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-ϵ polyhedron, where ϵ 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.
All Science Journal Classification (ASJC) codes
- Renewable Energy, Sustainability and the Environment
- Materials Science(all)