Medium-Range Order in Sol-Gel Prepared Al2O3-SiO2 Glasses: New Results from Solid-State NMR

The medium-range order of 0.5Al(2)O(3)-xSiO(2) glasses (1 <= x <= 6) prepared via a new sol-gel route from the Al lactate precursor has been studied by Si-29 and Al-27 single- and double-resonance solid-state NMR techniques. For high-alumina samples Si-O-Al connectivities are detected by Si-29 MAS NMR as well as by Si-29{Al-27} rotational echo adiabatic passage double-resonance (REAPDOR) spectroscopy. To boost the signal-to-noise ratio, the REAPDOR experiment was combined with a Carr-Purcell-Meiboom-Gill (CPMG) echo train acquisition. While all five silicon units Q(mAl)((4)) (0 <= m <= 4) are detectable in appreciable concentrations for x = 1, the spectra indicate that the average number of Al species bound to silicon, (m(Al)), gradually decreases toward higher x values, as expected. The Al-27 MAS NMR spectra reveal four-, five-, and six-coordinated aluminum in these glasses. For x >= 3, the Al species detected are essentially independent of sample composition indicating a constant structural environment of Al. In contrast, for x = 1 and 2, an increase in the Al-27 isotropic chemical shifts suggests an increasing number of Al.. Al proximities. Consistent with this finding, two-dimensional Al-27-Al-27 double-quantum/single-quantum correlation spectroscopy reveals spatial proximities among and between all types of aluminum species present. On the basis of the complementary evidence from these single- and double-resonance experiments, a model for the medium-range order of these glasses is developed.