Oxygen ion diffusion and surface exchange properties of the ?- And ?-phases of Bi2O3

Abstract

Fast oxide ion conduction is a highly desirable property for materials in a wide range of applications. The fastest reported ionic conductor, representing the current state of the art and an oft-proposed effective limit of oxide ion conductivity, is the high temperature fluorite-structured ? phase of Bi2O3. Here, the ionic nature of this conduction is, for the first time, directly determined through oxygen tracer diffusion measurements. This phase also presents a remarkably high oxygen surface exchange coefficient, competitive with the highest performance solid oxide fuel cell (SOFC) cathodes yet counterintuitively in a material with negligible electronic conduction. The low temperature ?-Bi2O3 polymorph is also investigated, revealing a remarkable drop in diffusivity of over 7 orders of magnitude with a temperature drop of just =150 �C. Surprisingly, the diffusion studies also reveal a secondary, significantly faster migration pathway in the ? phase. This is attributed to grain boundary conduction and shown to be 3-4 orders of magnitude higher than in the bulk. This previously unobserved property could present an exciting opportunity to tailor ionic conductivity levels through manipulating microstructure down to the nanoscale. � 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.