First-principles study of strain effect on the formation and electronic structures of oxygen vacancy in SrFeO₂

Motivated by recent experimental observations of metallic conduction in the quasi-two-dimensional SrFeO₂, we study the epitaxial strain effect on the formation and electronic structures of oxygen vacancy (V_o) by first-principles calculations. The bulk SrFeO₂ is found to have the G-type antiferromagnetic ordering (G-AFM) at zero strain, which agrees with the experiment. Under compressive strain the bulk SrFeO₂ keeps the G-AFM and has the trend of Mott insulator-metal transition. Different from most of the previous similar work about the strain effect on V_o, both the tensile strain and the compressive strain enhance the V_o formation. It is found that the competitions between the band energies and the electrostatic interactions are the dominant mechanisms in determining the V_o formation. We confirm that the V_o in SrFeO₂ would induce the n-type conductivity where the donor levels are occupied by the delocalized d_x²-_y² electrons. It is suggested that the vanishing of n-type conductivity observed by the Hall measurement on the strained films are caused by the shift of donor levels into the conduction band. These results would provide insightful information for the realization of metallic conduction in SrFeO₂.