Solid-State 95Mo and 93Nb NMR Study of Ba7Nb4MoO20-Based Materials and Ba7Ta3.7Mo1.3O20.15

Hexagonal perovskite-related oxides, Ba₇Nb₄MoO₂₀, Ba₇Nb_4–xMo_1+xO_20+(1/2)x (x = 0.1), Ba₇Nb_4–yW_yMoO_20+(1/2)y (y = 0.15), and Ba₇Ta_4–zMo_1+zO_20+(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, Ba₇Nb_3.9Mo_1.1O_20.05, Ba₇Nb_3.85W_0.15MoO_20.075, and Ba₇Ta_3.7Mo_1.3O_20.15, using solid-state ⁹⁵Mo and ⁹³Nb NMR analysis. The ⁹⁵Mo NMR similar predominant peaks revealed in Ba₇Nb_3.9Mo_1.1O_20.05, Ba₇Nb_3.85W_0.15MoO_20.075, and Ba₇Ta_3.7Mo_1.3O_20.15 are also attributed to the MoO₄ tetrahedron near the oxide-ion conducting layer owing to the small quadrupolar coupling constant, |C_Q|. Furthermore, a minor peak of ⁹⁵Mo has been observed in Ba₇Ta_3.7Mo_1.3O_20.15, 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, |P_Q|, 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 Ba₇Nb_4–xMo_1+xO_20+(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.