Orienting Oxygen Vacancy Channels in Brownmillerite Strontium Ferrite Thin Films Using Strain: Implications for Facile Oxygen Ion Transport

The control of oxygen vacancy channels (OVCs) in brownmillerite (BM) oxides has a substantial impact on tuning their physical and chemical properties. Here, we demonstrated a strain mechanism for directing the OVCs in BM-SrFeO 2.5 (SFO) synthesized via a two-step process in which perovskite (P)-structured SrFeO 3−δ (P-SFO 3−δ ) thin film is first prepared and then converted to the BM phase through the post-vacuum-annealing (PVA) treatment. The initial strain state for the first-step-prepared P-SFO 3−δ thin films plays a determining role in the orientation of the OVCs of the final BM phase with the systematic manipulation of both chemical strain (lattice expansion) and epitaxial strain by tuning the oxygen deficiency and using different substrates, respectively. Larger tensile strains in the initial P-SFO 3−δ thin films are inclined to facilitate the formation of BM with horizontal OVCs after PVA, while small tensile or compressive strains favor the vertical OVCs. Our results offer insights into the selective control of the orientation of the OVCs of BM-SrFeO 2.5 thin films, which are of benefit to the design for functionalities related to the ion transport and migration.