Oxide-ion conductors and proton conductors have attracted considerable attention because of their potential applications such as fuel cells, and gas sensors. Exploring novel ionic conductors with high ionic conductivity is an important but challenging task. Our group tackled this task by several strategies, and successfully discovered novel ionic conductors. We also investigate ion migration path and mechanism through the precise structure analysis.

Although Dion−Jacobson phases have been investigated in various fields, there are no reports on oxide-ion conduction. Through the screening by the bond valence method, we have found that Dion−Jacobson phase CsBi₂Ti₂NbO_10−δ exhibits high oxide-ion conductivity.[1] From the high temperature neutron diffraction study, we revealed that the high conductivity is attributable to the large anisotropic thermal motions of oxygen atoms, the presence of oxygen vacancies, and the formation of oxide-ion conducting layers in the crystal structure. Ba₃Y₂O₅Cl₂ is discovered as the first examples of oxide-ion conducting oxychlorides.[2] Ruddlesden–Popper (RP) oxychlorides were expected to show high oxide-ion conductivity, because they have larger free and lattice volumes than RP oxides, leading to low activation energies. Two dimensional oxide-ion migration in Ba₃Y₂O₅Cl₂ is indicated from the DFT calculations. By investigating Ba₇Nb_4–xMo_1+xO_20+x/2 materials, we found that Ba₇Nb_3.8Mo_1.2O_20.1 exhibits high oxide-ion and proton conductivity.[3] Total direct current conductivity at 400 °C in wet air of Ba₇Nb_3.8Mo_1.2O_20.1 was 13 times higher than that of Ba₇Nb₄MoO₂₀. Ab initio molecular dynamics (AIMD) simulations, neutron-diffraction experiments at 800 °C, and neutron scattering length density analyses of Ba₇Nb_3.8Mo_1.2O_20.1 indicated that the excess oxygen atoms are incorporated by the formation of both 5-fold coordinated (Nb/Mo)O₅ monomer and its (Nb/Mo)₂O₉ dimer with a corner-sharing oxygen atom and that the breaking and reforming of the dimers lead to the high oxide-ion conduction. Ba₂LuAlO₅ exhibits high proton conductivities, high diffusivity and high chemical stability without chemical doping.[4] Ba₂LuAlO₅ is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba₂LuAlO₅·x H₂O. Ba₇Nb₄MoO₂₀ is one of the promising ionic conductors. However, the chemical order/disorder of Mo and Nb atoms was not revealed because Mo and Nb have similar scattering power for both X-ray and neutron. Therefore, we have combined resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations to reveal the chemical order in Ba₇Nb₄MoO₂₀.[5] NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively.