Coordination Environment of Fluorine in the CaO-SiO₂-CaF₂ Glasses Bearing Na₂O: A Solid-state ¹⁹F MAS NMR Study

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The coordination environment of fluorine in the CaO-SiO₂-CaF₂-Na₂O glasses has been measured by ¹⁹F MAS NMR over a wide composition range including the similar compositions to those of mould fluxes so as to determine the composition dependence of the coordination environment of F⁻. Most of F⁻ are fourfold-coordinated by Ca²⁺ for the samples having the value of n_Na/(n_Na+n_Ca)≤0.3, where n represents the mole fraction of each cation. On the other hand, the samples having the value of n_Na/(n_Na+n_Ca)≥0.4 contain F⁻ coordinated with both Na⁺ and Ca²⁺ and F⁻ sixfold-coordinated by Na⁺.