Hybrid Hexagonal Perovskite Ba₃Ti_0.7Mo_1.3O_8.3: Integrating High Cationic Disordering with Enhanced Tetrahedral Proportion to Achieve High Oxide Ionic Conductivity

The advancement of crystalline solid-state oxide-ion conductors is essential for the development of clean energy applications, particularly in the context of solid-oxide fuel cells. Although numerous oxide-ion conducting materials exist, there is a pressing need for electrolyte materials that exhibit high oxide ionic conductivity at intermediate temperatures (300–600 °C). Herein, we report a fast oxide ionic conductor Ba3Ti0.7Mo1.3O8.3, which possesses a hybrid perovskite identified by three-dimensional electron diffraction, neutron powder diffraction, neutron pair distribution functions, and in situ synchrotron X-ray diffraction. Ba3Ti0.7Mo1.3O8.3 exhibits the highest degree of M-site disordering among the Ba3M′M″O8.5 ± δ hexagonal perovskite family while simultaneously maintaining a high proportion of the tetrahedral coordination geometry within the structure. The high level of M-site disordering and the high tetrahedral percentage in Ba3Ti0.7Mo1.3O8.3 facilitate oxide-ion diffusion along the c-axis and ab-plane, respectively, thereby resulting in a high bulk conductivity of 1.4 mS/cm at 500 °C, which is significantly higher than those of the Ba3M′M″O8.5 ± δ family members and other oxide ionic conductors. Our findings demonstrate that achieving a balance between M-site disordering and tetrahedral percentage is an effective strategy for improving the oxide ionic conductivity of Ba3M′M″O8.5 ± δ-type conductors.