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

High-resolution F-19 NMR, with sample spinning rates to 25 kHz, can provide quantitative, new information about the structure of fluoride sites in F-d oped silicate glasses. Here we present results for sodium-, potassium-, cal cium-, lanthanum- and mixed cation silicate glasses, containing 1-2 wt% F. In the end member compositions, most fluoride is in sites that are structur ally similar to the corresponding simple metal fluoride, although a few per cent of Si-F bonds can be detected in the sodium silicates. In the mixed ca tion 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 con siderable disorder is present for all cation pairs (Na-K, Na-Ca, Na-La) stu died. (C) 2000 Elsevier Science B.V. All rights reserved.