Enhanced Oxygen Ion Conductivity and Mechanistic Understanding in Ba₃Nb_1–xV_xMoO_8.5

Significant oxide ion conductivity has recently been reported in the cation-deficient hexagonal perovskite derivative Ba₃NbMoO_8.5. This system exhibits considerable anion and cation disorder. Oxygen disorder enables the ionic conduction and is generated by the competitive occupation of two available average tetrahedral/octahedral oxygen positions within the palmierite-like layers of the average crystal structure. A random distribution of cationic vacancies leads to the formation of complex disordered stacking configurations of the constituting polyhedral units. Here, we report on the electrical and structural properties of the series Ba₃Nb_1–xV_xMoO_8.5 (x = 0.0, 0.1, 0.2, 0.3, 0.4). Neutron diffraction data evidence that substitution of Nb⁵⁺ with V⁵⁺ leads to an increase in the average concentration of lower coordination M1O_x units, which is also accompanied by an increase in polyhedral distortion. Bond-valence site energy (BVSE) analysis on the average structure reveals that the ionic migration along the palmierite-like layers is comprised of two energy barriers relative to the populations of the average oxygen crystallographic sites and to the distortion of the flexible M1O_x units. The compound with x = 0.1, Ba₃Nb_0.9V_0.1MoO_8.5, shows the lowest activation energy and a high bulk ionic conductivity of ∼0.01 S cm^–1 at 600 °C, almost one order of magnitude higher than the bulk conductivity of the parent compound. Ba₃Nb_0.9V_0.1MoO_8.5 presents predominant ionic conductivity and good stability in a wide oxygen partial pressure range, making it a promising candidate for solid electrolyte applications.