Fluorine sites in calcium and barium oxyfluorides: F-19NMR on crystalline model compounds and glasses

As part of an ongoing effort to apply Fluorine-19 NMR to understanding the effects of fluoride on the structures and properties of silicate glasses and melts, the local environment of fluorine in several fluorosilicate and fluoroaluminate crystals was explored using F-19 MAS NMR. Upon combining this work with recent data, we found the chemical shift generally increases with the radius of the adjoining cation. Also, we found that, in F-La(n) and F-Ca(n) sites, increasing the coordination of the fluorine atom lowers the frequency of the chemical shift. In fluoroaluminates, however, increasing coordination increases the chemical shift. Using this compilation of crystalline data, we analyzed F-19 NMR spectra for several silicate and aluminosilicate glasses. The major feature in the Ba silicate glass was a broad peak assigned to F-Ba(n), and a low intensity shoulder was attributed to Si-F-Ba(n). It was impossible to assign the main peak in the Ba aluminosilicate glass to a particular structure, but it was clear that there was a negligible amount of FBa(n) present. The Ca aluminosilicate glass had F-Ca(n) units suggested by additional intensity in the higher frequency spinning sidebands, implying that Ca2+ is more effective than Ba 2, in competing with Al3+ for fluorine bonds. (C) 2002 Elsevier Science B.V. All rights reserved.