LAUSR

Abstract Solid solution formation in the system Bi 3 Y 1 −  x W x O 6 + 3 x /2 has been studied using a combination of X-ray and neutron powder diffraction and a.c. impedance spectroscopy. Compositions in the solid solution adopt the δ-Bi 2 O 3 type structure, with single phases evident from x  = 0.00 to x  = 0.20. Evidence for dopant clustering is presented and discussed. Models for the defect structure derived from diffraction studies are presented. Tungsten is proposed to adopt a tetrahedral coordination geometry, with a distorted octahedral geometry adopted by yttrium. Calculated coordination numbers for bismuth of around five are consistent with stereochemical activity of the Bi 6 s 2 lone pairs of electrons. Despite a significant lowering of the nominal vacancy concentration with respect to δ-Bi 3 YO 6 , as well as enhanced vacancy trapping by W 6+ , tungsten doping is found to have very little influence on the total conductivity of δ-Bi 3 YO 6. This is attributed to the compensating effect of enhanced oxide ion mobility caused by lattice expansion.