EXAFS and XANES Spectra of Calcium Silicate Glasses

According to conventional views of the structure of oxide glasses, alkali and alkaline earth ions occupy holes of random size and variable symmetry provides by the silicon-oxygen framework. The nearest-neighbour shell of oxygens around Ca would thus be expected to be broad — perhaps so broad as to be undetectable by normal structural techniques. Although some indirect evidence has been accumulated against this view, the most direct and persuasive arguments for an alternative viewpoint derives from the EXAFS studies of GREAVES et al.[1]. The extended fine structure above the Na and Ca K edges corresponds to reasonably well-defined shells of oxygens with mean distances and coordination numbers similar to those observed in crystals. The standard deviation, σ, of the Na-O and Ca-O distribution is in the range 0.07 – 0.17 A [1,2].