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

Abstract

The results of high-resolution 23Na, 27Al and 29Si NMR spectroscopy of aluminosilicate glasses with MO/ Al 2 O 3 or L 2O Al 2O 3 = 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.