First Principles Investigation of Oxygen Vacancies in Columbite MNb₂O₆ (M = Mn, Fe, Co, Ni, Cu)

This work presents a computational study on the stability and electronic properties of oxygen vacancies in candidate mixed electronic−ionic conducting columbites, MNb 2 O 6 (M = Mn, Fe, Co, Ni, Cu). Calculations are based on density functional theory with the DFT+U method. Three distinct oxygen vacancy types are investigated, differing by the coordination of the oxygen atoms removed: oxygen atoms coordinated by three, two, or one Nb ion. Calculation shows that the energy of oxygen vacancy formation depends on the transition metal ion, with values ranging from 6 eV in MnNb 2 O 6 to 3.5 eV in CuNb 2 O 6 (data refers to one oxygen vacancy per four formula units, or 4.16% vacancies). For a given transition metal the location of the vacancy formation energies on the different sites vary by a maximum range of 0.85 eV. In both MnNb 2 O 6 and CuNb 2 O 6 the oxygen vacancy formation induces (a) a narrowing of the band gap and (b) an electronic density redistribution leading to the reduction of cations to lower oxidation states. For MnNb 2 O 6 reduction affects mostly Nb 5+ ions, while for CuNb 2 O 6 the Cu 2+ ions are reduced to Cu 1+ . The potential improvement of electronic conductivity in CuNb 2 O 6− x together with the moderate vacancy formation energy makes this material a potential mixed electronic−ionic conductor.