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 F-19 MAS spectrum of chiolite (Na5Al3F14) 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 F-19 MAS NMR spectrum of the isostructural compound Na5W3O9F5. Thus, a combination of F-19 MAS, F-19 --> Na-23 CP, and Na-23 --> F-19 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.