Direct evidence for two-dimensional oxide-ion diffusion in the hexagonal perovskite-related oxide Ba3MoNbO8.5–δ
Developments of 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. Research for SOFCs with lower operating temperature and higher energy efficiency has stimulated the discovery of new oxide-ion conductors and the understanding of oxide-ion diffusion mechanism. 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 have not been known yet. In the present work, we report the first experimental visualization of oxide-ion diffusion pathways of the hexagonal perovskite-related material Ba3MoNbO8.5–δ by in situ neutron diffraction (21–1100 oC) and maximum-entropy method. Here δ is the content of oxygen deficiency. Oxide ions were found to migrate two-dimensionally through mixed O2 octahedral and O3 tetrahedral oxygen sites on the O2–O2–O2 face of (Mo/Nb)O5–ε polyhedron where ε is the content of oxygen deficiency. The (Mo/Nb)–O distance is not kept constant due to the interexchange between the O2 octahedral and O3 tetrahedral coordinations during the O2–O3 migration, while the Ba–O distance is kept constant to some extent on the O2–O3 diffusion pathways. The O2–O3 migration and O2/O3 disorders of oxide ions 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.