Imaging tunnel atoms in intergrowth tungsten bronzes

Abstract

Crystals of caesium and rubidium intergrowth tungsten bronze (ITB) have been studied by high-resolution electron microscopy. The structure of these consists of slabs of WO3 type, intergrown with slabs of hexagonal tungsten bronze (HTB) type containing hexagonal tunnels in which the alkali atoms are accommodated. In the images of thicker parts the hexagonal tunnels are clearly revealed. In the thin parts the HTB structure mostly appears as a hexagonal pattern of dots of equal intensity and the tunnels are not recognizable. This applies particularly to Cs ITB but also in many instances to Rb ITB. Image simulations, assuming known or estimated degrees of filling of the tunnels with alkali, show a clear difference in contrast at the tungsten and alkali positions, especially at certain focus settings. The discrepancy is thought to be due to surface reconstruction, whereby tungsten atoms are trapped in the tunnels or the surface layers are shifted, making the surface structure similar to that of WO3.⅓H2O.