A HIGH-RESOLUTION SI-29 AND AL-27 NMR-STUDY OF ALKALINE-EARTH ALUMINOSILICATE GLASSES

The abundance of chain- (Q2), sheet- (Q3), and framework-like (Q4) tetrahedral units, and the distribution of Al among those units in CaOAl2O3SiO2 and MgOAl2O3SiO2 glasses, has been investigated by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR). In each system two compositional series were studied: (i) average polymerization is held constant at one non-bridging oxygen per tetrahedron (Q3), and (ii) silica content held constant at 50 mol% (average polymerization varying from Q2.67 to Q4). The results indicate that (1) Al is entirely tetrahedrally coordinated and is largely in Q4 sites, except in Al-rich Q3 glasses where it is also in Q3 sites, and (2) at constant polymerization of Q3 the 'reaction' Q2 + Q4 = 2Q3 goes to the right with increasing Al content. 29Si chemical shifts calculated from 27Al peak positions assuming that all Al is tetrahedrally coordinated and in Q4 sites, and that only Q2, Q3, and Q4 sites are present in the glass, give reasonable agreement with measured values (±2 ppm), but this model applies only to sufficiently silicon-rich compositions in which there is negligible Al in Q3 sites. © 1990.