First principles investigations of structure, stability and electronic properties of polar Ca2Fe2O5(0 1 0) surfaces

Abstract

The brownmillerite structure of a technologically important oxide Ca2Fe2O5 (CFO) consists of alternating layers of octahedra and tetrahedra of Fe and O, with the tetrahedral layers having intrinsic O vacancies. In this work, using first principles density functional theory based calculations, we have studied the stability, structure and electronic properties of the polar (0 1 0) surface of CFO. In contrast with those observed in case of the polar surfaces of conventional perovskites, we find that the stoichiometric, unreconstructed, Fe containing terminations are more stable than the non-stoichiometric reconstructed ones. In particular, our calculations show that depending on temperature and oxygen partial pressure, there are two possible stable terminations: (a) the layer containing the Fe and O atoms in the octahedron and (b) the layer containing the Fe and O atoms in the tetrahedron. While the former is metallic, the later has a band gap. The unreconstructed terminations are stabilized by dipole compensation through charge rearrangement in the surface layers during structural relaxations.