Bronze tungsten

Tungsten Bronze. Tungsten bron2es (30,31) constitute a series of well-defined non stoichiometric compounds of the general formula WO, where x is a variable between 0 and 1, and M is some other metal, generally an alkaU metal, although many other metals can also be substituted. 

Simple oxotungstates (M2WO4, M = Li, Na, K) may be reduced with H2 at high temperature to give unreactive and intensely colored substances that look like bronze. Tungsten bronzes (M tW03, 0 < X < 1) have semimetallic properties and good electrical conductivity forx >0.3 and they are semiconductors for x < 0.3. 

Diphosphate tungsten bronzes (DTB) Cs Hg (P 04)4(W03)gai e chai ge-density wave (CDW) conductors. The framework of Cs contains columns of corner-sharing WO octahedra internked through P O group. Properties of Cs can be tuned via out-of-... 

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

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... 

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. 

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]. 

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

LiNbC>3 type crystals and compounds that crystallize in a tungsten bronze-type structure. 

The second group is the group of oxyfluorides that are derived from ferroelectric oxides by means of fluorine-oxygen substitution. The basic oxides are usually perovskite, tetragonal tungsten bronze, pyrochlore, lithium tantalate etc. [400]. 

Abbreviations CC=cluster connectivity Of=overall framework HTB=hexagonal tungsten bronze HLF = Honeycomb-like layered framework [Tl5(Tl2Cl5)][(Nb6Cli204)3(Ti3CU)2]. 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

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