High-resolution 19F MAS and 19F-113Cd REDOR NMR study of oxygen/fluorine ordering in oxyfluorides

The ordering of fluoride ions on the anion sites in the oxyfluorides Ba2WO3F4, Ba2MoO3F4, CdWO3F2, NaMoO3F, and K2NbO3F has been studied with very fast magic angle spinning F-19 NMR. F-19 MAS NMR of Ba2WO3F4 shows that the fluoride ions are disordered within the four equatorial anion positions on the W-O/F chains, but that the anion positions between the chains are fully occupied by fluorine. No difference in fluoride-ion ordering is observed between samples synthesized under a wide variety 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 isostructural compound Ba2MoO3F4 are almost completely ordered both between and on the W-O/F chains. Two-dimensional NMR is, however, used to demonstrate that a weak F-19 resonance corresponding to approximate to 0.35% of the total fluoride-ion content is not due to a BaF2 impurity 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, consistent with earlier studies. The F-19 and F-19-Cd-113 REDOR NMR of CdWO3F2 show that the fluoride ions are ordered on two anion sites, each equidistant from a cadmium ion, in contrast to the isostructural compound CuWO3F2 where ordering on only one anion site has been proposed. A new model for the structure of CdWO3F2 is proposed. (C) 1998 Academic Press.