Tungsten trioxide

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 is monoclinic with space group P21/n.

The pure compound is an electric insulator, but oxygen-deficient varieties, such as = , are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and the dioxide at 1000 °C in vacuum.

Possible signs of superconductivity with critical temperatures Tc = 80–90 K were claimed in sodium-doped and oxygen-deficient crystals. If confirmed, these would be the first superconducting materials containing no copper, with Tc higher than the boiling point of liquid nitrogen at normal pressure.

Crystallography

Tungsten trioxide exists in multiple polymorphs whose structures have been precisely determined using X-ray crystallography and neutron diffraction. Each phase exhibits a distinct arrangement of distorted octahedra, which affect its electronic and optical behavior.

Tungsten trioxide () is a polymorphic compound whose crystal structure changes depending on temperature. It adopts several forms, including:

• Tetragonal above 740 °C
• Orthorhombic from 330 to 740 °C
• Monoclinic from 17 to 330 °C
• Triclinic from −50 to 17 °C
• A second monoclinic phase below −50 °C
• A hexagonal form synthesized under specific conditions

The most common ambient phase is monoclinic with space group P21/n, featuring distorted octahedra linked at their corners. Each polymorph exhibits variations in symmetry, lattice parameters, and atomic positions, making structural determination important for understanding the material's physical and electronic properties.

Tetragonal WO₃

This high-temperature phase is observed above 740 °C, but specific crystallographic data are often not tabulated separately in modern studies. It exhibits relatively symmetric octahedra.

Orthorhombic WO₃

• Space group: Pmnb (No. 62)
• Lattice parameters (Å): a = 7.341(4), b = 7.570(4), c = 7.754(4)
• Angles (°): α = β = γ = 90°
• Cell volume: 430.90 Å3
• Z: 8
• Temperature: 873 K
• Pressure: Atmospheric
• R-value: 0.061
• Reference: Salje, E. (1977). Acta Crystallographica Section B, 33(2), 574–577.

Monoclinic WO₃

• Space group: P1/c1 (No. 7)
• Lattice parameters (Å): a = 5.27710(1), b = 5.15541(1), c = 7.66297(1)
• Angles (°): α = γ = 90°, β = 91.7590(2)
• Cell volume: 208.38 Å3
• Z: 4
• Temperature: 5 K
• Pressure: Atmospheric
• R-value: 0.09
• Reference: Salje, E.K.H. et al. (1997). Journal of Physics: Condensed Matter, 9, 6563–6577.

Triclinic WO₃

• Space group: P−1 (No. 2)
• Lattice parameters (Å): a = 7.309(2), b = 7.522(2), c = 7.678(2)
• Angles (°): α = 88.81(2), β = 90.92(2), γ = 90.93(2)
• Cell volume: 421.92 Å3
• Z: 8
• Temperature: Room temperature
• Pressure: Atmospheric
• R-value: 0.05
• Reference: Diehl, R. et al. (1978). Acta Crystallographica Section B, 34, 1105–1111.

Hexagonal WO₃

A less common hexagonal polymorph of has been reported and characterized using powder X-ray diffraction. It exhibits higher symmetry and potentially distinct electronic properties.

• Space group: P6/mmm (No. 191)
• Lattice parameters (Å): a = 7.298(2), c = 3.899(2)
• Angles (°): α = β = 90°, γ = 120°
• Cell volume: 179.84 Å3
• Z: 3
• Temperature: Room temperature
• Pressure: Atmospheric
• R-value: 0.055
• Reference: Gérand, B. et al. (1979). Journal of Solid State Chemistry, 29, 429–434.

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, , or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to and water at high temperatures.

: :

Laboratory

Another common way to synthesize WO3 is by calcination of ammonium paratungstate (APT) under oxidizing conditions:

:

Reactions

Tungsten trioxide can be reduced with carbon or hydrogen gas yielding the pure metal.

: (high temperature) : (550–850 °C)

Uses

Tungsten trioxide is a starting material for the synthesis of tungstates. Barium tungstate is used as an X-ray screen phosphors. Alkali metal tungstates, such as lithium tungstate and cesium tungstate , 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.

References

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