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

Abstract

3 d metal K-shell X-ray absorption spectra of perovskites with the composition La 1− x Ca x CoO 3− δ ( x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La 1− x Sr x CoO 3− δ ( x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La 1− x Sr x FeO 3− 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 La 1− x Ca x CoO 3− δ 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 LaCoO 3 sample. Virtually no Co K-edge shift was observed for the La 1− x Ca x CoO 3− δ and La 1− x Sr x CoO 3− δ compounds and the Co–O distances are also not significantly reduced when La 3+ is partially substituted by Ca 2+ or Sr 2+. 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 La 1− x Sr x FeO 3− 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 La 1− x Sr x FeO 3− δ perovskites in fact increase as much as x increases.