High-resolution NMR data for lithium and sodium in silicate, aluminosilicate, germanate, borate, and tellurite glasses and glass melts all show systematic effects of composition on isotropic chemical shift. The most reliable interpretation of these data is through well-established correlations of increased (higher frequency) chemical shift with smaller cation site size. Recent studies of Li and Na in mixed (Li, Na, K, Rb) alkali glasses and liquids, where systematic compositional effects were also reported, therefore suggest that as a smaller cation is substituted for a larger (e.g. Li for Na), the mean size of the sites for the larger cation actually increases. The opposite effect is clearly observed for substitution of a larger cation for a smaller one. This is an example of other systematic effects of cation field strength and is likely to be due to `competition' among cations for negatively charged oxygen sites. The role of these structural changes in cationic conduction is discussed.
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Jonathan F. Stebbins (1998) studied this question.