We investigated ${\mathrm{SrFeO}}_{3-x}$ thin films on a ${\mathrm{SrTiO}}_3$ (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 (${\mathrm{BM}}_{\mathrm{oop}}$ and ${\mathrm{BM}}_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel. In the ${\mathrm{BM}}_{\mathrm{oop}}$ 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 ${\mathrm{FeO}}_6$ octahedron. Our results provide important information regarding the ionic conduction mechanism in ${\mathrm{SrFeO}}_{3-x}$ material systems.

• Received 28 September 2017
• Revised 4 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.075104