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

LAURENCE A.J. GARVIE
Department of Geology, Arizona State University, Tempe, Arizona 85287-1404, U.S.A.

PETER R. BUSECK
Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287-1404, U.S.A.

ALAN J. CRAVEN
Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, U.K.

Abstract

We used parallel electron energy-loss spectroscopy (PEELS) in a scanning 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 the excited atoms. In many cases, the ELNES exhibits a shape that is dependent upon the nearest-neighbor coordination and represents a "fingerprint" of this coordination. Gaudefroyite, Ca4Mn3(BO3)3(CO3)(O,OH)3, contains several light elements and, as such, is ideal for a demonstration of the fingerprint technique. We studied the core-loss edges from gaudefroyite in the 0 to 1000 eV energy-loss range, and illustrate the uniqueness of the spectra by comparison with those from materials possessing a range of structural and electronic properties. The B K-edge from gaudefroyite is consistent with 3-fold-coordinated B and distinct from the edge for 4-fold-coordinated B. The C K-edge is distinct from that in elemental C, but consistent with that attributed to the carbonate anion. The Mn L2,3-edge exhibits ELNES indicative of octahedrally 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 contribution from the predominantly ionic Ca-O bonds.

Keywords: parallel electron energy-loss spectroscopy, PEELS, electron-loss near-edge structure, ELNES, coordination fingerprint, valence fingerprint, gaudefroyite.