Fluoride sites in aluminosilicate glasses: High-resolution F-19 NMR results

We present here high-resolution, F-19 NMR spectra, collected with sample sp inning rates to 25 kHz, for sodium and calcium silicate and aluminosilicate glasses. Several distinct fluoride ion sites are well resolved and can be assigned to various coordination environments based on clear similarities t o crystalline model compounds. In aluminosilicates, fluoride with one Al an d several Na or Ca neighbors predominate, but silicon-fluoride groups may a lso be significant. Small concentrations of the latter can also be detected in sodium silicate glasses, suggesting a possible role in reduction of vis cosity. Fluoride sites with no Si neighbors are, however, predominant in Al -free sodium silicates and probably in Ca silicates as well.