A multinuclear MAS NMR study of the short-range structure of fluorophosphate glass
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R. J. Kirkpatrick Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 15 October 1991; accepted 25 March 1992)
We have examined the bonding arrangements in Na-P-O-F and Na-Al-P-O-F glasses using 19 F, 27A1, and 31 P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. For the Al-free series of glasses, the 19F 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 31 P 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 fiuorophosphate glass. For the Na-Al-P-O-F glasses, 27A1 NMR detects only octahedral Al-sites, the 19F NMR spectra include a second peak near —12 ppm due to F bonded to Al, and the 31 P NMR spectra contain signals due to Q1-sites with one or more Al next-nearest neighbors. The relative intensity of the two 19 F 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 NH 4 HF 2 .
I. INTRODUCTION Substantial amounts of fluorine (>25 at. %) can be incorporated into alkali and alkaline earth phosphate glasses by remelting the base compositions in NH4HF2.1 Such treatments significantly reduce the glass transition temperatures and increase the thermal expansion coefficients,1'2 changes that suggest that F weakens the phosphate glass structure by replacing bridging oxygens with terminating P-F bonds. Raman spectroscopic studies of fluorinated-NaPO3 glasses support this structural depolymerization model,3 as does our recent x-ray photoelectron spectroscopic (XPS) study,4 which quantitatively measured the decrease in bridging-to-nonbridging oxygen ratios in F-substituted NaPC>3. The effect of fluorination on the structure of aluminophosphate glass is more complex. XPS analyses indicate that F-Al, as well as F-P, species are present in relative concentrations that are dependent on both the cation ratios and the total F-content.4'5 For example, F-Al sites are predominant in aluminophosphate base glasses with low F-contents, and the proportion of F-P bonds significantly increases after remelting the base glass in NH 4 HF 2 . Raman spectra of XA1F 3 • (1 — x) NaPO 3 glasses fluorinated by reaction with NH 4 HF 2 clearly show the presence of P-F bonds.3'4 Band assignments to Al-polyhedra, however, are more ambiguous because of overlapping vibrations from phosphate, a
'Current address: Arizona Materials Lab, Tucson, Arizona.
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J. Mater. Res., Vol. 7, No. 7, Jul 1992 Downloaded: 13 Mar 2015
aluminophosphate, and 4- and 6-coordinated aluminofluoride polyhedra. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy resolves some of these ambiguities. NMR chemical shifts are sensitive to different polyhedral arrangements in solids, including glasses, that pr
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