High oxide ion and proton conductivity in a disordered hexagonal perovskite.

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 that 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.

Hexagonal perovskite derivatives: a new direction in the design of oxide ion conducting materials.

Various structural families have been reported to support oxide ion conductivity; among these, perovskite conductors have received particular attention. The perovskite structure is generally composed of a framework of corner-sharing octahedral units. When the octahedral units share their faces, hexagonal perovskites are formed. Mixed combinations of corner-sharing and face-sharing octahedral units can give rise to a variety of hexagonal perovskite derivatives. However, the ionic conducting properties of these materials have not been well explored. In this feature article, we review the conducting properties of the most significant hexagonal perovskite derivatives, with special focus on Ba3MM'O8.5. Ba3MM'O8.5 is the first hexagonal perovskite derivative to exhibit substantial oxide ion conductivity, and here we outline the structural features that are key for the oxide ion conduction within this system. The results demonstrate that further investigation of hexagonal perovskite derivatives could open up new directions in the design of oxide ion conductors.

Relationship between the Crystal Structure and Electrical Properties of Oxide Ion Conducting Ba3W1.2Nb0.8O8.6.

The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesized as part of an investigation into the new class of Ba3M'M''O8.5 (M' = W, Mo; M'' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba3W1+ xNb1- xO8.5+ x/2 leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 °C, the ionic conductivity of Ba3W1.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5, and Ba3W1.2Nb0.8O8.6 are very similar (σb = 0.0022, 0.0017, and 0.0016 S cm-1, respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba3MoNbO8.5 above 300 °C, but the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba3M'1+ xM''1- xO8.5+ x/2 (M' = W, Mo; M'' = Nb) phases and the number of tetrahedrally coordinated M' and M ″ cations present within the crystal structure.