CATION DISTRIBUTION IN MIXED-ALKALI DISILICATE GLASSES

We have investigated the structure of five disilicate glass compositio ns-K2O . 2SiO(2), (Na0.25K0.75)(2)O . 2SiO(2,) (Na0.50K0.50)(2)O . 2Si O(2), (Na0.75K0.25)(2)O . 2SiO(2,) and Na2O . 2SiO(2)-using 2D O-17 Dy namic Angle Spinning (DAS) Nuclear Magnetic Resonance (NMR). The nonbr idging and bridging oxygen sites can be resolved in the isotropic O-17 DAS spectrum. The bridging oxygen isotropic resonance shifts from 16. 1 ppm in K2O . 2SiO(2) to 11.8 ppm in Na2O . 2SiO(2) and remains almos t identical in width. The nonbridging oxygen isotropic line shapes in the mixed compositions are broader with a maximum intensity that conti nuously shifts from the position found in the pure potassium compositi on to the position found in the pure sodium composition. The evolution of the nonbridging oxygen isotropic line shape with changing alkali c omposition is consistent with a model of the glass structure where fou r alkali cations are distributed in random combinations around each no nbridging oxygen.