Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses: A multinuclear NMR study

The incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O-CaO aluminosilicate glasses with similar to 65 mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In F-19 MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2 mol% F the following F environments were identified: F-Ca(n) at similar to-113 ppm, Si-F-Na(n) or Al-F-Ca(n) at similar to-146 ppm, Al-F-Al at similar to-168 ppm, Al-F-Na(n) at similar to-188 ppm and F-Na(n) at similar to-225 ppm ("n" indicates that the number of atoms is variable or uncertain). F-Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F-Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2 mol%. The addition of up to 16.5 mol% (5.3 wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F-Al sites compared to F-Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH. In the peraluminous glasses containing up to 18.3 mol% F only three F environments Si-F-Na(n) or Al-F-Ca(n) at similar to-149 ppm, Al-F-Al at similar to-170 ppm and Al-F-Na(n) at similar to-190 ppm are observed. Al-F-Na(n) is the most abundant site with a fraction of 54-61%. The F speciation also changes with the F concentration, with a minimum in Al-F-Na(n) sites between 3.5 and 9.7 mol% F. Fluorine has only a small effect on the Na-23 and Si-29 MAS NMR spectra. Al-27 MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses similar to 5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F. (C) 2014 Elsevier Ltd. All rights reserved.