# Tungsten trioxide

Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO$_3$. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid H$_2$WO$_4$. It is a light yellow crystalline solid.
Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO$_3$·H$_2$O, meymacite WO$_3$·2H$_2$O and hydrotungstite (of the same composition as meymacite, however sometimes written as H$_2$WO$_4$). These minerals are rare to very rare secondary tungsten minerals.

## History

In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate. He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.

## Structure and properties

The crystal structure of tungsten trioxide is temperature dependent. It is tetragonal at temperatures above 740 °C, orthorhombic from 330 to 740 °C, monoclinic from 17 to 330 °C, triclinic from −50 to 17 °C, and monoclinic again at temperatures below −50 °C. The most common structure of WO$_3$ is monoclinic with space group P2$_1$/n.
The pure compound is an electric insulator, but oxygen-deficient varieties, such as WO$_{2.90}$ = W$_{20}$O$_{58}$, are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and the dioxide WO$_2$ at 1000 °C in vacuum.
Possible signs of superconductivity with critical temperatures T$_c$ = 80–90 K were claimed in sodium-doped and oxygen-deficient WO$_3$ crystals. If confirmed, these would be the first superconducting materials containing no copper, with T$_c$ higher than the boiling point of liquid nitrogen at normal pressure.

## Preparation
### Industrial

Tungsten trioxide is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores can be treated with alkalis to produce soluble tungstates. Alternatively, CaWO$_4$, or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to WO$_3$ and water at high temperatures.
    CaWO$_4$ + 2 HCl → CaCl$_2$ + H$_2$WO$_4$
    H$_2$WO$_4$ → H$_2$O + WO$_3$

### Laboratory

Another common way to synthesize WO$_3$ is by calcination of ammonium paratungstate (APT) under oxidizing conditions:
    (NH$_4$)$_{10}$[H$_2$W$_{12}$O$_{42}$] · 4 H$_2$O → 12 WO$_3$ + 10 NH$_3$ + 10 H$_2$O

## Reactions

Tungsten trioxide can be reduced with carbon or hydrogen gas yielding the pure metal.
    2 WO$_3$ + 3 C → 2 W + 3 CO$_2$ (high temperature)
    WO$_3$ + 3 H$_2$ → W + 3 H$_2$O (550–850 °C)

## Uses

Tungsten trioxide is a starting material for the synthesis of tungstates. Barium tungstate BaWO$_4$ is used as a x-ray screen phosphors. Alkali metal tungstates, such as lithium tungstate Li$_2$WO$_4$ and cesium tungstate Cs$_2$WO$_4$, give dense solutions that can be used to separate minerals. Other applications, actual or potential, include:
* Fireproofing fabrics
* Gas and humidity sensors.
* Ceramic glazes where it gives a rich yellow color.
* Electrochromic glass, such as in smart windows, whose transparency can be changed by an applied voltage.
* Photocatalytic water splitting.
* Substrate for surface-enhanced Raman spectroscopy replacing noble metals.