The solid state system Bi 2 O 3 :ZrO 2 has been investigated up to 28 mol% ZrO 2 using X-ray and neutron powder diffraction, ac impedance spectroscopy and differential thermal analysis. X-Ray powder diffraction has shown that a solid solution is formed with general composition Bi 22x Zr x O 3zx/2 (0.05¡x¡0.17) for samples calcined at 850 ‡C. Members of the solid solution adopt a new b-Bi 2 O 3 type structure termed b III , which is closely related to Pb 2 F 2 O. The defect structure of a sample of composition x~0.15 has been determined by combined high-resolution neutron and X-ray powder diffraction. High temperature powder X-ray diffraction has been used to confirm the structure of the high temperature polymorph, which is a cubic d-Bi 2 O 3 analogue. In the b III -phase, solid solution formation proceeds through an anion interstitial mechanism with respect to the hypothetical end member b III -Bi 2 O 3 , with interstitial oxide ions located in channels parallel to the c-axis. The defect structure involves a pair of Zr atoms coordinated to interstitial ions in the channels. A possible conduction mechanism is proposed. Differential thermal analysis and ac impedance measurements indicate a complex phase transition to a highly conducting polymorph (s 700 ~1.02 S cm 21 ) at temperatures above ca. 690 ‡C on heating. Crystal parameters: T~298 K, Bi 1.85 Zr 0.15 O 3.075 , M~449.49, tetragonal, P4 2 /nmc, a~7.7206(8), c~5.6370(6) A ˚, Z~4, U~336.0(1) A ˚3, D c ~8.889(3) g cm 23 ; T~1023 K, cubic, Fm3 ¯m, a~5.6277(4) A ˚, U~178.24(4) A ˚3, Z~2, D c ~8.379(2) g cm 23 .{Electronic supplementary information (ESI) available: crystal data, data collection and structural refinement data, atomic coordinates, isotropic thermal parameters, bond lengths, bond angles and contact distances for b III -Bi 1.85 Zr 0.15 O 3.075 at 298 K and for d-Bi 1.85 Zr 0.15 O 3.075 at 1023 K. See
