Tungsten(VI) oxide, also known astungsten trioxide is a chemical compound ofoxygen and the transition metaltungsten, withformulaWO3. The compound is also calledtungstic anhydride, reflecting its relation totungstic acidH2WO4. It is a light yellow crystalline solid.[1]
Tungsten(VI) oxide occurs naturally in the form ofhydrates, which include minerals:tungstiteWO3·H2O,meymaciteWO3·2H2O, andhydrotungstite (of the same composition as meymacite, however sometimes written asH2WO4). These minerals are rare to very rare secondary tungsten minerals.
In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide andsodium tungstate.[2] He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.[2]
The crystal structure of tungsten trioxide is temperature dependent. It istetragonal 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 ofWO3 is monoclinic withspace group P21/n.[2]
The pure compound is an electric insulator, but oxygen-deficient varieties, such asWO2.90 =W20O58, are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and thedioxideWO2 at 1000 °C in vacuum.[1][4]
Possible signs ofsuperconductivity with critical temperatures Tc = 80–90 K were claimed in sodium-doped and oxygen-deficientWO3 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]
Tungsten trioxide exists in multiple polymorphs whose structures have been precisely determined usingX-ray crystallography andneutron diffraction. Each phase exhibits a distinct arrangement of distortedWO6 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:
Ahexagonal form synthesized under specific conditions
The most common ambient phase is monoclinic with space groupP21/n, featuring distortedWO6 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.
This high-temperature phase is observed above 740 °C, but specific crystallographic data are often not tabulated separately in modern studies. It exhibits relatively symmetricWO6 octahedra.
A less common hexagonal polymorph ofWO3 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]
Tungsten trioxide is obtained as an intermediate in the recovery of tungsten from its minerals.[7] Tungsten ores can be treated withalkalis to produce solubletungstates. Alternatively,CaWO4, orscheelite, is allowed to react withHCl to producetungstic acid, which decomposes toWO3 and water at high temperatures.[7]
Tungsten trioxide is a starting material for the synthesis oftungstates.Barium tungstateBaWO4 is used as ax-ray screenphosphors. Alkali metal tungstates, such aslithium tungstateLi2WO4 andcesium tungstateCs2WO4, give dense solutions that can be used to separate minerals.[1] Other applications, actual or potential, include:
^abcdLassner, Erik and Wolf-Dieter Schubert (1999).Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds. New York: Kluwer Academic.ISBN978-0-306-45053-2.
^H. A. Wriedt (1898): "The O-W (oxygen-tungsten) system".Bulletin of Alloy Phase Diagrams., volume 10, pages 368–384.doi:10.1007/BF02877593
^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.doi:10.1007/s100510050735S2CID121476634
^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.doi:10.1088/0957-0233/13/6/314
^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
^S. Hurst (2011). "Utilizing Chemical Raman Enhancement: A Route for Metal Oxide Support Based Biodetection".The Journal of Physical Chemistry C.115 (3):620–630.doi:10.1021/jp1096162.
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