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

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.