjacent R- and T-like phases was observed in both high-resolution TEM images and selected-area electron diffraction patterns. Superlattice reflection spots were observed along the [110] direction in the R-like phase, which were attributed to the antiphase tilting of oxygen octahedra about the pseudocubic [111] axis of the R-like phase. A structural phase transition from the T-like phase to the R-like one at around 435 °C was observed by TEM during in situ heating and cooling experiments.