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We investigated SrFeO$_{3-x}$ thin films on a SrTiO$_3$ (001) substrateprepared via pulsed laser epitaxy using an optical spectroscopy technique. Theoxygen vacancy level ($x$) was controlled by post-annealing processes atdifferent oxygen partial pressures. We achieved a brownmillerite(BM) structureat $x =$ 0.5 and observed the evolution of the crystal structure from BM intoperovskite(PV) as the oxygen concentration increased. We observed the evolutionof infrared-active phonons with respect to the oxygen concentration, which wasclosely related to the structural evolution observed via X-ray diffraction. Weidentified the phonons using the shell-model calculation. Furthermore, westudied temperature-dependent behaviors of the phonon modes of threerepresentative samples: PV, and two BMs (BM$_{\mathrm{oop}}$ andBM$_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel.In the BM$_{\mathrm{oop}}$ sample, we observed a phonon mode, which exhibitedan unusual red-shift with decreasing temperature; this behavior may have beendue to the apical oxygen instability in the FeO$_6$ octahedron. Our resultsprovide important information regarding the ionic conduction mechanism inSrFeO$_{3-x}$ material systems.

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