Oxygen permeation and oxidation states of transition metals in (Fe, Nb)-doped BaCoO3 − δ perovskites

The influence of cation substitution on the transition metal oxidation states, oxygen nonstoichiometry and oxygen transport properties of Ba(Co, Fe, Nb)O3 − δ perovskites was investigated with room temperature XRD, X-ray absorption near edge structure and high temperature oxygen permeation measurements. Results show that substitution of Fe for Co is accompanied by a decrease in the lattice constant, whereas Nb substitution leads to the opposite trend. With increasing Fe and/or Nb content, the oxygen permeation flux decreases, whereas the apparent activation energy of the oxygen permeation increases. The oxidation state of Co is smaller than +3 and susceptible to the variation of cation composition. In contrast, the oxidation states of Fe and Nb are larger than +3 and close to +5, respectively, and almost independent of the composition. The influence of Fe and Nb contents on the oxygen transport properties is discussed in terms of the oxygen stoichiometry and metal–oxygen bonding.