Solid oxide fuel cell electrolyte: The synthesis and X-ray diffraction refinement of the Ba3MoNb1-xVxO8.5 series

Abstract

The Ba3MoNb1-xVxO8.5 family exhibits significant oxide ion conductivity, and is thus interesting in the potential use of solid oxide fuel cells as an electrolyte. Its structure contains palmierite-like layers consisting of metallic cations forming octahedral and tetrahedral polyhedra, through which the oxide ions can be conducted. Furthermore, the cationic vacancies present in the structure lead to complex stacking configurations beneficial to the conductivity. In this work the Ba3MoNb1-xVxO8.5 family, with x = 0.0, 0.1, 0.2, 0.3, 0.4, have been synthesised and analysed with the use of X-ray diffraction and subsequent refinement of their structures. The synthesis for each sample was performed successfully, and the first calcination step was further identified to bring a higher yield under higher temperature and longer time of the calcination. From the LeBail refinement the a lattice parameter was shown to correlate with Vegard's law, decreasing with an increase of substitution of Nb5+ with V5+, whilst the c parameter expands upon substitution of the smaller cation. This is hypothesised to be caused by repulsion, either coming from distortions caused by second-order Jahn Teller effect, or due to more tetrahedral coordination. Finally, the Rietveld refinement has given insight into the location and occupation of the metallic cations. V5+ has been incorporated into the solid solution, and sits on the same crystallographic sites as the other metallic cations, choosing to split off to either one of the two sites, thereby contradicting earlier literature. Furthermore, the vanadium doping also seems to move the overall concentration of Mo6+ and Nb5+ to one site in particular. These results have thereby given new insight into this family of solid electrolytes.