CATION SITES IN MIXED-ALKALI OXIDE GLASSES - CORRELATIONS OF NMR CHEMICAL-SHIFT DATA WITH SITE SIZE AND BOND DISTANCE

High-resolution NMR data for lithium and sodium in silicate, aluminosi licate, germanate, berate, and tellurite glasses and glass melts all s how systematic effects of composition on isotropic chemical shift. The most reliable interpretation of these data is through well-establishe d correlations of increased (higher frequency) chemical shift with sma ller cation site size. Recent studies of Li and Na in mixed (Li, Na, K , Rb) alkali glasses and liquids, where systematic compositional effec ts were also reported, therefore suggest that as a smaller cation is s ubstituted for a larger (e.g. Li for Na), the mean size of the sites f or the larger cation actually increases. The opposite effect is clearl y observed for substitution of a larger cation for a smaller one. This is an example of other systematic effects of cation field strength an d is Likely to be due to 'competition' among cations for negatively ch arged oxygen sites. The role of these structural changes in cationic c onduction is discussed. (C) 1998 Published by Elsevier Science B.V. Al l rights reserved.