M. Schmucker et al., NMR-STUDIES ON RAPIDLY SOLIDIFIED SIO2-AL2O3 AND SIO2-AL2O3-NA2O-GLASSES, Journal of non-crystalline solids, 217(1), 1997, pp. 99-105
Al-27 and Si-29 MAS NMR studies were performed on roller-quenched SiO2
-Al2O3-glasses with Al2O3 contents ranging from 10 to 60 mol% and on S
iO2-Al2O3-Na2O glasses containing 10 mol% Al2O3 and 2.5 to 10 mol% Na2
O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm.
The frequency distribution of the different Al-sites is not affected
by the glass composition. In glasses of the system SiO2-Al2O3-Na2O the
30 ppm peak decreases to zero as the Na2O content increases. The 30 p
pm peak is assigned to distorted triclustered Al-O-tetrahedra, rather
than to fivefold coordinated Al. Triclustering of tetrahedra may provi
de for charge neutrality in glasses with molar excess of Al2O3 over Na
2O. As charge balance is increasingly achieved by addition of alkali i
ons, the tendency of tetrahedral triclustering is reduced, reflected b
y the disappearance of the 30 ppm peak in glasses containing greater t
han or equal to 7.5 mol% Na2O. (C) 1997 Elsevier Science B.V.