Tungsten and oxygen
In the fully oxidized state (W6+) the octahedral
coordination dominates (WO6). The tungsten atoms are situated in the center of
octahedrons and are surrounded by six oxygen atoms arranged at the corners. As
long as all tungsten ions are hexavalent, these octahedrons are only
corner-sharing.
WO6 octahedron; (a) Locations of the centers of the
atoms; the black circle is tungsten, the white circles oxygen. (b) Atoms shown
in full size; the tungsten atom is the small circle.
However, as the oxygen-to-tungsten ratio decreases, which
occurs on reduction (transition to lower oxides), the octahedra become more and
more intricately joined in combinations of corner-, edge-, and face-sharing.
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