Tetragonal tungsten bronze structure

S.Fabbrici, E.Montanaii, G.Calestani and A. Migliori Charge ordering and tilt modulation in multiferroic KxM"xM "i xF3 (0.4transition metal fluorides with tetragonal tungsten bronze structure Chem. Mater. 2004 16(16) 3007-3019... 

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 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.

FIGURE 4 12 Tetragonal tungsten bronze structure the circles represent possible metal atom sites and the shaded squares WOs octahedra [4.30]. 

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. 

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Fig. 40. Structure of MxMe02+xF/.x type compounds — tetragonal tungsten bronze- type structure.

Finally, a number of other mixed oxides that do not have the perovskite structure have also been examined. For example, niobium titanates with the rutile structure,tetragonal tungsten bronze... 

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

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). 

The preparation under high pressure (65 kbar) of Mo bronzes with tungsten bronze structures has been described for example, cubic Na (Mo03 and K MoOs (x 0-9) and tetragonal K MoOs (x The Mo bronzes prepared by the electrolysis... 

---