First Principles Investigation of Oxygen Vacancies in Columbite MNb2O6 (NI = 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, MNb2O6 (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)on, with values ranging from 6 eV in MNb2O6 to 3.5 eV in CuNb2O6 (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 it maximum range of 0.85 eV. In both MNb2O6 and CuNb2O6 the oxygen vacancy formation induces (a) a narrowing of the band gap and (b) ail electronic density redistribution leading to the reduction of cations to lower oxidation states. For MNb2O6 reduction affects mostly Nb5+ ions, while for CuNb2O6 the Cu2+ ions are reduced to Cu1+. The potential improvement of electronic conductivity in CuNb2O6-x together with the moderate vacancy formation energy makes this material a potential mixed electronic-ionic conductor.