Cation ordering at fluoride sites in silicate glasses: a high-resolution 19F NMR study

High-resolution 19F NMR, with sample spinning rates to 25 kHz, can provide quantitative, new information about the structure of fluoride sites in F-doped silicate glasses. Here we present results for sodium-, potassium-, calcium-, lanthanum- and mixed cation silicate glasses, containing 1–2 wt% F. In the end member compositions, most fluoride is in sites that are structurally similar to the corresponding simple metal fluoride, although a few percent of Si–F bonds can be detected in the sodium silicates. In the mixed cation glasses, partially resolved peaks are observed for series of distinct mixed cation coordination environments for fluoride. Fluoride ions seem to be preferentially bonded to higher field-strength modifier cations, but considerable disorder is present for all cation pairs (Na–K, Na–Ca, Na–La) studied.