Medium-range order in sol-gel prepared Al2O3-SiO2 glasses: new results from solid-state NMR

Abstract

The medium-range order of 0.5Al2O3-xSiO2 glasses (1 ≤ x ≤ 6) prepared via a new sol-gel route from the Al lactate precursor has been studied by 29Si and 27Al single- and double-resonance solid-state NMR techniques. For high-alumina samples Si-O-Al connectivities are detected by 29Si MAS NMR as well as by 29Si{27Al} 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(4) mAl (0 ≤ m ≤ 4) are detectable in appreciable concentrations for x = 1, the spectra indicate that the average number of Al species bound to silicon, <mAl>, gradually decreases toward higher x values, as expected. The 27Al 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 27Al isotropic chemical shifts suggests an increasing number of Al···Al proximities. Consistent with this finding, two-dimensional 27Al-27Al 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 mediumrange order of these glasses is developed.