Composition and orientation dependence of the O Κ and Fe L2,3 EELS fine structures in Ca2(AlxFe1-x)2O5

Oxygen K edge and iron L-2,L-3 edges electron-energy-loss spectra have been collected on brownmillerites [Ca-2(Al-x,Fe1-x)(2)O-5] synthesized with different Al/Fe compositions (0<x<0.67). The intensity of the pre peak in the O K edge is directly correlated with the proportion of Fe-O-Fe bonds present in the brownmillerite. The effect of natural linear dichroism on Fe L-2,L-3 edges in these compounds has been evaluated together with its possible influence on the determination of the Fe 3d-band occupation. These results have been used as a reference for the study of the Fe 2p spectra in Si and Ti substituted brownmillerites, which are present on a nanometer scale in cements. They demonstrate that substitution by tetravalent cations into brownmillerite is not accompanied by a significant reduction of the valence of the iron atoms. This suggests that charge compensations operate through the increase of oxygen content in the structure.