23Na NMR chemical shifts and local Na coordination environments in silicate crystals,melts and glasses

In order to decipher information about the local coordination environments of Na in anhydrous silicates from ²³Na nuclear magnetic resonance spectroscopy (NMR), we have collected ²³Na magic angle spinning (MAS) NMR spectra on several sodium-bearing silicate and aluminosilicate crystals with known structures. These data, together with those from the literature, suggest that the ²³Na isotropic chemical shift correlates well with both the Na coordination and the degree of polymerization (characterized by NBO/T) of the material. The presence of a dissimilar network modifier also affects the ²³Na isotropic chemical shift. From these relations, we found that the average Na coordinations in sodium silicate and aluminosilicate liquids of a range of compositions at 1 bar are nearly constant at around 6–7. The average Na coordinations in glasses of similar compositions also vary little with Na content (degree of polymerization). However, limited data on ternary alkali silicate and aluminosilicate glasses seem to suggest that the introduction of another network-modifier, such as K or Cs, does cause variations in the average local Na coordination. Thus it appears that the average Na coordination environments in silicate glasses are more sensitive to the presence of other network-modifiers than to the variations in the topology of the silicate tetrahedral network. Further studies on silicate glasses containing mixed cations are necessary to confirm this conclusion.