Tungsten Oxides Colours (I)

Details: Category: Tungsten Information; Published on Monday, 02 February 2015 18:22; Written by cq; Hits: 1623

There are several tungsten oxides, but only four of them are of major importance. They all have attractive colours: the yellow WO3, the dark blue W20O58 (WO2.9), the violet W18O49 (W2.72) and the chocolate brown WO2. So-called higher tungsten oxides are structurally related to both the WO3 and W20O58 structures and exhibit chemical formulas between WO2.99 and WO2.889. They belong to the group of non-stoichiometric tungsten oxides, and are of interest mainly to structural chemists. They exhibit colours from yellow to green to very dark blue. The colour changes are a result of a slight loss of oxygen which generates an additional valence state in the WO3 parent structure, either W5+ or W4+. Cation-to-cation charge transfer between the parent W6+ and a reduced ion is responsible for the change in colour.

Tungsten oxides are important starting materials for the production of tungsten metal powder by hydrogen reduction. These oxides are formed out of ammonium paratungstate by a thermal decomposition process called “calcination” and are depending on the decomposition conditions (more or less reducing), a mixture of different higher tungsten oxides, including amorphous oxide phases. As described before, they exhibit colours between yellow and dark blue (TBO,“Tungsten Blue Oxide”).

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