Oxygen vacancy-induced structural evolution of SrFeO${}_{3-x}$ epitaxial thin film from brownmillerite to perovskite

Abstract

We investigated SrFeO${}_{3-x}$ thin films on a SrTiO${}_3$ (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 (BM${}_{\mathrm{oop}}$ and BM${}_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel. In the BM${}_{\mathrm{oop}}$ sample, we observed a phonon mode, which exhibited an unusual red-shift with decreasing temperature; this behavior may have been due to the apical oxygen instability in the FeO${}_6$ octahedron. Our results provide important information regarding the ionic conduction mechanism in SrFeO${}_{3-x}$ material systems.

18 pages, 7 figures