Ba 3 VWO 8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10 −5 S cm −1 at 600 °C. Ba 3 VWO 8.5 is anomalous to the other Ba 3 M′M″O 8.5 (M′ = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba 3 VWO 8.5 , which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba 3 M′M″O 8.5 are dictated by water absorption. The significant water uptake in Ba 3 M′M″O 8.5 (M′ = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V 5+ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.
The
hexagonal perovskite derivatives Ba3NbMoO8.5, Ba3NbWO8.5, and Ba3VWO8.5 have recently been reported to exhibit significant oxide ion conductivity.
Here, we report the synthesis and crystal structure of the hexagonal
perovskite derivative Ba3–x
VMoO8.5–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″O8.5 family, Ba3–x
VMoO8.5–x
is not
stoichiometric and both barium and oxygen vacancies are present. Although
synthesized in air at elevated temperatures, Ba3–x
VMoO8.5–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 Ba3–x
VMoO8.5–x
.
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