Colours tungsten bronzes

Tungsten bronzes can be prepared by a variety of reductive techniques but probably the most general method consists of heating the normal tungstate with tungsten metal. They are extremely inert chemically, being resistant both to alkalis and to acids, even when hot and concentrated. Their colours depend in the proportion of M and W present. In the case of sodium... 

The gold paint with which lamp-posts and other ornamental metalwork are decorated contains no gold, because of its prohibitive expense. The reflective gold-coloured substitute is sodium tungsten bronze, Nao.3W03. So why is NaojWOs reflective ... 

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,... 

A remarkable series of reduction products is obtained from the tungstates of the alkali and alkaline earth metals. These have the empirical formula R 20.(W03). W02, and are known as tungsten bronzes, since they are vividly coloured and usually possess a bronze-like superficial lustre (see p. 248). 

A still more extreme example shows that the perovskite structure can even occur with some of the A sites unoccupied. Sodium tungsten bronze has the ideal composition NaWOs, with the perovskite structure, but this compound shows very variable composition and colour, and is better represented by the formula Na WOg with 1 > x > 0. In the sodium-poor varieties the structure remains essentially unaltered but some of the sites normally occupied by sodium are vacant. To preserve neutrality one tungsten ion is converted from W5 to W6+ for every site so unoccupied, and this change in ionization gives rise to the characteristic alteration in colour and explains its association with the sodium content. In the extreme case, when no sodium is present, we have W03, the structure of which is closely related to that of A1F3. We have already shown how this structure, in its turn, is related to that of perovskite. 

L1XV2O5 is a non-stoichiometric compound (x 1) and acts as a Li atom store. When a small potential is applied across the cell, Li ions migrate from the lithium polymer electrolyte into the WO3 layer forming a tungsten bronze (see equation 22.42 and discussion). Its formation results in a colour change from colourless to blue. 

Tungsten bronzes are inert materials M WOy (0 < x < 1) with defect perovskite structures Figure 5.23). Their colour depends on x golden for x a 0.9, red for x 0.6, violet for x a 0.3. Bronzes with x > 0.25 exhibit metallic conductivity owing to a band-like structure associated with W(V) and W(VI) centres in the lattice those with x < 0.25 are semiconductors (see Section 5.8). Similar compounds are formed by Mo, Ti and V. ... 

Protonic conduction is verified if hydrogen is evolved at the cathode in an electrolysis experiment with a hydrogen source at the anode. This has been used very elegantly by Kreuer, who used the blue colouring of tungsten bronzes when hydrogen is intercalated as a proof of proton conduction in lithium hydrazinium sulphate, HUAs and HUP. Several other workers have used the formation of hydrogen as a proof of proton conduction (Murphy (1964), Takahashi et al. (1976,1979) Chandra (1985)22 Iwahara et al. (1986) ). 

Perhaps the most fascinating exemplar of hydrogen spillover, and certainly the most visually convincing is the process whereby it reacts with certain oxides to be incorporated in their structures without the elimination of water. The products, which are known collectively as hydrogen bronzes, are formed especially by the oxides of tungsten, molybdenum, vanadium and rhenium (see Further Reading section). The highly coloured products (W blue Mo violet) contain ions of lower oxidation state (e.g. W", Mo", V ") formed for example as... 

Cadmium sulfides and selenides group Tin sulfides group Tungsten group Berndtite Cassiterite Chromium tin oxide Cobalt tin oxide Copper tin oxide Herzenbergite Lead antimony tin oxide Lead tin oxide Lead tin silicon oxide Ottemannite Romarchite Tin chromate Tin(IV) oxide Tin(IV) sulfide Bronze colours-, Egyptian blue Lead tin yellow Mosaic gold Mosaic silver, Tin white... 

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