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Stoichiometric compounds such as W20O58, W18O49 or WO2 are formed intermediately during the reduction process and can be isolated by interrupting the reduction process at the selected reduction state.
All , characteristic crystal symmetries, and also peculiar crystal forms. They are commonly not visible to the naked eye, as the powder particles are too small for a simple visual detection.
However, they can be made visible by looking at the powders with the help of an electron microscope where distinct differences of the crystal forms can be clearly recognised.
Colours of the tungsten oxides, prepared by interrupting the reduction process at selected reduction stages: yellow WO3, blue W20O58, violet W18O49, chocolate-brown WO2, and grey W metal.