X-ray absorption investigation of the valence state and electronic structure of La_(1-x)Ca_xCoO_(3-δ) in comparison with La_(1-x)Sr_xCoO_(3-δ) and La_(1-x)Sr_xFeO_(3-δ)

摘要

3d metal K-shell X-ray absorption spectra of perovskites with the composition La_(1-x)Ca_xCoO_(3-δ) (x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La_(1-x)SrxCoO_(3-δ) (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La_(1-x)Sr_xFeO_(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_xCoO_(3-δ) also at temperatures as low as 1020 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)CaxCoO_(3-δ) and La_(1-x)SrxCoO_(3-δ) compounds and the CoO 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 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 La_(1-x)Sr_xFeO_(3-δ) perovskites the Fe K-edge is shifted, the pre-edge features intensity is increasing and the FeO bond length is decreasing with increasing x. The valence states of the iron in the La_(1-x)Sr_xFeO_(3-δ) perovskites in fact increase as much as x increases.