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.

No abstract

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.

We report on the unconventional magnetism in the cubic B-site ordered double perovskite Ba 2 YMoO 6 , using ac and dc magnetic susceptibility, heat capacity and muon spin rotation. No magnetic order is observed down to 2 K while the Weiss temperature is $ À 160 K. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate Mo 5þ s ¼ 1=2 spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare example of a valence bond glass.

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.

A sizeable negative magnetoresistance (MR) has been observed for oxypnictides LnOMnAs (Ln = La,Nd). MR up to -24% is observed at 200 K for LaOMnAs which is unprecedented for divalent Mn(2+). Both materials are weak ferromagnets with transition temperatures above room temperature.

A variable temperature neutron and synchrotron diffraction study has been performed on the giant magnetoresistant oxypnictides LMnAsO (L = La, Nd). The low-temperature magnetic structures have been studied, and results show a spin reorientation of the Mn 2+ spins below T N (Nd) for NdMnAsO. The Mn 2+ spins rotate from alignment along c to alignment into the basal plane, and the Mn 2+ and Nd 3+ moments refine to 3.54(4) μ B and 1.93(4) μ B , respectively, at 2 K. In contrast, there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K; however, discontinuities in the cell volume and L-O and Mn-As bond lengths are detected at ∼150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.