Composition and orientation dependence of the O Kappa and Fe L-2,L-3 EELS fine structures in Ca-2(AlxFe1-x)(2)O-5

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 dif ferent 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 presen t 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 i nfluence on the determination of the Fe 3d-band occupation. These results h ave been used as a reference for the study of the Fe 2p spectra in Si and T i substituted brownmillerites, which are present on a nanometer scale in ce ments. They demonstrate that substitution by tetravalent cations into brown millerite is not accompanied by a significant reduction of the valence of t he iron atoms. This suggests that charge compensations operate through the increase of oxygen content in the structure.