Relationship between the Crystal Structure and Electrical Properties of Oxide Ion Conducting Ba

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.