ELECTRON-LOSS NEAR-EDGE STRUCTURE (ELNES) AS A PROBE OF VALENCE AND COORDINATION-NUMBER

We used parallel electron energy-loss spectroscopy (PEELS) in a scanni ng transmission electron microscope (STEM) equipped with a cold field- emission gun to acquire high-resolution spectra. Detailed analysis of electron-loss near-edge structure (ELNES) of core-loss edges provides chemical information about the oxidation state and site symmetry of th e excited atoms. In many cases, the ELNES exhibits a shape that is dep endent upon the nearest-neighbor coordination and represents a ''finge rprint'' of this coordination. Gaudefroyite, Ca4Mn3(BO3)(3)(CO3)(O,OH) (3), contains several light elements and, as such, is ideal for a demo nstration of the fingerprint technique. We studied the core-loss edges from gaudefroyite in the 0 to 1000 eV energy-loss range, and illustra te the uniqueness of the spectra by comparison with those from materia ls possessing a range of structural and electronic properties. The B K -edge from gaudefroyite is consistent with 3-fold-coordinated B and di stinct from the edge for 4-fold-coordinated B. The C K-edge is distinc t from that in elemental C, but consistent with that attributed to the carbonate anion. The Mn L(2,3)-edge exhibits ELNES indicative of octa hedrally coordinated Mn3+; it is distinct from the edge shapes for the other Mn oxidation states. The O K-edge is rich in structure arising from O in the BO33-, CO32-, and Mn3+O6n- groups, with little contribut ion from the predominantly ionic Ca-O bonds.