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...