F-19 NMR study of the ordering of high field strength cations at fluoride sites in silicate and aluminosilicate glasses

Several glasses were synthesized to explore cation ordering at fluorine sites in silicate and aluminosilicate glasses. Utilizing ¹⁹F NMR, we found a significant (at least ∼30% of the total intensity) amount of the fluorine signal to be due to F-Mg(n) (fluorine with an unknown number of exclusively Mg²⁺ nearest neighbors) bonding in the Mg-aluminosilicates. By combining this with previous data on Ca- and Ba-aluminosilicate glasses, we demonstrated a clear trend of an increasing amount of F-M(n) type bonding with increasing field strength of the network modifying cation. This indicates that the higher field strength cations can more effectively compete with Al³⁺ for fluorine bonds. The mixed-modifier (Na,La)-silicate glasses have primarily F-M(n) type bonding, with a pronounced preference for bonding to the higher field strength La³⁺ over Na⁺. In addition, Si-F bonding was found in the (Na,La)-silicate glasses at a level (∼2% of the total intensity) comparable to that found in other silicate glasses, suggesting that Si-F bonding is consistently present in silicate glasses. The (Na,La)-silicate glasses also had unusually short spin-spin relaxation times, suggesting short (similar to crystalline fluorides) fluorine-fluorine distances in environments associated with La³⁺.