Tungsten and oxygen(7)

Tungsten and oxygen

Commercially available WO3 consists almost entirely of γ-WO3.

The reason for the numerous modifications originates in the structure, which is a three-dimensional array of corner-sharing metal-oxygen octahedra (Fig. 1.1).

The idealized structure (cubic symmetry; ReO3-type) is drawn in Fig. 1.2, showing the octahedra as squares in projection. The real structure is distorted. This means that the relatively small tungsten atoms tend to be displaced from the octahedral center and these displacements are temperature-dependent. Moreover, the symmetry can be influenced by small amounts of impurities.

FIGURE 1.1. 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.

FIGURE 1.2. The idealized WO3 structure represented as an infinite array of corner-shared octahedra; in the real structure small distortions of the octahedra reduce the symmetry from cubic to monoclinic at room temperature.

two of the hardest materials made of the tungsten diamond ring