Hexagonal tungsten bronze structure

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. 

Known oxyfluoroniobates (-tantalates) with compositions that correspond to X Me = 3 crystallize in typical structures of ReC>3, pyrochlore, and hexagonal and tetragonal tungsten bronze. Table 35 presents structural parameters of such compounds. 

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 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. 

Framework (skeleton) structures of oxides have been identified for fast ion conduction of Na" and other ions (Goodenough et al., 1976). One-, two- or three-dimensional space is interconnected by large bottlenecks in these oxide hosts. While the tungsten bronze and j8-alumina structures contain one- and two-dimensional interstitial space, the hexagonal framework of NaZr2(P04)3 has a three-dimensional... 

A second example of the intergrowth structure is tungsten bronze, M,. WO3 (M = H, Li, Na, K, Rb, Cs, Ca, Sr, Ba, In, Tl, Ge, Sn, Pb, Cu, Ag etc. nowadays many bronzes such as Ti, V, Mo are well known ), so named because its colour is similar to that of alloy bronze CuZn. It has been shown to have a kind of intergrowth structure in a limited composition (x) range. Generally, the stability of the structural types (hexagonal, tetragonal, and cubic) depends both on the ionic radius (rj and the composition (x) of M. For instance,... 

Figure 2.90 shows the structure of the cubic M, WO3 (CTB, cubic tungsten bronze). The centre holes are partially and randomly occupied by M. This structure is the same as cubic BaTiO3, apart from the partial occupation of M sites. The structure of the hexagonal M WOj (HTB, hexagonal tungsten... 

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.

M = V. Cr. Fe. Ga. Ti. Nb. or W X = F or F + O x = 0.35—0.55 0 < y < 1) have been obtained by heating mixtures of the corresponding fluorides and oxides. These materials all crystallize with the hexagonal tungsten bronze structure and their lattice constants have been determined.98... 

A good deal of work is still required to clarify the tetragonal and hexagonal tungsten bronze structures for which only the metal positions are known (32, 34). Superstructures of both have been observed, but little is known about them except in the case of the hexagonal phase at the composition BOs, where they are due to the ordering of molybdenum, an impurity substituent for tungsten (26). 

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