A multinuclear MAS NMR study of the short-range structure of fluorophosphate glass

We have examined the bonding arrangements in Na–P–O–F and Na–Al–P–O–F glasses using¹⁹F,²⁷Al, and³¹P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. For the Al-free series of glasses, the¹⁹F NMR spectra are dominated by peaks near +90 ppm, representative of F terminating P-chains. The formation of these bonds has little effect on the³¹P chemical shifts, indicating that F preferentially replaces bridging oxygen on the phosphate tetrahedra, consistent with previous NMR studies of crystalline fluorophosphates and other spectroscopic studies of fluorophosphate glass. For the Na–Al–P–O–F glasses,²⁷Al NMR detects only octahedral Al-sites, the¹⁹F NMR spectra include a second peak near −12 ppm due to F bonded to Al, and the³¹P NMR spectra contain signals due to Q¹-sites with one or more Al next-nearest neighbors. The relative intensity of the two¹⁹F peaks correlates well with previous spectroscopic studies and shows that a greater fraction of F–P bonds forms when the base glass is remelted in NH₄HF₂.