NMR studies on rapidly solidified SiO 2Al 2O 3 and SiO 2Al 2O 3Na 2O-glasses

Abstract

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