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 H₂WO₄. It is a light yellow crystalline solid.^[1]

Tungsten trioxide

Sample of Tungsten(VI) Oxide
Names
IUPAC name Tungsten trioxide
Other names Tungstic anhydride Tungsten(VI) oxide Tungstic oxide
Identifiers
CAS Number	1314-35-8 Y
3D model (JSmol)	Interactive image
ECHA InfoCard	100.013.848
PubChem CID	14811
RTECS number	YO7760000
UNII	940E10M08M Y
CompTox Dashboard (EPA)	DTXSID7032262
InChI InChI=1S/3O.W
SMILES O=[W](=O)=O
Properties
Chemical formula	WO3
Molar mass	231.84 g/mol
Appearance	Canary yellow powder
Density	7.16 g/cm3
Melting point	1,473 °C (2,683 °F; 1,746 K)
Boiling point	1,700 °C (3,090 °F; 1,970 K) approximation
Solubility in water	insoluble
Solubility	slightly soluble in HF
Magnetic susceptibility (χ)	−15.8·10−6 cm3/mol
Structure
Crystal structure	Monoclinic, mP32
Space group	P121/n1, No. 14
Coordination geometry	Octahedral (WVI) Trigonal planar (O2– )
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Irritant
Flash point	Non-flammable
Safety data sheet (SDS)	External MSDS
Related compounds
Other anions	Tungsten trisulfide
Other cations	Chromium trioxide Molybdenum trioxide
Related tungsten oxides	Tungsten(III) oxide Tungsten(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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 WO_2.90 = W₂₀O₅₈, are dark blue to purple in color and conduct electricity. They can be prepared by combining the trioxide and the dioxide WO₂ at 1000 °C in vacuum.^[4][1]

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.^[5][4]

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.^[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 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.^[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)
• 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.^[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 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.^[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]

CaWO₄ + 2 HCl → CaCl₂ + H₂WO₄

H₂WO₄ → H₂O + WO₃

Laboratory

Another common way to synthesize WO₃ is by calcination of ammonium paratungstate (APT) under oxidizing conditions:^[2]

(NH₄)₁₀[H₂W₁₂O₄₂] · 4 H₂O → 12 WO₃ + 10 NH₃ + 10 H₂O

Reactions

Tungsten trioxide can be reduced with carbon or hydrogen gas yielding the pure metal.^{[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 BaWO₄ is used as a x-ray screen phosphors. Alkali metal tungstates, such as lithium tungstate Li₂WO₄ and cesium tungstate Cs₂WO₄, give dense solutions that can be used to separate minerals.^[1] Other applications, actual or potential, include:

• Fireproofing fabrics^[8]
• Gas and humidity sensors.^[9][1]
• Ceramic glazes where it gives a rich yellow color.^[7][1]
• Electrochromic glass, such as in smart windows, whose transparency can be changed by an applied voltage.^[10][11][1]
• Photocatalytic water splitting.^{[12][13][14][15]}
• Substrate for surface-enhanced Raman spectroscopy replacing noble metals.^{[16][17][18][19]}