Oxygen Ion Diffusion and Surface Exchange Properties of the α- and δ-phases of Bi2O3

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 delta 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 alpha-Bi2O3 polymorph is also investigated, revealing a remarkable drop in diffusivity of over 7 orders of magnitude with a temperature drop of just approximate to 150 degrees C. Surprisingly, the diffusion studies also reveal a secondary, significantly faster migration pathway in the alpha 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.