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

Abstract

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 $\mathrm{Fe}<\mathrm{Co}<\mathrm{Al}$. The electronic band gaps of ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ are shown to remain largely unchanged as Al is substituted into the lattice forming ${M}_{2}\mathrm{Al}{\mathrm{O}}_{4}$ $(M=\mathrm{Fe},\mathrm{Co})$, but increase greater than $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for $M{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ as the octahedral $M$ metal sites are lost. However, for stoichiometric $\mathrm{Fe}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, 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.