Electrical and Structural Characterization of Ba₃Mo_1–xNb_1+xO_8.5–x/2: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba₃MoNbO_8.5

The electrical and structural properties of the series Ba₃Mo_1–xNb_1+xO_8.5–x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba₃Mo_1–xNb_1+xO_8.5–x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)­O₄ and (Mo/Nb)­O₆ units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)­O₄ tetrahedra to (Mo/Nb)­O₆ octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10^–6 S cm^–1 for x = 0.0 to 1.1 × 10^–7 S cm^–1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba₃MoNbO₈ preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)­O₄ tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba₃MoNbO_8.5 system.