The relationship between oxide-ion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba₃VWO_8.5

Significant oxide ionic conductivity has recently been reported in cation-deficient hexagonal perovskite Ba3M’M’’O8.5 derivatives (M’ = Nb; M’’ = Mo, W), with disordered hybrid 9R-palmierite average structures. Here, we present a study of the crystal structure and electrical properties of the related compound Ba3VWO8.5. Electrical characterization demonstrates that Ba3VWO8.5 is also an oxide ion conductor with a bulk conductivity of 2.0 × 10-3 S cm-1 in air at 900 °C, thus revealing that it is possible to obtain oxide ion conducting Ba3M’M’’O8.5 materials with a variety of different M’M’’ combinations. Whilst Ba3NbMoO8.5 and Ba3NbWO8.5 present a random distribution of cationic vacancies, X-ray and neutron diffraction experiments demonstrate that the cationic vacancies are ordered on the M2 sites in Ba3VWO8.5, resulting in a structure where M1Ox palmierite-like layers are separated by empty octahedral cavities. Bond-valence site energy (BVSE) analysis on the different phases reveals that ordering of the cationic vacancies hinders long-range oxygen diffusivity parallel to the c-axis in Ba3VWO8.5 explaining the reduced ionic conductivity of this compound. These results suggest that, together with the dominant 2-dimensional conduction pathway along the palmierite-like layers, additional diffusion routes parallel to the c-axis provide a relevant contribution to the conductivity of these Ba3M’M’’O8.5 systems by creation of a complex 3-dimensional ionic percolation network, the topology of which depends on the particular arrangement of cation and anion vacancies.