NMR studies on rapidly solidified SiO2-Al2O3 and SiO2-Al2O3-Na2O-glasses

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 SiO2-Al2O3-Na2O glasses containing 10 mol% Al2O3 and 2.5 to 10 mol% Na2O. 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 ppm peak is assigned to distorted triclustered Al-O-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al2O3 over Na2O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing greater than or equal to 7.5 mol% Na2O. (C) 1997 Elsevier Science B.V.