Direct evidence for two-dimensional oxide-ion diffusion in the hexagonal perovskite-related oxide Ba3MoNbO8.5−δ

Developments in solid oxide-ion (O2−) conductors have led to various energy and environmental technologies, such as gas sensors, solid oxide fuel cells (SOFCs) and oxygen-separation membranes. The research on SOFCs with lower operating temperature and higher energy efficiency has stimulated the discovery of new oxide-ion conductors and improved understanding of oxide-ion diffusion mechanisms. Although there exist a variety of structure types in hexagonal perovskite-related materials, oxide-ion conductors are quite rare and their oxide-ion diffusion pathways are unclear. In the present work, we report the first experimental visualization of oxide-ion diffusion pathways in the hexagonal perovskite-related material Ba3MoNbO8.5−δ by in situ neutron diffraction (21–1100 °C) and the maximum-entropy method, where δ is the oxygen vacancy concentration. Oxide ions were found to migrate two-dimensionally through mixed O2 octahedral and O3 tetrahedral oxygen sites over the O2–O2–O2 face of the (Mo/Nb)O5−e polyhedron, where e is the oxygen vacancy concentration. The (Mo/Nb)–O distance is not kept constant due to the interexchange between the O2 octahedral and O3 tetrahedral coordinations during O2–O3 migration, but the Ba–O distance is kept constant to some extent in the O2–O3 diffusion pathways. The oxide-ion O2–O3 migration and O2/O3 disorders in the mixed tetrahedral and octahedral geometry are responsible for high oxide-ion conductivity. These unique features of the diffusion pathways provide insights into its unique structural rearrangements and O2− mobility, leading to further developments of the oxide-ion conductors.