Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties and the discovery of new materials is a challenging research focus. Here we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1) and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell which will combine the advantages of both oxide ion and proton conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

Oxide ion conductors are important materials with a range of technological applications and are currently used as electrolytes for solid oxide fuel cells (SOFCs) and solid oxide electrolyser cells (SOECs). Here we report the crystal structure and electrical properties of the hexagonal perovskite derivative Ba3MoNbO8.5. Ba3MoNbO8.5 crystallises in a hybrid of the 9R hexagonal perovskite and palmierite structures. This is a new and so far unique crystal structure that contains a disordered distribution of (Mo/Nb)O6 octahedra and (Mo/Nb)O4 tetrahedra.Ba3MoNbO8.5 shows a wide stability range and exhibits predominantly oxide ion conduction over a pO2 range of 10 -20 -1 atm with a bulk conductivity of 2.2 x 10 -3 S cm -1 at 600 C. The high level of conductivity in a new structure family suggests that further study of hexagonal perovskite derivatives containing mixed tetrahedral and octahedral geometry could open up new horizons in the design of oxygen conducting electrolytes.

A variable temperature neutron diffraction study of the novel oxide ion conductor Ba 3 MoNbO 8.5 has been performed between 25 °C and 600 °C. Non-monotonic behaviour of the cell parameters, bond lengths and angles are observed indicating a structural rearrangement above 300 °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 °C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)O 4 tetrahedra within the average structure of Ba 3 MoNbO 8.5 is observed. The increase in the number of (Mo/Nb)O4 tetrahedra upon heating from 300-600 °C most likely offers more low energy transition paths for transport of the O 2ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, enhancing the ionic conductivity. The second order Jahn-Teller effect most likely further enhances the distortion of the MO 4 /MO 6 polyhedra as distortions created by both electronic and structural effects are mutually supportive.

The crystal structure of the novel oxide ion conductor Ba3WNbO8.5.

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 Nb 5+ by V 5+ 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. ASSOCIATED CONTENTSupporting Information. The supporting information includes figures and tables of crystallographic data, figures showing electrical data, SEM micrographs and thermogravimetric data.