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The presence of tungsten oxide has many forms, mainly can be classified into four species:  yellow tungsten oxide, blue tungsten oxide, violet tungsten oxide, and brown tungsten oxide.

Molecular formula as follow:

Yellow tungsten oxide (WO₃）

Blue tungsten oxide（WO_2.9）

Violet tungsten oxide（WO_2.72）

Brown tungsten oxide（WO₂）

Different oxides have different colors.

In the system of W/O, there exist four stable oxides such as yellow tungsten oxide, blue tungsten oxide, violet tungsten oxide, and brown tungsten oxide.

Yellow tungsten oxide (WO₃）has the appearance of yellow crystal power. The density is about 7.2~7.4g/cm³; the melting point reaches 1470℃;and the boiling point is between 1700℃ and 2000℃. When the temperature is higher than 800℃,the phenomenon of sublimation will happen.

According to the different size of the particles, it represents two different colors.

When the crystal size of yellow tungsten oxide is about 1.5um. It shows yellow; and when the crystal size of yellow tungsten oxide is about 15um, it shows green. Although the color presents differently, the properties are still similar.

Blue tungsten oxide which is short of BTO is a compound of tungsten (VI) and tungsten (V) mixed valence state showing blue crystal powder. Generally speaking, it forms in solution. In addition, blue tungsten oxide is also the result of reducing agent and tungsten compound.

Violet tungsten oxide （WO_2.9）is abbreviated as VTO and the appearance of is violet or blue violet powder. The crystal has octahedral structure looks like rod and needle. The violet tungsten oxide is produced under the precise temperature and reduction environment. It mainly uses the APT as a raw material to produce tungsten carbide and tungsten metal powder.

The appearance of brown tungsten oxide (WO₂) is brown powder; the density is 10.82g/cm³; melting point is between 1500℃ and 1600℃; the boiling point is 1730℃; it can’t dissolve in water, alkali solution, hydrochloric acid and dilute sulfuric acid. The brown tungsten oxide easily disproportionate in an inactive gas and generates tungsten and tungsten trioxide. However, it is very stable in the 40% ~ 55% hydrogen environment and at 45%~60% water vapor. As usual, tungsten oxide is often produced under the temperature between 575℃ and 600℃.