Journal article

X-ray absorption investigation of the valence state and electronic structure of La 1-xCa xCoO 3−δ in comparison with La 1-xSr xCoO 3−δ and La 1-xSr xFeO 3−δ

3d metal K-shell X-ray absorption spectra of perovskites with the composition La1-xCaxCoO3-delta (x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La1-xSrxCoO3-delta (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La1-xSrxFeO3-delta (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-delta also at temperatures as low as 10-20 K. Going to low-temperature the measurements indicate an increase in t(2g)* and a decrease in e(g)* orbital occupancy, which is most accentuated in the LaCoO3 sample. Virtually no Co K-edge shift was observed for the La1-xCaxCoO3-delta and La1-xSrxCoO3-delta 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 t(2g)* orbitals are less, and the e(g)* 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-delta 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-delta perovskites in fact increase as much as x increases. (C) 2011 Elsevier Inc. All rights reserved.


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