Ion Conduction / Self-Diffusion / Model / Materials ScienceUnderstanding the mechanisms of translational and localised ionic movements in disordered materials has seen intense activity spanning several decades. This article attempts to convey a concise overview of our contribution to this field over the period from 2005 to 2010 and to place it in its broad context.

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The thermal behavior of the oxide ion-conducting solid electrolyte Bi4YbO7.5 was investigated using a combination of variable temperature X-ray and neutron powder diffraction, thermal analysis (DTA and TGA), and ac impedance spectroscopy. The title compound shows a fluorite-type structure throughout the measured temperature range (20–850 °C), with a phase separation at ca. 600 °C into a cubic δ-type phase and an orthorhombic phase of assumed stoichiometry Bi17Yb7O36. This type of transition is a relatively common feature in bismuth oxide-based systems and can limit their practical application. Here, the transition was carefully studied using isothermal measurements, which showed that it is accompanied by changes in oxide-ion stoichiometry, as well as significant disorder in the oxide ion sublattice in the δ-type phase. These results correlate with the observed electrical behavior. Analysis of the total neutron scattering through reverse Monte Carlo (RMC) modeling reveals details of the coordination environments for both cations. The oxide-ion vacancy distribution seems to be consistent with a favoring of ⟨100⟩ vacancy pairs, although ⟨110⟩ vacancy pairs exhibit the highest frequency as they have the maximum likelihood. A vacancy ordering model based on three vacancies per cell is presented.

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

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