AL AND SI COORDINATION IN SIO2-AL2O3 GLASSES AND LIQUIDS - A STUDY BY NMR AND IR SPECTROSCOPY AND MD SIMULATIONS

Structural properties of a series Of SiO2-Al2O3 glasses with up to 59 mole% Al2O3 were investigated by magic angle spinning NMR and infrared absorption spectroscopies. The NMR results showed that Al is present in IV- and VI-coordinated sites. The relative proportions of these species depends on composition and quench rate. The Al-27 NMR results have been used to "calibrate" the IR absorption spectra, in that features in the IR spectra have been assigned to Al-O stretching vibrations of different AlOx polyhedral species. This will be useful for future in situ studies of these glasses and melts at high pressures or high temperature, when NMR experiments are difficult to perform, or do not give information on structural sites. For comparison with these results, molecular dynamics simulations Of SiO2-Al2O3 liquids were carried out for several compositions along the join. The pair distribution functions were analyzed to give information on the distribution of Al and Si coordination species in the liquids. At high silica content, both Si and Al predominantly occupy IV-coordinated sites, though the presence of some III-coordinated Al is indicated. With increasing Al2O3 content, the proportion of Al(V) and Al(VI) species is increased, and the average Al coordination number for Al2O3 liquid approaches 5.0. Concomitant increases in Si(V) coordination sites are observed, and the average Si coordination increases to approximately 4.7 near pure Al2O3.