Hexagonal Tungsten Trioxide

Tungsten Trioxide (WO₃) has different morphologies and structures at different temperatures. WO₃ has at least five structural phase transitions between -40 and 740 degrees Celsius, and the hexagonal phase is a stable morphological structure between 200 and 400 degrees Celsius.

The crystal form of hexagonal tungsten trioxide is distorted cubic ReO3 structure, which is also called layered structure. The crystal structure of tungsten trioxide is a distorted anorthite structure, that is, the absence of cations appears in the formal structure. However, the hexagonal tungsten trioxide crystal structure is produced by the absence of A cation in perovskite structure (ABO₃ type) under mild conditions. The vertex connection of each octahedron will form a six-membered ring, and the adjacent six-membered rings can form a triangle with each other, which will form a one-dimensional three-way channel (the growth direction of the hexagonal tungsten trioxide layer is the crystal axis direction).

Hexagonal Tungsten Trioxide exists stably in the range of 200 to 400 degrees Celsius. Temperature will affect the formation of the crystal structure of Tungsten Trioxide. When the calcination temperature is 200 degrees Celsius, the product is orthorhombic Tungsten Trioxide. When the temperature rises to 300 degrees Celsius, the characteristic peak of hexagonal Tungsten Trioxide begins to appear. When the temperature reaches 450 degrees Celsius, the characteristic peak of hexagonal Tungsten Trioxide disappears completely, and it becomes monoclinic.

The production of hexagonal Tungsten Trioxide roughly comprises the following five steps.

1. Dissolve 3.68 grams of sodium tungstate in 20 ml of deionized water;

2. 3 mol/L hydrochloric acid is slowly added dropwise while stirring until the pH = 1; the value of the solution equals 1;

3. Stir until yellow precipitate is no longer generated, and add a proper amount of additive potassium oxalate or potassium sulfate (K2C2O4 or K2SO4);

4. Stir for 30 min, transfer the solution to a 100 ml stainless steel reaction kettle with PTFE liner, and add deionized water to 3/4 of the volume of the reaction kettle;

5. After sealing, react at 150 degrees Celsius for 12 hours, naturally cool to room temperature, centrifugally wash with deionized water and absolute ethanol   many times, and dry at 60 degrees Celsius to obtain the final product hexagonal Tungsten Trioxide.

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