Characterization of local chemistry and disorder in synthetic and natural alpha-Al2O3 materials by X-ray absorption near-edge structure spectroscopy

X-ray absorption fine spectra at the Al K edge were measured experimentally and calculated theoretically via the multiple-scattering formalism for che mically pure and physically perfect synthetic alpha-Al2O3 (alpha-alumina), natural ruby/sapphire (corundum) and a series of artificial corundums produ ced for technical purposes and used as geochemical standards. The Al K-edge spectra differ, despite the identical coordination (short-range arrangemen t) assumed by O around Al, and vary slightly in relation to the slightly di fferent chemistries of the materials (substitutional defects) as well as on account of the location taken by foreign atoms in the structural lattices (positional defects). A quantitative treatment of the observed changes is m ade in terms of short-range distortion of the coordination polyhedron and o f medium to long-range modifications in the overall structure, both induced by substitutions. In some technical corundums, the impurities of admired b eta-alumina, where Al is both in fourfold and sixfold coordination, produce another small but detectable effect on Al K edges. Therefore, XAFS spectro scopy proves its potential for both measuring a light element such as Al an d detecting minor coordination changes and dilute substitutions (similar to 1-3 wt% as oxide) of the absorber by other atoms, at least under the favou rable conditions occurring in this system.