Raman scattering and group-theoretical studies of the paraelectric-ferroelectric phase transition in the chiolite-type-crystal Na₅W₃O₉F₅

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

“…22 Sodium mobility in chiolite was shown to be limited to within the layers of edge-sharing NaF 6 octahedra (Na2 in Figure 1), and no exchange between the Na1 and Na2 was observed. No significant change of the 19 F spectra was seen, up to 150 °C, indicating the lack of any oscillatory motions of the AlF 6 octahedra in chiolite in this temperature range.…”

“…[1][2][3][4] For example, we have used 125 Te MAS NMR to distinguish between disorder originating from stacking faults (one-dimensional disorder) or from substitution (three-dimensional disorder), in a series of layered tellurates. 5,6 We have also applied fast 19 F MAS NMR to study disorder in crystalline fluorides, systems in which the disorder of the fluorine ions plays an important role in controlling ionic conductivity. 7,8 Oxygen/fluorine ordering is very difficult to determine from both X-ray and neutron diffraction experiments, due to the similar scattering factors of O and F in both experiments.…”

“…12 In a previous paper we showed that the different local environments for fluorine could be resolved in a series of molybdenum, tungsten, and niobium oxyfluorides, allowing the extent of O/F ordering to be determined. 13 In this paper, very fast 19 F MAS NMR is used to study O/F ordering in a tungsten oxyfluoride, Na 5 W 3 O 9 F 5 .…”

“…1996, 100, 1539-1549. 27 Al, 23 Na, and 19 F solid-state NMR have been used to study the sodium fluoroaluminates, chiolite, and cryolite (Na 3 AlF 6 ). [20][21][22] The 23 Na MAS spectra of chiolite were originally reported by Stebbins et al 20 Two overlapping resonances were observed at room temperature with intensity ratios 1:2.…”

“…21 The chemical shifts of the 27 Al octahedra were found to be controlled by the number of bridging Al-F-Al atoms, a shift to lower frequency being observed as the number of bridging fluorine atoms increased. The variable-temperature 27 Al, 23 Na, and 19 F static NMR spectra were also collected in order to probe the cation diffusion and the nature and causes of the phase transitions in both cryolite and chiolite. 22 Sodium mobility in chiolite was shown to be limited to within the layers of edge-sharing NaF 6 octahedra (Na2 in Figure 1), and no exchange between the Na1 and Na2 was observed.…”