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

TL;DR

This study explores how oxygen vacancies induce structural changes in SrFeO$_{3-x}$ thin films, transitioning from brownmillerite to perovskite, and examines associated phonon behaviors to understand ionic conduction mechanisms.

Contribution

It provides detailed insights into the structural evolution and phonon dynamics of SrFeO$_{3-x}$ films as oxygen content varies, using optical spectroscopy and shell-model calculations.

Findings

Structural transition from brownmillerite to perovskite with increasing oxygen

Observation of temperature-dependent phonon mode shifts, including an unusual red-shift

Identification of phonons related to oxygen vacancy configurations

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.