Investigation of the Crystal Structure and Ionic Pathways of the Hexagonal Perovskite Derivative Ba_3–xVMoO_8.5–x

The hexagonal perovskite derivatives Ba₃NbMoO_8.5, Ba₃NbWO_8.5, and Ba₃VWO_8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba_3–xVMoO_8.5–x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1O_x polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba₃M′M″O_8.5 family, Ba_3–xVMoO_8.5–x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba_3–xVMoO_8.5–x is unstable at lower temperatures, as illustrated by the formation of BaCO₃ and BaMoO₄ by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba_3–xVMoO_8.5–x.