Enhanced Oxygen Ion Conductivity and Mechanistic Understanding in Ba<sub>3</sub>Nb<sub>1–<i>x</i></sub>V<i><sub>x</sub></i>MoO<sub>8.5</sub>
Sacha Fop, Kirstie McCombie, Ronald I. Smith, Abbie C. Mclaughlin
Abstract
Significant oxide ion conductivity has recently been reported in the cation-deficient hexagonal perovskite derivative Ba3NbMoO8.5. 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 Ba3Nb1–xVxMoO8.5 (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 M1Ox 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 M1Ox units. The compound with x = 0.1, Ba3Nb0.9V0.1MoO8.5, 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. 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.