Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

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Hexagonal perovskite derivatives Ba 3 MNbO 8.5 (M: W and Mo) are attracting much interest due to high oxide-ion conductivity and potential use for many applications. This work shows the electrical conductivities of Ba 3 WNbO 8.5 (3.7 × 10 −2 S cm −1 ) and Ba 3 MoNbO 8.5 (8.8 × 10 −2 S cm −1 ) at 900 °C and confirms higher activation energy for conductivity of Ba 3 WNbO 8.5 than that of Ba 3 MoNbO 8.5 . Key factors governing the conductivity and activation energy are the ratio of tetrahedral O3 to octahedral O2 oxide ions and diffusion pathways in Ba 3 MNbO 8.5 . However, the O2/O3 disorders and oxide-ion diffusion paths are unresolved important issues in Ba 3 MNbO 8.5 . Here, Rietveld and maximumentropy method (MEM) analyses of in situ neutron-diffraction data up to 800 °C were performed to obtain the crystal structure and neutron scattering length densities (NSLDs) of Ba 3 WNbO 8.5 . MEM NSLDs show two-dimensional oxide-ion migration through the octahedral O2 and tetrahedral O3 sites in the intrinsically oxygen-deficient layer. Numbers of the interstitial O3 and lattice O2 atoms n(O3) and n(O2) increase and decrease, respectively, with increasing temperature, which indicates that the O2/O3 disorder is more prominent at high temperatures. The O2/O3 disordering makes the minimum NSLD on the O2−O3 path higher, which enhances oxide-ion conductivity, leading to higher activation energies of Ba 3 WNbO 8.5 compared with Ba 3 MoNbO 8.5 .

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba7Nb4MoO20 by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. 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. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

The oxide-ion conductor Ba3MoNbO8.5, the oxide-ion and proton conductor Ba7Nb4MoO20, and their related oxides are important groups of materials because of their high ionic conductivity. The structure of the ion-conducting layer of these materials has not been clarified because of their complex structure and the difficulty in distinguishing between Mo and Nb. In this study, we separately detected 95Mo and 93Nb by solid-state nuclear magnetic resonance (NMR) measurements to directly observe the Mo and Nb coordination in the high-oxide-ion conductor Ba3MoNbO8.5. The results showed that the number of revealed peaks was different for 93Nb and 95Mo. For the two chemical shifts from 93Nb NMR, the more intense peak was attributed to a NbO6 octahedron in the conducting layer, while the less intense peak was ascribed to a NbO4 tetrahedron in the conducting layer or a NbO6 octahedron in the nonconducting layer. Four peaks were observed in the 95Mo NMR of the 95Mo-enriched sample. One peak was attributed to the MoO6 octahedron in the nonconducting layer. The other three peaks attributed to the conducting layer were only interpreted by assigning either one or two of them to the MoO5 polyhedra, which are speculated to play an important role in ionic conduction. Presumably, these are the first results supporting the presence of MoO5 in the ion-conducting layer of oxide-ion conductors, and Mo likely plays an important role in ionic conduction. The present work has demonstrated that the analysis of the local structure of Mo–O and Nb–O polyhedra by NMR is an important tool for understanding the nature of ionic conduction because it provides element-independent information. It is therefore expected to contribute to the further development of oxide-ion conductors.

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