Fast grain boundary oxygen ion diffusion in the α-phase of Bi2O3

The low temperature stable α-phase of pure Bi2O3 is known to have an oxygen ion diffusivity that is over 7 orders of magnitude lower than the high temperature fluorite-structured δ-phase. Tracer oxygen-18 diffusion studies of polycrystalline α-Bi2O3 at 600 °C using an ion microscope with lateral resolutions of ~ 50 nm for the surface distributions of the oxygen isotopes have resolved the secondary fast migration pathways as well as the normal bulk diffusion profile in the grains. A grain boundary pathway for the oxygen migration is distinguished from other extended defects, some of which are also active in the overall oxygen diffusivity. This experimental study highlights the potential manipulation of the micro-structure of this material for enhanced oxygen ion conduction in intermediate temperature solid oxide fuel cells as has been shown for perovskite MIEC electrode materials.