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

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Abstract
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.