Fluorine in Complex Alumino-Boro-Silicate Glasses: Insight into Chemical Environment and Structure

Fluorine incorporation into silicate glasses is important for technical fields as diverse as geophysics, extractive metallurgy, reconstructive dentistry, optical devices, and radioactive waste management. In this study, we explored the structural role of fluorine in alkaline alumino-borosilicate glass, with increasing amounts of fluorine up to 25 mol % F while maintaining the glass composition. Glasses were characterized by X-ray diffraction (XRD), ²⁷Al and ¹⁹F magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and electron probe microanalysis. Results showed that essentially all F was retained; however, between 12 and 15 mol % F (∼3.6 and 4.5 wt % F), excess fluorine partitions to CaF₂ and then NaF and Na-Al-F crystalline phases. Even prior to crystallization, there exist five distinct F sites, three of which evolve into crystalline phases. The two persistent glassy sites likely involve ^[4]Al-F-Ca/Na local structures. We propose a general understanding of the expected chemical shift of ¹⁹F NMR in systems containing Al, Ca, and Na.