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 deriv-ative Ba3NbMoO8.5. This system exhibits considerable anion and cation disorder. Oxygen disorder enables the ionic conduc-tion 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 struc-tural properties of the series Ba3Nb1-xVxMoO8.5 (x = 0.0, 0.1, 0.2, 0.3, 0.4). Neutron diffraction data evidence that substitution of Nb5+ by V5+ leads to an increase in the average concentration of lower coordination M1Ox units, which is also accompanied by an increase in polyhedral distortion. Bond-valence site energy (BVSE) calculations on the average structure reveal that the ionic migration along the palmierite-like layers is comprised by two energy barriers relative to the populations of the average oxygen crystallographic sites and to the distortion of the flexible M1Ox units. The compound with x = 0.1, Ba3Nb0.9V0.1MoO8.5, shows the lowest activation energy, and high bulk ionic conductivity: ~ 0.01 S cm-1 at 600 °C, almost one order of magnitude higher than the bulk conductivity of the parent compound. Ba3Nb0.9V0.1MoO8.5 presents predominant ionic conductivity and good stability in a wide oxygen partial pressure range, making it a promising candidate for solid electrolyte applications.