An x-ray spectroscopic study of A2FeMoO6 and Sr2Fe1-xCrxMoO6 double perovskites

The electronic and geometrical local structures of A(2)FeMoO(6) (A = Ca, Sr, Ba, Ba1/2Sr1/2) and Sr2Fe1-xCrxMoO6 (x = 0, 1/4, 1/2, 3/4, 1) compounds have been investigated by means of x-ray absorption spectroscopy at the iron, chromium and molybdenum K edges. The Mo K-edge spectra show a similar electronic state for Mo in all samples with similar Mo-O distances. At the Fe K edge, instead, XANES and EXAFS spectra showed significant differences along the A(2)FeMoO(6) series, denoting an influence of the divalent A metal in the electronic state of iron. Both the chemical shift at the iron K edge and the interatomic Fe-O distances closely correspond to Fe3+ for Sr2FeMoO6 and Ca2FeMoO6 while a mixed valence state (between +2 and +3) is deduced for the Ba samples. We have not observed differences in the spectra of the Sr2Fe1-xCrxMoO6 series for any of the absorption edges. This demonstrates that substitution of Fe with Cr is homovalent. Our work suggests that the description in terms of a band model is a better approximation than an ionic model.