Hexagonal tungsten bronze form

Suspensions of R-NiFs at 20 °C slowly evolve F2 over several days to give yellow-brown NiF2. This loss of F2 in aHF at 20 °C may be the cause of the formation of a hexagonal tungsten bronze form of the trifluoride (H-NiFt) when the tetrafluoride is decomposed, in aHF, at 20 °C rather than at 0 °C ... 

The compound K0 3NbF3 has an average niobium valency of 2.7 and forms a crystal structure that is referred to as hexagonal tungsten bronze [239]. 

FIGURE 5.35 (a) The tetragonal tungsten bronze structure (b) the hexagonal tungsten bronze structure. The shaded squares represent WOe octahedra, which are linked to form pentagonal, square and hexagonal tunnels. These are able to contain a variable population of metal atoms, shown as open circles. 

Fig. 2.91 Structure of hexagonal tungsten bronze (HTB) M WOs. A group of six WOg forms tunnels of hexagonal prisms by sharing corners. This structure is composed of the structural elements A ( = B) and E, as shown by the arrows. Metal sites (open circles) are randomly and partially occupied.

Alkali tungsten bronzes of the type M WOs, M = K, Rb, or Cs with 0.16 < x 0.3 may adopt the hexagonal tungsten bronze (HTB) stmcture in which the large Cs atoms reside in hexagonal tunnels formed by six WOe octahedra in 12-fold coordination. The limit of x, as determined by the stmcture is 1/3. The stmcture is of interest since these compounds display superconductivity with values between 1 and 7K. It is also found for several mixed oxides of Nb and Ta with large A cations. 

A homologous series of cesium tungsten oxides having the formula Cs2 W4 iOi2 has been reported. The general stmcture consists of slabs composed of layers of the hexagonal tungsten bronze and the pyrochlore type. The alkali cations reside in between the layers and in the tunnels formed within the slabs. ... 

Because Nip4 (under aHF) loses 1/2 F2 above -60°C it remains poorly characterized but the trifluoride has been prepared in three forms approximately hexagonal-close-packed rhombohedral (Ji-NiFj), hexagonal tungsten bronze (//-NiFs) and pyrochlore (P-NiFs) the last two having small content (and corresponding Ni(II) incorporation). The NiFs forms can be kept indefinitely at 20°C in a dry atmosphere but all lose fluorine slowly in aHF at ambient temperature. 

In hexagonal tungsten bronzes comer-shared WOg octahedra are arranged to form hexagonal tunnels (Fig. 4.13.). The metal atoms are interpolated into the tunnels. Their composition range varies between x=0.15 to 0.33. They form when larger metal atoms are reacted with WO3 at high temperature. 

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