Quantitative electron microprobe analysis of Fe3+/{sum}Fe: Basic concepts and experimental protocol for glasses

摘要

The presence of an unfilled 3d electron shell in the transition metals of the first series is manifested in X-ray self-absorption effects that cause distortions in the L emission spectra. In particular, one observes peak shape changes and peak position shifts toward lower energies that are attributed to X-ray photon self-absorption in the specimen. Self-absorption corresponds to partial overlap of both L-emission and absorption transitions in the region of the Fermi level. Such an overlap is related to relaxation effects that follow the creation of the core hole in the studied ions. For Fe, the overlap between the FeL emission peak and the corresponding FeL_III absorption band is clearly higher for Fe2+ than for Fe3+, leading to an enhanced self-absorption induced shift of the FeL peak for Fe2+. The shift of the L peak between Fe2+ and Fe3+ can be measured with the electron microprobe and hence exploited to determine the Fe3+/Fe ratios in silicates and glasses. For that purpose, an empirical method has been established based on working curves constructed from reference materials. A complete set of working curves is presented for glasses as well as an original experimental protocol.