Superstructures and defect structures revealed by atomic-scale STM imaging ofWO3(001)

Atomic-scale images of the (001) surface of monoclinic ${\mathrm{WO}}_3$ have been obtained by scanning tunneling microscopy. Termination of the surface ${\mathrm{WO}}_2$ plane with half a monolayer of oxygen ions gives a surface periodicity √2 times that of the underlying ${\mathrm{ReO}}_3$-like framework. An additional periodicity is introduced by tilting of ${\mathrm{WO}}_6$ octahedra to produce alternately long and short O-O separations along the [110] direction. This produces a (2×2) superstructure approximating the space group c2mm. Prominent defect lines at the surface are attributed to missing oxygen ions.