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 acidH2WO4. It is a light yellow crystalline solid.[1]
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.[2] He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.[2]
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.[3] The most common structure of WO3 is monoclinic with space group P21/n.[2]
The pure compound is an electric insulator, but oxygen-deficient varieties, such as WO2.90 = W20O58, are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and the dioxideWO2 at 1000 °C in vacuum.[1][4]
Possible signs of superconductivity with critical temperatures Tc = 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 Tc higher than the boiling point of liquid nitrogen at normal pressure.[4][5]
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:
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 Å3
Z: 8
Temperature: 873 K
Pressure: Atmospheric
R-value: 0.061
Reference: Salje, E. (1977). Acta Crystallographica Section B, 33(2), 574–577.[6]
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.[6]
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)
Reference: Diehl, R. et al. (1978). Acta Crystallographica Section B, 34, 1105–1111.[6]
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 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.[6]
Preparation
Industrial
Tungsten trioxide is obtained as an intermediate in the recovery of tungsten from its minerals.[7] 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.[7]
Tungsten trioxide is a starting material for the synthesis of tungstates. Barium tungstateBaWO4 is used as a x-ray screen phosphors. Alkali metal tungstates, such as lithium tungstateLi2WO4 and cesium tungstateCs2WO4, give dense solutions that can be used to separate minerals.[1] Other applications, actual or potential, include:
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^Lee, W. J.; Fang, Y. K.; Ho, Jyh-Jier; Hsieh, W. T.; Ting, S. F.; Huang, Daoyang; Ho, Fang C. (2000). "Effects of surface porosity on tungsten trioxide(WO3) films' electrochromic performance". Journal of Electronic Materials. 29 (2): 183–187. Bibcode:2000JEMat..29..183L. doi:10.1007/s11664-000-0139-8. S2CID98302697.
^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. doi:10.1021/jz100233w
^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. doi:10.1016/j.cattod.2012.09.038
^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. doi:10.3390/ma9040258
^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. doi:10.1016/j.mssp.2015.08.042
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