Anisotropy in oxide ion conductivity of Bi 4V 2− xCo xO 11− δ

Abstract

Cobalt-substituted bismuth vanadates with composition Bi 4V 2− x Co x O 11− δ ( x=0.0–0.2) were synthesized in single- and polycrystalline forms. It was confirmed that 10 at.% ( x=0.2) substitution of cobalt for vanadium stabilized the high temperature γ-phase, which exibits a very high oxide ion conductivity, to room temperature by X-ray diffraction and differential thermal analysis. Results of X-ray photoelectron spectroscopy study revealed that there was no change in valence states of Bi and V due to the Co substitution; 3+ for Bi and 5+ for V, respectively. Conductivity measurements on single crystalline samples confirmed a high anisotropy of the ionic conductivity between the directions parallel and perpendicular to (Bi 2O 2) 2+ layer. 10 at.% substitution of cobalt for vanidium enhanced the oxide ion conductivity in the direction parallel to the (Bi 2O 2) 2+ layer ( ab-direction), more than 3 orders of magnitude below 400°C for single-crystalline samples, while 1.5 orders of magnitude for the polycrystalline sample.