We investigated the structure of five disilicate glass compositions: K2 • 2 SiO2, Na0.25K0.75)2 • 2 SiO2, Na0.50K0.50)2 • 2 SiO2, Na0.75K0.25)2 • 2 SiO2, Na2 • 2 SiO2, using 2D 17O Dynamic Angle Spinning (DAS) Nuclear Magnetic Resonance (NMR). The non-bridging and bridging oxygen sites can be resolved in the isotropic 17O DAS spectrum. The bridging oxygen isotropic resonance shifts from 16.1 ppm in K2 • 2 SiO2 to 11.8 ppm in Na2 • 2 SiO2 and remains identical in width. The non-bridging oxygen isotropic lineshapes 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 non-bridging oxygen isotropic lineshape with changing alkali composition is consistent with a model of the glass structure where four alkali cations are distributed in random combinations around each non-bridging oxygen. These results preclude significant clustering of similar cation types and are generally supportive of models of the mixed alkali effect that involve blocking of cation migration pathways by
Cation Distribution in Mixed Alkali Disilicate Glasses,