ALKALI-METAL LOCATION AND TUNGSTEN OFF-CENTER DISPLACEMENT IN HEXAGONAL POTASSIUM AND CESIUM TUNGSTEN BRONZES

The crystal structures of hexagonal tungsten bronze phases, AtWO3, with A=K and Cs and with t=0.20 and 0.32 have been refined, using the profile refinement procedure, from X-ray powder diffraction data obtained in Guinier-Hägg focussing cameras. The cesium atoms refine nicely in positions at the center of the tunnels, while there are strong indications that the potassium atoms are displaced and probably disordered over many off-center sites in the tunnels. The results for the cesium bronzes combine with those for other simple but distorted structures to give an approximately linear relationship between the amplitude of displacement of the tungsten atoms from the symmetrical sites and the formal valence of tungsten, this line also correlates with the lower compositional limit for the cubic sodium tungsten bronzes. The hexagonal potassium bronzes do not follow this pattern; they are anomalous also in other respects. © 1979.