Low Valence Cation Doping of Bulk Cr₂O₃: Charge Compensation and Oxygen Vacancy Formation

The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO₂, and CeO₂, the effect of substitutional metal dopants in bulk Cr₂O₃ and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard + U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal-doped bulk Cr₂O₃ to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr₂O₃ is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed "the active" oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr₂O₃ by lowering the energy cost.