High-resolution ²³Na, ²⁷Al and ²⁹Si NMR spectroscopy of framework Aluminosilicate glasses

The results of high-resolution ²³Na, ²⁷Al and ²⁹Si NMR spectroscopy of aluminosilicate glasses with MO/Al₂O₃ or L₂O Al₂O₃ = 1 (M = +2, L = +1 cations) shows that these glasses have a fully polymerized tetrahedral framework structure with only a defect level of non-bridging oxygens. The chemical shifts of the peak maxima for all three nuclides become less shielded (less negative or more positive) with decreasing Si/ (Si + Al). For ²⁹Si and ²⁷Al, this variation parallels the variation of framework crystalline silicates. The ²³Na chemicAl shifts for the glasses becomes less shielded (less negative) with decreasing Na/(Na + K), opposite to the trend for crystalline alkali feldspars. Few data exist for the ²³Na chemical shifts of crystalline samples, and the structural causes of variation in ²³Na chemical shifts are not well understood. The ²⁷Al and ²⁹Si chemical shifts of the glasses do not vary significantly with different large (modifier) cations. The ²⁹Si chemical shifts provide estimates of average Si-O-T (T = Si, Al)bond angles and Si-O bond distances and the ²⁷AL and ²⁹Si chemical shifts and peak breadths are consistent with a decrease in the tetrahedralring order (number of tetrahedra per ring) with decreasing Si/(Si + Al). The data presented here for fully polymerized glasses form a base from which the data for aluminosilicate glasses containing both fully polymerized sites and less polymerized sites can be interpreted.