Polymorphism and Oxide Ion Migration Pathways in Fluorite-Type Bismuth Vanadate, Bi₄₆V₈O₈₉

Abstract

We report the synthesis, structural characterization, and ionic conductivity measurements for a new polymorph of bismuth vanadate Bi₄₆V₈O₈₉, and an <i>ab initio</i> molecular dynamics study of this oxide ion conductor. Structure determination was carried out using synchrotron powder X-ray and neutron diffraction data; it was found that β-Bi₄₆V₈O₈₉ crystallizes in space group <i>C</i>2/<i>m</i> and that the key differences between this and the previously reported α-form are the distribution of Bi and V cations and the arrangement of the VO₄ coordination polyhedra in structure. β-Bi₄₆V₈O₈₉ exhibits good oxide ion conductivity, with σ = 0.01–0.1 S/cm between 600 and 850 °C, which is about an order of magnitude higher than yttria stabilized zirconia. The <i>ab initio</i> molecular dynamics simulations suggest that the ion migration pathways include vacancy diffusion through the Bi–O sublattice, as well as the O^2– exchanges between the Bi–O and the V–O sublattices, facilitated by the variability of the vanadium coordination environment and the rotational freedom of the VO_<i>x</i> coordination polyhedra