Tungsten trioxide

Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO₃. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light yellow crystalline solid.

Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO₃·H₂O, meymacite WO₃·2H₂O and hydrotungstite (of the same composition as meymacite, however sometimes written as H₂WO₄). 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 WO₃ 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 T_c = 80–90 K were claimed in sodium-doped and oxygen-deficient WO₃ 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.

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 WO₆ octahedra, which affect its electronic and optical behavior.

Tungsten trioxide (WO₃) 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 WO₆ 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 WO₆ 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 ų
* 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 ų
* 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 ų
* 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 WO₃ 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 ų
* 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, CaWO₄, or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to WO₃ and water at high temperatures.

:CaWO₄ + 2 HCl → CaCl₂ + H₂WO₄
:H₂WO₄ → + WO₃

Laboratory

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

:(NH₄)_{10 2}W₁₂O₄₂→ 12 WO₃ + 10 NH₃ + 10

Reactions

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

:2 WO₃ + 3 C → 2 W + 3 CO₂ (high temperature)
:WO₃ + 3 H₂ → W + 3 H₂O (550–850 °C)

Uses

Tungsten trioxide is a starting material for the synthesis of tungstates. Barium tungstate is used as a 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

K. J. Patel, M. S. Desai, C. J. Panchal, H. N. Deota, and U. B. Trivedi (2013):
All-Solid-Thin Film Electrochromic Devices Consisting of Layers ITO / NiO / ZrO2 / WO3 / ITO
. ''Journal of Nano-Electronics and Physics'', volume 5, issue 2, article 02023.

S. Reich and Y. Tsabba (1999): "Possible nucleation of a 2D superconducting phase on WO single crystals surface doped with Na". ''European Physical Journal B'', volume 9, pages = 1–4.

A. Shengelaya, K. Conder, and K. A. Müller (2020): "Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO_2.90". ''Journal of Superconductivity and Novel Magnetism'', volume 33, pages 301–306.

David E Williams, Simon R Aliwell, Keith F. E. Pratt, Daren J. Caruana, Roderic L. Jones, R. Anthony Cox, Graeme M. Hansford. and John Halsall (2002): "Modelling the response of a tungsten oxide semiconductor as a gas sensor for the measurement of ozone". ''Measurement Science and Technology''. volume 13. pages 923–931.

Yugo Miseki, Hitoshi Kusama, Hideki Sugihara, and Kazuhiro Sayama (2010): "Cs-Modified WO3 Photocatalyst Showing Efficient Solar Energy Conversion for O2 Production and Fe (III) Ion Reduction under Visible Light". ''Journal of Physical Chemistry Letters'', volume 1, issue 8, pages 1196–1200.

É. Karácsonyi, L. Baia, A. Dombi, V. Danciu, K. Mogyorósi, L. C. Pop, G. Kovács, V. Coşoveanu, A. Vulpoi, S. Simon, Zs. Pap (2013): "The photocatalytic activity of TiO2/WO3/noble metal (Au or Pt) nanoarchitectures obtained by selective photodeposition". ''Catalysis Today'', volume 208, pages 19-27.

István Székely, Gábor Kovács, Lucian Baia, Virginia Danciu, Zsolt Pap (2016): "Synthesis of Shape-Tailored WO3 Micro-/Nanocrystals and the Photocatalytic Activity of WO3/TiO2 Composites". ''Materials'', volume 9, issue 4, pages 258-271.

Lucian Baia, Eszter Orbán, Szilvia Fodor, Boglárka Hampel, Endre Zsolt Kedves, Kata Saszet, István Székely, Éva Karácsonyi, Balázs Réti, Péter Berki, Adriana Vulpoi, Klára Magyari, Alexandra Csavdári, Csaba Bolla, Veronica Coșoveanu, Klára Hernádi, Monica Baia, András Dombi, Virginia Danciu, Gábor Kovácz, Zsolt Pap (2016): "Preparation of TiO2/WO3 composite photocatalysts by the adjustment of the semiconductors' surface charge". ''Materials Science in Semiconductor Processing'', volume 42, part 1, pages 66-71.

H. A. Wriedt (1898): "The O-W (oxygen-tungsten) system". ''Bulletin of Alloy Phase Diagrams.'', volume 10, pages 368–384.

Merck (2006): "Tungsten trioxide." ''The Merck Index'', volume 14.

J. Christian, R.P. Singh Gaur, T. Wolfe and J. R. L. Trasorras (2011):
Tungsten Chemicals and their Applications
'. Brochure by International Tungsten Industry Association.

External links

International Tungsten Industry AssociationPreparation of tungsten trioxide electrochromic filmsSigma Aldrich (supplier)

{{DEFAULTSORT:Tungsten Trioxide
Tungsten compounds
Transition metal oxides