The relationship between oxide-ion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba3VWO8.5

Significant oxide ionic conductivity has recently been reported in cation-deficient hexagonal perovskite Ba3M ' M '' O(8.5)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 Ba(3)VWO(8.5)is also an oxide ion conductor with a bulk conductivity of 2.0 x 10(-3)S cm(-1)in air at 900 degrees C, thus revealing that it is possible to obtain oxide ion conducting Ba3M ' M '' O(8.5)materials with a variety of different M ' M '' combinations. Whilst Ba(3)NbMoO(8.5)and Ba(3)NbWO(8.5)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 M1O(x)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 thec-axis in Ba(3)VWO(8.5)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 thec-axis provide a relevant contribution to the conductivity of these Ba3M ' M '' O(8.5)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.