O-K and Co-L XANES Study on Oxygen Intercalation in Perovskite SrCoO3-δ

Evolution of the O-K and Co-L edge XANES spectra for SrCoO3-δ upon increasing oxygen content from 2.50 to 2.82 reveals that superoxide (O2−) related features are overwhelmingly strong in the surface region of highly oxygen-deficient samples. Moreover, should it be always present in the bulk, octahedrally coordinated cobalt is unobservable in the surface region of highly oxygen-deficient samples. On the other hand, the surface region hosts square-pyramidally coordinated cobalt even in brownmillerite SrCoO2.50. Observations in the present work support an oxygen-intercalation mechanism in which (i) O2 is first absorbed on the surface as O2−, (ii) then reductively split into Ox- associated with square-pyramidally coordinated cobalt, and (iii) finally in the bulk eventually reoxidized to Oz- (0 < z < x) being attached to octahedrally coordinated cobalt. The fact that only minute chemical shifts of the Co-L edge branches are seen indicates that electron transfer mostly takes place between oxygen atoms.