HIGH-RESOLUTION F-19 MAS AND F-19-CD-113 REDOR NMR-STUDY OF OXYGEN FLUORINE ORDERING IN OXYFLUORIDES/

The ordering of fluoride ions on the anion sites in the oxyfluorides B a2WO3F4, Ba2MoO3F4, CdWO3F2, NaMoO3F, and K2NbO3F has been studied wit h very fast magic angle spinning F-19 NMR. F-19 MAS NMR of Ba2WO3F4 sh ows that the fluoride ions are disordered within the four equatorial a nion positions on the W-O/F chains, but that the anion positions betwe en the chains are fully occupied by fluorine. No difference in fluorid e-ion ordering is observed between samples synthesized under a wide va riety of conditions (e.g., hydrothermally at 3 kbar at 800 degrees C, and at 227 degrees C at < 2 kbar.) In contrast, fluoride ions in the i sostructural compound Ba2MoO3F4 are almost completely ordered both bet ween and on the W-O/F chains. Two-dimensional NMR is, however, used to demonstrate that a weak F-19 resonance corresponding to approximate t o 0.35% of the total fluoride-ion content is not due to a BaF2 impurit y but that it results from a small amount of disorder in the tungsten chains. The fluoride ions order on one site in NaMoO3F and K2NbO3F, co nsistent with earlier studies. The F-19 and F-19-Cd-113 REDOR NMR of C dWO3F2 show that the fluoride ions are ordered on two anion sites, eac h equidistant from a cadmium ion, in contrast to the isostructural com pound CuWO3F2 where ordering on only one anion site has been proposed. A new model for the structure of CdWO3F2 is proposed. (C) 1998 Academ ic Press.