Evolution of the O-K and Co-L edge XANES spectra for SrCoO_3-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 SrCoO_2.50. 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 O^x- associated with square-pyramidally coordinated cobalt, and (iii) finally in the bulk eventually reoxidized to O^z- (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. The research was conducted at the NSRRC beamline BL20A1.

L. Karvonen, M. Valkeapaa, R. S. Liu, J. M. Chen, H. Yamauchi and M. Karppinen