Understanding oxygen vacancy migration and clustering in barium strontium cobalt iron oxide

In order to understand vacancy clustering and phase stability in oxygen-deficient barium strontium cobalt iron oxide (BSCF), we predict stability and activation energies for oxygen vacancy migration with plane wave Density Functional Theory. Using symmetry constrained search and Nudged Elastic Band method, we characterize the transition states for an oxygen anion moving into a nearby oxygen vacancy site that is surrounded by different cations and find the activation energies to vary in the range of 30-50 kJ/mol in good agreement with experimental data. The preference for oxygen vacancies to form square tetramers is confirmed, and found to have a low energy barrier to formation. The presence of Co and Ba cations is found to decrease the activation energy for oxygen vacancy migration. Based on this finding, we suggest that increased concentration of these cations will likely improve the ionic conductivity and allow the design of Solid Oxide Fuel Cells with a reduced operating temperature. (C) 2010 Published by Elsevier B.V.