Oxide Ion Dynamics in Hexagonal Perovskite Mixed Conductor Ba7Nb4MoO20: A Comprehensive Ab Initio Molecular Dynamics Study

Hexagonal perovskite Ba7Nb4MoO20-related materials are very promising solid electrolytes with high oxide ion conductivity and redox stability, making them potentially applicable in solid oxide fuel cells. Optimizing the properties of this family of materials necessitates atomic-level understanding of the oxide ion dynamics leading to high conductivity. Here we report extensive ab initio molecular dynamics simulations of Ba7Nb4MoO20 investigating oxide ion motions, which allowed the observation of a continuous diffusion pathway for oxide ions in the (ab) plane, but also revealed significant contribution of the oxygen atoms from crystallographic sites located outside this plane, to the long-range dynamics. To probe the timescale of oxide ion diffusion, complementary quasielastic neutron scattering experiments were carried out, and showed that oxide ion dynamics in Ba7Nb4MoO20, even at 950 oC, are too slow to be observable on a nanosecond timescale. Based on the atomic-level understanding of structure-property relationships afforded by this detailed computational study, we propose new materials design strategies with potential to significantly increase oxide ion conductivity in Ba7Nb4MoO20-related hexagonal perovskites, which target to simultaneously increase the number of oxide ion charge carriers and rotational flexibility of the (Nb/Mo)Ox polyhedra.