The tetragonal bronze structure

The tetragonal bronze structure, showing the three kinds of tunnel. 

We deal in more detail with this structure because it forms the basis of the structures of three groups of compounds, namely  

In the tetragonal bronze structure there are tunnels of three kinds (Fig. 13.13), of which only two S and F) are occupied in the bronzes (class (i)). The environment of an atom in a tunnel depends on its height relative to the atoms in the framework. In compounds of classes (i) and (ii) the tunnel atom is at height when the coordination groups of such atoms are  

T tricapped trigonal prism (9-coordination), site suitable only for very small ion 

In class (iii) a new principle is introduced. A metal atom is placed in a F tunnel, assumed now to have approximately regular pentagonal cross-section, at the height 0 (see Fig. 13.13) so that it is surrounded by a ring of five 0 atoms at the same height. An additional 0 atom is introduced with each metal atom in the tunnel, the 

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TABLE 13.17 Compounds with structures related to the tetragonal bronze structure Ferroelectric class (ii)) ... 

P. Lacorre, J. Pannetier, G. Ferey, The magnetic structure of the tetragonal bronze KMnFeFo J. Magn. Magn. Mat. 1991, 94, 331... 

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. 

Figure 1.16 Structure of the tetragonal tungsten bronze (TTB), K

Figure 9.1 Tetragonal tungsten-bronze structure of SiiBai-xNboOf, (Strontium barium nio-bate, SBN). Characteristic are the corner-linked niobium-oxygen-octahedra and 3 types of vacancies A,Bi,2,3 and C. Sr2+-ions are embedded in the A- and B2-positions and Ba2+ in the Bi-positions according to their ionic radii. The C-positions and B3-position remain empty.

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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