NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

Abstract 27 Al and 29 Si MAS NMR studies were performed on roller-quenched SiO 2 Al 2 O 3 -glasses with Al 2 O 3 contents ranging from 10 to 60 mol% and on SiO 2 Al 2 O 3 Na 2 O glasses containing 10 mol% Al 2 O 3 and 2.5 to 10 mol% Na 2 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 SiO 2 Al 2 O 3 Na 2 O the 30 ppm peak decreases to zero as the Na 2 O 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 Al 2 O 3 over Na 2 O. 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 ≥ 7.5 mol% Na 2 O.