Editing Tungsten trioxide (section)

=== 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 WO<sub>6</sub> 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 crystal family|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.<ref name="Sundberg 2144–2149">{{Cite journal |last=Sundberg |first=M. |date=1976-07-15 |title=The crystal and defect structures of W25O73, a member of the homologous series WnO3n−2 |url=https://journals.iucr.org/paper?S0567740876007280 |journal=Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry |language=en |volume=32 |issue=7 |pages=2144–2149 |doi=10.1107/S0567740876007280 |issn=0567-7408|url-access=subscription }}</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 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.<ref name="Sundberg 2144–2149"/>
[[File:WO3 triclinic structure.png|thumb|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 Å³  
* '''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 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.<ref name="Sundberg 2144–2149"/>