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

Abstract

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