Superstoichiometric Oxygen and Structural Instability of Ferrite CaBaFe₄O₇: ab Initio Approach

The pseudo-potential PAW method of the density functional theory is employed to study the localization of superstoichiometric oxygen in ferrite CaBaFe₄O₇ with the orthorhombic swedenborgite structure. The geometric characteristics of oxygen defects are determined by the crystal lattice relaxation procedure. It is shown that the formation of defects in calcium and barium coordination are energetically unfavorable. Oxygen intercalation into structural trigonal iron-oxygen and Kagomé layers is more favorable. In these layers, two edge-sharing Fe₂O₅ bipyramides are formed instead of FeO₄ tetrahedra connected via one oxygen atom. The structural instability of ferrite CaBaFe₄O₇ is shown to be caused by the population of anti-bonding 2p O states during oxygen intercalation. The asymmetric location of the intercalated oxygen ion relative to the neighboring iron ions promotes the appearance of local electric polarization, which can induce ferroelectric effects in the presence of ferromagnetic order.