Structural study of a new hexagonal form of tungsten trioxide

A new form of tungsten trioxide WO₃ has been obtained by dehydration of $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$ hydrate. The structural study was carried out from X-ray powder diffraction and selected area electron diffraction data. The crystallographic characteristics are: the hexagonal system; a = 7.298(2) Å, c = 7.798(3) Å; Z = 6. This hexagonal WO₃ is built up of slightly distorted (WO₆) octahedra sharing their corners arranged in six-membered rings in layers normal to the hexagonal axis; stacking of such layers leads to formation of large hexagonal tunnels. Some confirmations of this structure were made by high-resolution electron microscopy. Powder X-ray diffraction allowed us to determine an average structure. Absence of suitable single crystals has not permitted us to perform a complete structural determination. Although the existence of such a hexagonal structure for pure WO₃ had been considered as likely, it had not been hitherto observed.