Fluorine-19 NMR Study of the Environment of Fluorine in Silicate and Aluminosilicate Oxyfluoride Glasses.

Using a compilation of F-19 NMR chemical shift values for crystalline model compounds, we have explored the local fluorine environment in several silicate and aluminosilicate oxyfluoride glasses. The F-19 MAS NMR spectrum of the Mg-silicate glass consists of a broad peak centered at -170 ppm, which encompasses the F-Mg(3) environments (where "3" is the number of Mg neighbors) of crystalline MgF2 (-195 ppm) and phlogopite (-175 ppm). The predominant environment in the glass is consequently assigned to F-Mg(n), where "n" indicates an unknown number of nearest neighbors. The magnesium aluminosilicate glass spectra all consist of a major feature at about -175 ppm and a shoulder at approximately -145 ppm. Based on the model compound data, the shoulder lies in the region attributed to Al-F-M2+ sites, and it was observed that glasses with a higher Al/Mg ratio featured extra intensity in the high-frequency shoulder. The shoulder is consequently assigned to Al-F-Mg(n), while the major feature is attributed to F-Mg(n) sites. This indicates that Mg is decidedly more effective in bonding to fluorine than either Ca or Ba in aluminosilicate glasses, following the trend established in our previous work on the Ba- and Ca-aluminosilicate glasses (the higher field strength modifier cation is more effective in competing with Al). Fluorine preferentially bonding with the modifier cation over aluminum has, to our knowledge, never been previously observed in aluminosilicate glasses. Spectra for Na-La-silicate glasses are characterized by an extremely wide peak (hundreds of ppm) and a relatively narrow peak at -220 ppm. The narrow peak at -220 ppm is assigned to F-Na(n) due to its proximity to the chemical shift of F-Na(6) (-225 ppm, from crystalline NaF). The small relative area of the F-Na(n) peak indicates a pronounced preference for F-La bonds over F-Na bonds, which is consistent with the findings of the previously discussed aluminosilicates. The massive, broad feature encapsulates the region described by the mixed fluorine environments of crystalline NaLaF4 (-30 to -63 ppm), and has therefore been assigned to a wide variety of fluorine environments, consisting of F-La bonds in a mixed environment with some number of F-Na bonds. In order to explore La-F clustering we have employed "echo" experiments, which can provide information about F-F distances. In the Na-La-silicates, the broad feature decays much faster than the F-Na(n) peak, indicating a shorter F-F distance in the sites associated with La. This is not consistent with random anion distribution but can be easily explained by the formation of fluorine-rich clusters. This may provide insight into the mechanism for the onset of crystallization in glass ceramics. In the long echo-time experiments on the Na-La-silicate glasses, the intensity of the broad feature was virtually gone, and the intensity of the F-Na(n) peak was largely decreased. This allowed us to clearly observe another peak centered at -140 ppm. This is near the peak found for the Si-F-Na(2) structure (-152 ppm) in the model compounds. Further examination of the original one-pulse spectra revealed the presence of this peak in small quantities in all compositions. Its relative intensity (about 2 percent of the total intensity) is comparable to that of Si-F-Ba(n) and Si-F-Na(n) bonding found in the fluorine containing Ba-silicate and Na-silicate glasses (respectively) from our previous studies. This suggests that Si-F bonding is independent of composition and may be an intrinsic characteristic of fluorine-containing silicate glasses.