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.

Thermogravimetric analysis linked to mass spectrometry (TGA-MS) shows changes in mass and identifies gases evolved when a material is heated. Heating to 600 degrees C enabled samples of bone to be classified as having a high (cod clythrum, deer antler, and whale periotic fin bone) or a low (porpoise ear bone, whale tympanic bulla, and whale ear bone) proportion of organic material. At higher temperatures, the mineral phase of the bone decomposed. High temperature X-ray diffraction (HTXRD) showed that the main solids produced by decomposition of mineral (in air or argon at 800 degrees C to 1000 degrees C) were beta-tricalcium phosphate (TCP) and hydroxyapatite (HAP), in deer antler, and CaO and HAP, in whale tympanic bulla. In carbon dioxide, the decomposition was retarded, indicating that the changes observed in air and argon were a result of the loss of carbonate ions from the mineral. Fourier transform infrared (FTIR) spectroscopy of bones heated to different temperatures, showed that loss of carbon dioxide (as a result of decomposition of carbonate ions) was accompanied by the appearance of hydroxide ions. These results can be explained if the structure of bone mineral is represented by [Formula: see text] where V(Ca) and V(OH) correspond to vacancies on the calcium and hydroxide sites, respectively, and 2-x-y = 0.4. This general formula is consistent in describing both mature bone mineral (i.e., whale bone), with a high Ca/P molar ratio, lower HPO4(2-) content, and higher CO3(2-) content, and immature bone mineral (i.e., deer antler), with a low Ca/P ratio, higher HPO4(2-), and lower CO3(2-) content.

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.

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