INFERENCES ON THE STRUCTURE OF ALUMINOSILICATE GLASSES: A HIGH RESOLUTION SOLID-STATE SILICON-29 AND ALUMINUM-27 NUCLEAR MAGNETIC RESONANCE STUDY (MAGIC-ANGLE)

Framework-stoichiometry glasses and glasses in the system anorthite-diopside-forsterite were examined by high resolution solid-state silicon-29 and aluminum-27 nuclear magnetic resonance (NMR) spectroscopy. High resolution spectra were obtained at high magnetic field strengths (11.7 and 8.45 Tesla) using magic-angle sample-spinning (MASS) techniques.;The coordination state of silicon and aluminum can be determined by the NMR peak position and in all samples studied the NMR signal is from tetrahedral sites for both silicon and aluminum. The aluminum-27 peak position also indicates that most, if not all, of the aluminum is in fully polymerized, Q('4), sites. Both silicon and aluminum nuclei in framework-stoichiometry glasses become less shielded as aluminum content increases. For the silicon-29 chemical shift there is a linear correlation with Si/(Al+Si) ratio for ratios between 1 and 0.5, (delta) silicon-29 = -51.9 Si/(Al+Si) - 60.9. For the aluminum-27 chemical shift there is a linear correlation with Si/(Al+Si) ratio for ratios between 0.9 and 0.33 which at 11.7 Tesla, (delta) aluminum-27 = -21.6 Si/(Al+Si) + 70.5. Silicon-29 peak width increases with increasing structural disorder in the glasses. Empirical correlations of the silicon-29 chemical shift with structural parameters of crystals were employed to obtain estimates of the average Si-O-T bond angle and average Si-O bond length in the glasses studied. Modeling of the aluminum-27 spectra yield information on the polymerization state of silicon in the nonframework-stoichiometry glasses. There is a weak trend indicating that silicon nuclei are on average slightly more shielded in superheated samples and less shielded in supercooled samples. Also, it appears that glasses quenched from above the liquidus have greater structural disorder than those quenched from at or below the liquidus.