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 Template:Chem2. It is a light yellow crystalline solid.<ref name=chri2011/>

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.<ref name=lass1999/> He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.<ref name=lass1999/>

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.<ref name=wrie1989/> The most common structure of WO₃ is monoclinic with space group P2₁/n.<ref name=lass1999/>

The pure compound is an electric insulator, but oxygen-deficient varieties, such as Template:Chem2 = Template:Chem2, are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and the dioxide Template:Chem2 at 1000 °C in vacuum.<ref name=shen2020/><ref name=chri2011/>

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.<ref name=reic1999/><ref name=shen2020/>

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.<ref name="Sundberg 2144–2149">Template:Cite journal</ref>

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.<ref name="Sundberg 2144–2149"/>

File:WO3 triclinic structure.png

WO3 triclinic structure

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.<ref name="Sundberg 2144–2149"/>

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.<ref name="Sundberg 2144–2149"/>

Preparation

Industrial

Tungsten trioxide is obtained as an intermediate in the recovery of tungsten from its minerals.<ref name=prad2003/> 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.<ref name=prad2003/>

CaWO₄ + 2 HCl → CaCl₂ + H₂WO₄

H₂WO₄ → Template:H2O + WO₃

Laboratory

Another common way to synthesize WO₃ is by calcination of ammonium paratungstate (APT) under oxidizing conditions:<ref name=lass1999/>

(NH₄)₁₀[H₂W₁₂O₄₂]Template:Hydrate → 12 WO₃ + 10 NH₃ + 10 Template:H2O

Reactions

Tungsten trioxide can be reduced with carbon or hydrogen gas yielding the pure metal.Template:Citation needed

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 Template:Chem2 is used as a x-ray screen phosphors. Alkali metal tungstates, such as lithium tungstate Template:Chem2 and cesium tungstate Template:Chem2, give dense solutions that can be used to separate minerals.<ref name=chri2011/> Other applications, actual or potential, include:

• Fireproofing fabrics<ref name=merck2006/>
• Gas and humidity sensors.<ref name=will2002/><ref name=chri2011/>
• Ceramic glazes where it gives a rich yellow color.<ref name=prad2003/><ref name=chri2011/>
• Electrochromic glass, such as in smart windows, whose transparency can be changed by an applied voltage.<ref name=leew2000/><ref name=pate2013/><ref name=chri2011/>
• Photocatalytic water splitting.<ref name=mise2010/><ref name=kara2013/><ref name=szek2016/><ref name=baia2016/>
• Substrate for surface-enhanced Raman spectroscopy replacing noble metals.<ref name=oug2018/><ref name=hurst2011/><ref name=liuw2018/><ref name=zhou2019/>

References

<references>

<ref name=pate2013>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.</ref>

<ref name=reic1999>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. {{#invoke:doi|main}} Template:S2cid</ref>

<ref name=shen2020>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. {{#invoke:doi|main}}</ref>

<ref name=will2002>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. {{#invoke:doi|main}}</ref>

<ref name=mise2010>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. {{#invoke:doi|main}}</ref>

<ref name=kara2013>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. {{#invoke:doi|main}}</ref>

<ref name=szek2016>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. {{#invoke:doi|main}}</ref>

<ref name=baia2016>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. {{#invoke:doi|main}}</ref>

<ref name=oug2018>Template:Cite journal</ref>

<ref name=hurst2011>Template:Cite journal</ref>

<ref name=liuw2018>Template:Cite journal</ref>

<ref name=zhou2019>Template:Cite journal</ref>

<ref name=prad2003>Template:Cite book</ref>

<ref name=lass1999>Template:Cite book</ref>

<ref name=wrie1989>H. A. Wriedt (1898): "The O-W (oxygen-tungsten) system". Bulletin of Alloy Phase Diagrams., volume 10, pages 368–384. {{#invoke:doi|main}}</ref>

<ref name=merck2006>Merck (2006): "Tungsten trioxide." The Merck Index, volume 14.</ref>

<ref name=chri2011>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.</ref>

<ref name=leew2000>Template:Cite journal</ref>

</references>

External links

• International Tungsten Industry Association
• Preparation of tungsten trioxide electrochromic films
• Sigma Aldrich (supplier)