NMR Studies of Fluorine in Aluminosilicate–Lanthanum Fluoride Glasses and Glass‐Ceramics

Abstract

NMR determinations of fluorine environments in transparent oxyfluoride glass-ceramics were made to learn about the crystallization of LaF 3 , as well as to ascertain the structural role of fluorine in the surrounding glassy matrix. The fraction of fluorine in LaF 3 was measured as a function of heat treatments, demonstrating significant differences between glasses modified with barium and those containing sodium. The results of these measurements showed that not all of the fluorine formed LaF 3 in these glass-ceramics, with resolution of additional fluoride sites at ؊135 and ؊185 ppm, due primarily to Si-F and Al-F bonding, respectively. Not only is the evidence of Si-F bonding unexpected, given the presence of aluminum, but the amount of Si-F bonding is sensitive to the type of modifier in the glass. Samples containing barium oxide as the modifier showed a higher fraction of Si-F bonding than those modified with sodium oxide.

Characterisation of fluorine containing glasses by 19F, 27Al, 29Si and 31P MAS-NMR spectroscopy

“…The 19 F spectra of the A series glasses showed a single peak at about À100 ppm for the low fluorine content glasses and a second peak at À150 ppm for the higher fluorine content glasses (Fig. 3).…”

“…The two glasses containing strontium rather than calcium exhibited a peak with a chemical shift higher than À100 ppm found for the calcium containing glasses. Ketac Molar also had two peaks in its 19 F spectrum with a chemical shift of about À70 and À30 ppm.…”

“…11,12 X-ray powder diffraction had been carried out previously to identify the crystal phases formed. 11 19 F were referenced to CaF 2 taken as À108 ppm relative to the more common standard of CFCl 3 . In addition 27 Al spectra were run on a high field (Advance-600, Bruker, Germany) at a spinning rate of 15 kHz with a recycle time of 1 s.…”

“…10) corresponding to a pyrophosphate species for the same reasons as the B Series glasses, however the lower phosphate content glasses ART 9, ART 8 and ART 7 exhibit a sharp peak at about 2-3 ppm corresponding to an orthophosphate species. 15 ART 8 which has the most intense orthophosphate peak also exhibits a peak at À103 ppm in its 19 F spectra suggesting that this glass and possibly ART9 and ART7 have started to form fluorapatite like phases on cooling. It is important to note here that these glasses were all amorphous by X-ray diffraction so there was no evidence of any crystallinity, but the NMR data does suggest fluorapatite like clusters.…”

“…The 19 F spectra (Fig. 11) again consist of two broad peaks which in this case correspond to either F-Ca(n) and Al-F-Ca(n) or mixed calcium-strontium sites such as F-Ca/Sr(n) and Al-FCa/Sr(n).…”