A SI K-EDGE EXAFS/XANES STUDY OF SODIUM-SILICATE GLASSES

Si K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) have been obtained on a series of sodium silicate glasses containing 15-40 mol% Na2O. The XANES reveals that, with the addition of Na2O, the Si-Si bond distance distribution decreases and there are increased contributions from multiple scattering beyond the second coordination sphere. These responses imply that some degree of network ordering occurs with alkali addition. The EXAFS data indicate that the Si-O bond distance increases from 1.61 +/- 0.02 Angstrom for amorphous SiO2 to 1.66 +/- 0.02 Angstrom for 30 mol% added Na2O. For Na2O > 30 mol%, the Si-O bond distance decreases. The Si-O bond distance changes indicate that, for less than or equal to 30 mol% Na2O, network depolymerisation effects on the Si-O bond dominate any effect from increased non-bridging oxygen (NBO) formation. For compositions > 30 mol% Na2O, increased NBO formation has the dominant effect on the Si-O bond distance. This may indicate that the microsegregation of network modifiers from network formers, as predicted by molecular dynamics studies, is significant at greater than or equal to 30 mol% added Na2O.