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hermochromism

hermochromism is the property of substances to changecolour due to a change in temperature and WO3is an attractive example for this property. On cooling the oxide by liquid nitrogen down to -196°C, a sudden change occurs from yellow to white, which then alters to a bluish-white colour between -50 and -27°C. At room temperature it becomes pale lemon-yellow again. On furtherheating to 200 – 300°C, WO3becomes dark yellow, changingto a deep orange colour at 400 to 500°C . These reversible colour changes are linked to changes in the electronic properties of WO3 which alter due to changesof the internal symmetry of the WO3crystals (through alterations of the lattice arrangements).

Preparation

Yellow tungsten trioxide is a finely divided, yellow, crystalline powder. It is produced by roasting ammonium paratungstate at closely controlled temperatures to drive off combined water and ammonia.

Hazard

Toxic material.

Flammability and Explosibility

Notclassified

Safety Profile

Moderately toxic by ingestion. Can react violently with ClF3, Li, Cl2. See also TUNGSTEN COMPOUNDS.

Physical properties

Heavy yellow powder; turns dark orange on heating; reverts back to yellow on cooling; density 7.2 g/cm3; melts at 1,472°C; insoluble in water; slightly soluble in acids; soluble in caustic alkalies.

General Description

Tungsten(VI) oxide (WO3) is a semiconducting material with excellent electrochromic properties. It is a cost efficient material with a high optical modulation. It can be synthesized by a variety of methods which include chemical vapor deposition, electrodeposition, and other sol-gel based coating processes.