Different Local Structures of Mo and Nb Polyhedra in the Oxide-Ion-Conducting Hexagonal Perovskite-Related Oxide Ba₃MoNbO_8.5 Revealed by ⁹⁵Mo and ⁹³Nb NMR Measurements

The oxide-ion conductor Ba₃MoNbO_8.5, 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 Ba₃MoNbO_8.5. 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.