Structural, magnetic, and electronic properties of the Co-Fe-Al oxide spinel system: Density-functional theory calculations

A systematic study of nine binary and ternary spinel oxides formed from Co, Al, and Fe is presented by means of density functional theory. Analysis of the structural, magnetic, and electronic properties through the series of materials is carried out. Preference for the octahedral spinel sites are found in the order Fe < Co < Al. The electronic band gaps of Co3O4 and Fe3O4 are shown to remain largely unchanged as Al is substituted into the lattice forming M2AlO4 (M=Fe,Co), but increase greater than 1 eV for MAl2O4 as the octahedral M metal sites are lost. However, for stoichiometric FeAl2O4, the unsatisfied valence state of Fe results in partial occupation of the conduction band. The results and chemical trends are discussed in terms of atomic site and orbital energies, and in relation to potential photoelectrolysis activity for the splitting of water as a renewable means of hydrogen production.