Electrical and Structural Characterization of Ba<sub>3</sub>Mo<sub>1–<i>x</i></sub>Nb<sub>1+<i>x</i></sub>O<sub>8.5–<i>x</i>/2</sub>: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba<sub>3</sub>MoNbO<sub>8.5</sub>

The electrical and structural properties of the series Ba<sub>3</sub>Mo<sub>1–<i>x</i></sub>Nb<sub>1+<i>x</i></sub>O<sub>8.5–<i>x</i>/2</sub> (<i>x</i> = 0.0, 0.1, 0.2, 0.3) have been determined. Ba<sub>3</sub>Mo<sub>1–<i>x</i></sub>Nb<sub>1+<i>x</i></sub>O<sub>8.5–<i>x</i>/2</sub> crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)­O<sub>4</sub> and (Mo/Nb)­O<sub>6</sub> units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)­O<sub>4</sub> tetrahedra to (Mo/Nb)­O<sub>6</sub> octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10<sup>–6</sup> S cm<sup>–1</sup> for x = 0.0 to 1.1 × 10<sup>–7</sup> S cm<sup>–1</sup> for <i>x</i> = 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 Ba<sub>3</sub>MoNbO<sub>8</sub> preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)­O<sub>4</sub> tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba<sub>3</sub>MoNbO<sub>8.5</sub> system.