Jf. Stebbins, CATION SITES IN MIXED-ALKALI OXIDE GLASSES - CORRELATIONS OF NMR CHEMICAL-SHIFT DATA WITH SITE SIZE AND BOND DISTANCE, Solid state ionics, 112(1-2), 1998, pp. 137-141
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.