Hydrated Tungsten Oxide

Research on hydrated tungsten oxide dates to the 1940s. Up to now, there are four types of hydrated tungsten oxides that are commonly found in the world at present depending on the type of water they contain: WO₃ · 2H₂O, WO₃ · H₂O, WO₃ · 0.5H₂O and WO₃ · 0.33H₂O. Due to the different crystalline water, there is a clear difference between the crystalline structures of the different hydrous tungsten oxides. Tungsten dioxide dihydrate has a layered structure which, in common with monoclinic tungsten oxide, has the structural unit consisting of tungsten oxide octahedra.

The difference is that one oxygen atom located at the apex of the WO₃ · 2H₂O structural unit is replaced by a water molecule while the other water molecule is located between the layers. There is only van der Waals forces and hydrogen bonding interactions between layers. The existence of interlayer water not only increases the interlayer spacing, facilitates the stripping, and helps ion insertion and extraction, and has excellent electrochromic properties.

The crystal structure of monohydrate tungsten oxide is like that of dihydrate tungsten oxide with the only difference being the absence of interlayer water. Hemihydrate tungsten oxide is the least studied type of hydrated tungsten oxide. Its crystal is cubic pyrochlore structure, composed of a total of tungsten oxide, water molecules exist in a total of tungsten oxide channel structure. The study found that WO₃ 0.33H₂O contains two co-vertex tungsten oxide octahedral structure. The first type is that the central tungsten atom is surrounded by six oxygen atoms. The second type is where two oxygen atoms are replaced by a shorter terminal tungsten-oxygen bond and a longer tungsten-hydroxyl bond. The WO₃ · 0.33H₂O crystal structure is formed by stacking these two types on top of each other.

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