Tungsten bronze cubic structure

The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

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. 2.90 Structure of cubic tungsten bronze (CTB) projected on (001). Metal sites (open circles) are partially and randomly occupied.

A clear crystallochemical interpretation of hardness anisotropy measurement results, especially for monocrystals, makes it possible to estimate the structural homogeneity of crystal. Button et al. (1979) testing the hardness of cubic sodium tungsten bronzes Na W03 (where 0.4 < x < 0.75) with the Knoop indenter, found the hardness of W03 to rise from 450 to 844 within a highly differentiated hardness anisotropy for various values of the Na+ ion. This variation is the outcome of differences in atomic spacings in crystals. 

Introduction of sodium into empty 12-coordinate sites of the WO3 lattice results in a series of oxide bronzes. These have general formula NaxWOa, where 0.0sodium tungsten bronzes retain the monoclinic structure of the parent WO3 and they are n-type semiconductors [276,277]. With increasing Na content the structure evolves through two distinct tetragonal phases and for x>0.43 the bronzes adopt an essentially cubic perovskite structure [278] closely related to that of ReOs (fig. 18). The Na 3s levels lie about 10 eV above the bottom of the W 5d bands and each added Na atom is therefore ionised to Na, with donation of one electron into the W 5d band of local t2g symmetry [279]. For x values of less than 0.26, the 5d electrons are localised, probably by an interplay between polaronic effects, disorder... 

When the B atom sites of perovskites are occupied by atoms of different valences (as in BajUOg) the structure is distorted but with retention of cubic symmetry here the oxygens are closer to U than to Ba. The tungsten bronzes, Na WOj, also crystallize in the perovskite structure when 0.32 < x < 0.93 Na MoOj- and K Mo03-type systems (where 0.9 < x < 0.97) are also perovskites. In these bronzes not all the A sites are fliled. 

FIG. 13.12. Projections of the structures of tungsten bronzes (a) perovskite structure of cubic bronzes, (b) the same showing only alkali-metal ions (shaded circles) and W atoms (small circles), (c) and (d) the W frameworks and alkali-metal ions in the tetragonal and hexagonal bronzes. 

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... 

In hydrogen tungsten bronzes x values between 0.03 and 0.53 and y values from 0.015 to 0.3 can be found. They have quite different structures (tetragonal, orthorhombic, monocline, hexagonal, and cubic) and can also be detected in tungsten blue oxides. 

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