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

Abstract

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 Ba₃VWO_8.5. Electrical characterization demonstrates that Ba₃VWO_8.5 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′′O_8.5 materials with a variety of different M′M′′ combinations. Whilst Ba₃NbMoO_8.5 and Ba₃NbWO_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 Ba₃VWO_8.5, 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 the c-axis in Ba₃VWO_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 the c-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.