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 using F-19, Al-27, and P-31 solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. For the Al-free series of glasses, the F-19 NMR spectra are dominated by peaks near +90 ppm, representative of F terminating P-chains. The formation of these bonds has little effect on the P-31 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, Al-27 NMR detects only octahedral Al-sites, the F-19 NMR spectra include a second peak near -12 ppm due to F bonded to Al, and the P-31 NMR spectra contain signals due to Q1-sites with one or more Al next-nearest neighbors. The relative intensity of the two F-19 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.