Composition and orientation dependence of the O K and Fe $L_{2,3}$ EELS fine structures in ${\mathrm{Ca}}_2({\mathrm{Al}}_x{\mathrm{Fe}}_{1-x}{)}_2{\mathrm{O}}_5$

Oxygen K edge and iron $L_{2,3}$ edges electron-energy-loss spectra have been collected on brownmillerites $\left[{\mathrm{Ca}}_2\right({\mathrm{Al}}_x,{\mathrm{Fe}}_{1-x}{)}_2{\mathrm{O}}_5]$ synthesized with different Al/Fe compositions $\mathrm{}(0<x<\mathrm{}0.67\mathrm{}).$ The intensity of the prepeak 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 $L_{2,3}$ 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.