Liquid-like Ionic Diffusion in Solid Bismuth Oxide Revealed by Coherent Quasielastic Neutron Scattering

Abstract

The exceptional oxide ionic conductivity of the high-temperature phase of bismuth oxide gives rise to a characteristic “quasielastic” broadening of its neutron scattering spectrum. We show that the oscillating form of this broadening can be fit using a modified version of a jump-diffusion model previously reserved for liquid ionic conductors. Fit parameters include a quantitative jump distance and a semiquantitative diffusion coefficient. In the case presented here, the results show that diffusion is isotropic (liquid-like) even though some directions present shorter oxygen–vacancy distances, an insight corroborated by computational dynamics simulations. More broadly, the results show for the first time that quasielastic neutron scattering can be directly analyzed to yield quantitative insights into the atomic-scale mechanisms of solid-state ionic conduction, even when the diffusing species is a coherent neutron scatterer such as oxygen. This shows its power as a tool for studying functional solid-state materials, notably for solid-oxide fuel cells and, potentially, lithium-ion batteries.