Sixfold coordinated Si in alkali and alkali-CaO silicophosphate glasses by Si K-edge XANES spectroscopy

Alkali and alkali-CaO silicophosphate glasses (R2O . 2SiO(2) . 3P(2)O(5) and yR(2)O . yCaO . 2SiO(2) . 3P(2)O(5); R = Li, Na, K, Rb, Cs; y = 0.5) have been investigated by Si K-edge X-ray absorption near-edge structure (XANES) spectroscopy for evidence of sixfold coordinated Si (Si-[6]) and its dependence on alkali metal cations. The XANES spectra were collected by fluorescence yield (FY), which is sensitive to the interior of the sample. The proportion of Si-[6] varies from about 50 to 60% and is greatest for K-and Na-bearing glasses and marginally greater for glasses quenched at 300 degrees C rather than room temperature. [6]Si determined by XANES for the alkali silicophosphate glasses is in agreement with Si-[6] determined from Si-29 nuclear magnetic resonance (NMR) spectra. The proportion of Si-[6] does not vary appreciably with alkali metal and substitution of Ca2+ for 2R(+), suggesting that Si-[6] is largely independent of the details of the large cation sites in the glasses and the proportion of these sites occupied. XANES spectra collected by total electron yield (TEY) reflect diminution in Si-[6] through near-surface sample degradation and do not properly represent the bulk glass structure, e.g., the TEY-XANES of Cs-bearing glass quenched at 300 degrees C indicated no detectable Si-[6]. (C) 1997 Elsevier Science B.V.