X-ray absorption investigation of the valence state and electronic structure of La1−xCaxCoO3−δ in comparison with La1−xSrxCoO3−δ and La1−xSrxFeO3−δ

3d metal K-shell X-ray absorption spectra of perovskites with the composition La1−xCaxCoO3−δ (x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La1−xSrxCoO3−δ (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La1−xSrxFeO3−δ (x=0, 0.2, 0.4, 0.5, 0.6, 0.8) are compared on the basis of pre-edges, white line features and extended fine structures. The measurements were performed at 300 K and for La1−xCaxCoO3−δ also at temperatures as low as 10–20 K. Going to low-temperature the measurements indicate an increase in t2g⁎ and a decrease in eg⁎ orbital occupancy, which is most accentuated in the LaCoO3 sample. Virtually no Co K-edge shift was observed for the La1−xCaxCoO3−δ and La1−xSrxCoO3−δ compounds and the Co–O distances are also not significantly reduced when La3+ is partially substituted by Ca2+ or Sr2+. From the pre-edge features of these perovskites we are tended to conclude that the t2g⁎ orbitals are less, and the eg⁎ orbitals are more occupied with increasing x in the Ca and Sr substituted compounds, whereas the total d-electron density is not changing. These results indicate that cobalt prefers a valence state of 3+ in these Co perovskites. This could also be confirmed with iodometric titrations. The Fe perovskites behave differently. In contrast to the Co perovskites, for La1−xSrxFeO3−δ perovskites the Fe K-edge is shifted, the pre-edge features intensity is increasing and the Fe–O bond length is decreasing with increasing x. The valence states of the iron in the La1−xSrxFeO3−δ perovskites in fact increase as much as x increases.

Co K and Fe K pre-edge of La1−xCaxCoO3−δ and La1−xSrxFeO3−δ perovskites one of the evidences in favor of δ=x/2 for the Co-perovskites and δ=0 for the Fe-perovskites .Figure optionsDownload as PowerPoint slideHighlights
► XAS a valuable tool to evaluate the valence states of Co and Fe perovskites.
► For La1−xCaxCoO3−δ and La1−xSrxCoO3−δ perovskites δ is close to x/2.
► For La1−xSrxFeO3−δ series δ is close to 0.
► Discussion of the x dependency of the pre-edge bands.