Oxygen vacancy induced structural evolution of SrFeO3-x epitaxial thin film from brownmillerite to perovskite

We investigated SrFeO3-x thin films on a SrTiO3 (001) substrate prepared via pulsed laser epitaxy using an optical spectroscopy technique. The oxygen vacancy level (x) was controlled by post-annealing processes at different oxygen partial pressures. We achieved a brownmillerite (BM) structure at x = 0.5 and observed the evolution of the crystal structure from BM into perovskite (PV) as the oxygen concentration increased. We observed the evolution of infrared-active phonons with respect to the oxygen concentration, which was closely related to the structural evolution observed via x-ray diffraction. We identified the phonons using the shell-model calculation. Furthermore, we studied temperature-dependent behaviors of the phonon modes of three representative samples: PV and two BMs (BMoop and BMip) with different orientations of the oxygen vacancy channel. In the BMoop sample, we observed a phonon mode, which exhibited an unusual redshift with decreasing temperature; this behavior may have been due to the apical oxygen instability in the FeO6 octahedron. Our results provide important information regarding the ionic conduction mechanism in SrFeO3-x material systems.