Brownmillerite phase formation and evolution in epitaxial strontium ferrite heterostructures

By controlling the synthesis and process conditions, we show that the oxygen vacancy channels (OVCs) of brownmillerite-structured SrFeO_2.5+δ (0 ≤ δ< 0.5) thin films can be oriented differently on the same LaAlO₃(001) substrate due to either a bottom up or top down synthesis route. The dynamic brownmillerite structure formation and evolution processes yield distinctively different optical and electronic properties, owing to the difference in both the oxygen stoichiometry and the OVC configuration in the resultant films. With the established recipe, we generated insulating SrFeO_2.5 with vertically aligned OVCs and metallic SrFeO₃ standards so that the other intermediate states can be directly compared and understood. Our results offer further insight into the phase stability and oxygen-diffusion mechanisms, which is important for the predictive synthesis of novel functional materials.