Investigation of the Relationship between the Structure and Conductivity of the Novel Oxide Ionic Conductor Ba3MoNbO8.5

A variable temperature neutron diffraction study of the novel oxide ion conductor Ba3MoNbO8.5 has been performed between 25 and 600 degrees C. Nonmonotonic behavior of the cell parameters, bond lengths, and angles are observed indicating a structural rearrangement above 300 degrees C. The oxygen/vacancy distribution changes as the temperature increases so that the ratio of (Mo/Nb)O-4 tetrahedra to (Mo/Nb)O-6 octahedra increases upon heating above 300 degrees C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)O-4 tetrahedra within the average structure of Ba3MoNbO8.5 is observed. The increase in the number of (Mo/Nb)O-4 tetrahedra upon heating from 300-600 degrees C most likely offers more low energy transition paths for transport of the O2- ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, increasing the ionic conductivity. The second order Jahn Teller effect most likely further enhances the distortion of the MO4/MO6 polyhedra as distortions created by both electronic and structural effects are mutually supportive.