Coordination of sodium ions in NaAlO2–SiO2 melts: a high temperature 23Na NMR study
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The coordination environment of the sodium ion in the melts of several simple ionic liquids and an Na2O–Al2O3–SiO2 mixture has been investigated by high temperature 23Na NMR measurements. A new high temperature NMR probe was utilized for the measurements of the compositional and temperature dependence of the 23Na NMR chemical shift at temperatures up to 1600 °C.
23Na NMR spectra of ionic liquids, NaCl, NaBr and NaNO3, show two peaks at their solid to liquid transition, corresponding to the solid and liquid state, respectively. The 23Na NMR peak shift in passing from the liquid to the solid is positive. This suggests a decrease in the coordination number for the molten state compared to the crystalline state.
The 23Na peak position for the Na2O–Al2O3–SiO2 melts of the composition range Na/Al≥1 shifted almost linearly in the positive direction as a function of both the increased degree of depolymerization, NBO/T, and [Al]/([Al]+[Si]). 23Na MAS-NMR measurement for crystalline silicate compounds of known structure provided a revised relationship between the mean Na–O distances and 23Na chemical shifts. Comparison of the 23Na chemical shift of the melts with that of crystalline silicate compounds suggests that the coordination number of Na in those melts is around 6–8 with little compositional dependence. The 23Na peak position shifted in the negative direction with increasing temperature for sodium silicates, whereas that of aluminosilicates did not show any temperature dependence. The activation energy from the temperature dependence of the 23Na line width shows little compositional dependence, and the value (51∼58 kJ/mol) was close to that of the trace Na ion diffusion in NaAlSi3O8 glass.
KeywordsIonic Liquid Coordination Number Aluminosilicate Na2O Sodium Silicate
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