Cation Distribution in Mixed Alkali Disilicate Glasses

Abstract

We have investigated the structure of five disilicate glass compositionsK2O·2SiO2, (Na0.25K0.75 )2O·2SiO2, (Na0.50K0.50 )2O·2SiO2, (Na0.75K0.25)2O·2SiO2, and Na2O·2SiO2using 2D 17O Dynamic Angle Spinning (DAS) Nuclear Magnetic Resonance (NMR). The nonbridging and bridging oxygen sites can be resolved in the isotropic 17O DAS spectrum. The bridging oxygen isotropic resonance shifts from 16.1 ppm in K2O·2SiO2 to 11.8 ppm in Na2O·2SiO2 and remains almost identical in width. The nonbridging oxygen isotropic line shapes in the mixed compositions are broader with a maximum intensity that continuously shifts from the position found in the pure potassium composition to the position found in the pure sodium composition. The evolution of the nonbridging oxygen isotropic line shape with changing alkali composition is consistent with a model of the glass structure where four alkali cations are distributed in random combinations around each nonbridging oxygen.