Collective diffusion within the superionic regime of Bi 2 O 3 Chris E. Mohn a∗ 1 Centre for Earth Evolution and Dynamics, University of Oslo, N-0315 Oslo, Norway Marcin Krynski b 2 Department of Chemistry, University of Cambridge, Cambridge, UK (Dated: January 24, 2020) The delta phase of Bi 2 O 3 has the highest known value of oxide ion conductivity within the solid state and therefore remains a benchmark for the development of fu- ture generations of electrolyte materials to fuel cell technologies. Conventionally, the high value of conductivity in δ-Bi 2 O 3 has been explained by a large concentration of inherent vacancies, together with a strongly polarisable Bi-O bond. We show from ab initio molecular dynamics simulations that short “chains” of collective migrating oxygens contribute strongly to the high value of conductivity with the single particle Nernst–Einstein (N.E.) conductivity to collective (d.c.) conductivity σ N.E. /σ d.c. ∼ 0.5 at 1033 K. The nature of collective events is investigated from a hopping model, the distinct part of the Van Hove function and from the extent of dynamical hetero- geneities in the superionc regime. Results from this analysis indicates that the main contribution to collective ionic diffusion in δ-Bi 2 O 3 involves short collinear chains of 2 or 3 oxygens. These chains are either initiated by an oxygen that jumps into an already occupied oxygen cavity (where they co-exist for a very short time before the residential oxygen is kicked out of its cavity), or from a jump into a vacant cavity which trigger a next nearest neighbouring oxygen to migrate. Since δ-Bi 2 O 3 is easily stabilised in a range of environments, the nature of these collective chains can give important insight into the design of δ-Bi 2 O 3 -based fuel cells for the future. Atomistic insight into the nature of ionic diffusion in the superionic regime has unveiled strong evidence for collective “multi ion” migration [1–13]. Although first suspected in