Tungsten bronzes tunnel structures

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) structure in which the large Cs atoms reside in hexagonal tunnels formed by six WOg octahedra in 12-fold coordination. The limit of x, as determined by the structure is 1/3. The structure 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. 

Figure 13 Projections of (a) tetragonal(II), and (b) hexagonal "tungsten bronze" structure. A polyhedral representation showing the large pentagonal and hexagonal tunnels, respectively. From Reference 55.

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.

Fig. 2.94 Structure model deduced from Fig. 2.93. The arrows I show the rows of tunnels of hexagonal prisms. The structure can be regarded as the intergrowth of HTB and DRO, which was named intergrowth tungsten bronze (1TB) by Kihlborg and co-workers.

The final example of a three-dimensional tunnel structure is that of hexagonal Alkali metal tungsten bronzes... 

FIGURE 4.14. Intergrowth tungsten bronze structure of the pyrochlore type (single tunnel) [4.30],... 

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