Directing Oxygen Vacancy Channels in SrFeO 2.5 Epitaxial Thin Films.

Transition metal oxides with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels, which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possess a great potential for practical applications, and would provide a better visualization of the diffusion pathways of ions in transition metal oxides. In this study, the orientations of the oxygen vacancy channels in BM-SrFeO are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the oxygen vacancy channel orientation further leads to modifications in the hydrogenation into the BM-SrFeO thin films. The results demonstrate strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.