Al-O-Al oxygen sites in crystalline aluminates and aluminosilicate glasses: High-resolution oxygen-17 NMR results

We report O-17 magic-angle spinning (MAS) NMR data for crystalline NaAlO2 and CaAl2O4 at external magnetic fields of 9.4 and 14.1 T, as model compounds for Al-O-Al sites in tetrahedral networks. The former contains one peak with isotropic chemical shift (delta(ISO)) = 30.9 ppm and quadrupolar coupling constant (C-Q) approximate to 1.8 MHz. The latter contains several peaks with delta(ISO) ranging from 39 to 87 ppm and C-Q approximate to 1.5 to 2.4 MHz. Triple-quantum MAS (3QMAS) spectra of sodium and calcium aluminosilicate glasses with Si/Al < 1 show clearly resolved peaks for Al-O-Al sites, removing ambiguity in the use of such spectra to explore the extent of aluminum avoidance in both glassy and crystalline materials. We also report Na-23 and Al-27 NMR data for the crystalline phases.