Solid-State ⁹⁵Mo and ⁹³Nb NMR Study of Ba₇Nb₄MoO₂₀-Based Materials and Ba₇Ta_3.7Mo_1.3O_20.15

Hexagonal perovskite-related oxides, Ba7Nb4MoO20, Ba7Nb4–xMo1+xO20+(1/2)x (x = 0.1), Ba7Nb4–yWyMoO20+(1/2)y (y = 0.15), and Ba7Ta4–zMo1+zO20+(1/2)z (z = 0.3), have recently been reported to exhibit high oxide-ion and proton conductivity. These materials are of great interest in industrial applications, such as solid oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) and are known for their unusual structures. Although the structures of Ba7Nb4MoO20 and their related materials were primarily analyzed by assuming an even distribution of Mo and Nb at each M (=Mo/Nb) site, solid-state nuclear magnetic resonance (NMR) spectra have revealed that Mo and Nb are unevenly distributed in Ba7Nb4MoO20. As it is crucial to determine whether the contributions to oxide-ion and proton conduction are the same for Mo and Nb, we focused on the signal differences among these as-prepared materials, namely, Ba7Nb4MoO20, Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, and Ba7Ta3.7Mo1.3O20.15, using solid-state 95Mo and 93Nb NMR analysis. The 95Mo NMR similar predominant peaks revealed in Ba7Nb3.9Mo1.1O20.05, Ba7Nb3.85W0.15MoO20.075, and Ba7Ta3.7Mo1.3O20.15 are also attributed to the MoO4 tetrahedron near the oxide-ion conducting layer owing to the small quadrupolar coupling constant, |CQ|. Furthermore, a minor peak of 95Mo has been observed in Ba7Ta3.7Mo1.3O20.15, which is presumed to be a MoO5 polyhedron, MoO5 monomer, or (Mo/Ta)2O9 dimer, formed by the binding of the excess oxygen, represented by (1/2)z (z = 0.3) in the chemical formula. One shoulder peak in the 93Nb NMR spectrum of Ba7Nb4MoO20 could be attributed to the NbO4 tetrahedron near the ion conducting layer from its small quadrupolar coupling product, |PQ|, but its intensity is smaller than that considered from the occupancy factors. The small signal intensity is plausible because many are not regular NbO4 tetrahedrons in Ba7Nb4MoO20. In Ba7Nb4–xMo1+xO20+(1/2)x (x = 0.1), the intensity of NbO4 tetrahedron has been further reduced, indicating that the decrease is caused by the transformation of the residual NbO4 tetrahedron to NbO5 polyhedron, NbO5 monomer, or (Mo/Nb)2O9 dimer, by the binding of excess oxygen, represented by (1/2)x (x = 0.1) in the chemical formula. Thus, the solid-state NMR analysis of the local structure of Mo and Nb oxide polyhedra is a vital tool in analyzing nonstoichiometric ion conductors because it provides information on individual Mo and Nb local structures near the conducting layers of the disordered materials. Therefore, it will potentially contribute to further developing applications using ion conductors.