Direct Observation of Oxide Ion Dynamics in La₂Mo₂O₉ on the Nanosecond Timescale

Quasielastic neutron scattering (QENS), underpinned by ab initio molecular dynamics (AIMD) simulations, has been used to directly observe oxide ion dynamics in solid electrolyte La 2 Mo 2 O 9 on the nanosecond timescale, the longest timescale probed in oxide ion conductors by neutron scattering to date. QENS gives the activation energy of 0.61(5) eV for this process, while AIMD simulations reveal that the exchange processes, which ultimately lead to long-range oxide ion diffusion in La 2 Mo 2 O 9, rely on the flexibility of the coordination environment around Mo 6+, with oxide ions jumps occurring between vacant sites both within and between Mo coordination spheres. Simulations also differentiate between the crystallographic sites which participate in the oxide ion exchange processes, offering the first atomic-level understanding of the oxide ion dynamics in La 2 Mo 2 O 9, which is consistent with the macroscopic experimental observations on this material.