✍ Scribed by Lin-Shu Du; Francis Wang; Clare P. Grey
No coin nor oath required. For personal study only.
has been studied with very fast magic angle spinning 19 F NMR. 19 F MAS NMR of Ba 2 WO 3 F 4 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°C, and at 227°C at (2 kbar.) In contrast, fluoride ions in the isostructural compound Ba 2 MoO 3 F 4 are almost completely ordered both between and on the W-O/F chains. Two-dimensional NMR is, however, used to demonstrate that a weak 19 F resonance corresponding to +0.35% of the total fluoride-ion content is not due to a BaF 2 impurity but that it results from a small amount of disorder in the tungsten chains. The fluoride ions order on one site in NaMoO 3 F and K 2 NbO 3 F, consistent with earlier studies. The 19 F and 19 F-113 Cd REDOR NMR of CdWO 3 F 2 show that the fluoride ions are ordered on two anion sites, each equidistant from a cadmium ion, in contrast to the isostructural compound CuWO 3 F 2 where ordering on only one anion site has been proposed. A new model for the structure of CdWO 3 F 2 is proposed.
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