19F/23Na Double Resonance MAS NMR Study of Oxygen/Fluorine Ordering in the Oxyfluoride Na5W3O9F5

Three resonances corresponding to the three crystallographically distinct fluorine sites were resolved in the high-resolution 19 F MAS spectrum of chiolite (Na 5 Al 3 F 14 ) collected at a field strength of 19.6 T and at a spinning speed of 40 kHz. In contrast, only one broad resonance is observed in the 19 F MAS NMR spectrum of the isostructural compound Na 5 W 3 O 9 F 5 . Thus, a combination of 19 F MAS, 19 F → 23 Na CP, and 23 Na → 19 F HETCOR NMR experiments was required to resolve the resonances from the different local environments in this compound. Fluorine was shown to preferentially occupy two of the three anion sites.