Oxygen triclusters in crystalline CaAl4O7 (grossite) and in calcium aluminosilicate glasses: O-17 NMR

We present O-17 MAS NMR data for crystalline calcium dialuminate (grossite) , CaAl4O7 and monoaluminate, CaAl2O4. The first of these contains an oxygen tricluster site and serves as a model compound for sites of this type in a luminosilicate glasses. Tricluster site NMR parameters are distinct from th ose of bridging O atoms (Al-O-Al), allowing partial resolution in triple qu antum MAS NMR spectra. Such spectra for calcium aluminosilicate glasses are consistent with the presence of a small fraction of tricluster sites. Obse rved chemical shifts for non-bridging oxygen (NBO) atoms in an impurity pha se in the CaAl2O4 sample are distinct from those for NBO in Ca-aluminosilic ate glasses, indicating that the latter are primarily bonded to Si, not Al.