Low-temperature reduction of brownmillerite CaFeO2.5 in LaAlO3/CaFeO2.5 heterostructures made on SrTiO3

Abstract

When LaAlO₃/CaFeO_2.5 thin-film heterostructures made on SrTiO₃ were annealed with CaH₂ at low temperatures below 300 °C, the brownmillerite CaFeO_2.5 layer was reduced to CaFeO₂ with an infinite-layer structure while both the LaAlO₃ capping layer and the SrTiO₃ substrate remained intact. The reduction behaviour strongly depends on the lattice matching of LaAlO₃ to CaFeO_2.5, suggesting that oxygen ions migrate through the coherently grown LaAlO₃ layer of the heterostructure predominantly in the out-of-plane direction. The structural defects near the interface in the relaxed-structure LaAlO₃ capping layer prevent the oxygen ions from migrating.