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

• Published in: Chemistry of materials 2010-02, Vol.22 (3), p.906-913
• Main Authors: Arroyo y de Dompablo, M. E, Lee, Yueh-Lin, Morgan, D
• Format: Article
• Language: English
• Subjects: Inorganic Solids and Ceramics; Ionic Conductors (including Solid (Inorganic) and Polymer Electrolytes); Theory and Modeling
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm901723j

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 ion, with values ranging from 6 eV in MnNb2O6 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 a maximum range of 0.85 eV. In both MnNb2O6 and CuNb2O6 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 MnNb2O6 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.