Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A 3 V 2 O 8 (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO 4 units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t H ∼ 0.6−0.8) and high protonic mobility. In particular, the proton conductivity of Sr 3 V 2 O 8 is 1.0 × 10 −4 S cm −1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the threedimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO 4 units, which can stabilize oxygen defects via formation of V 2 O 7 dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

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 .

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy related technologies. Here we report on the discovery of significant proton and oxide ion conductivity in palmierite ox-ides A3V2O8 (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO4 units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (tH = 0.6 – 0.8) and high protonic mobility. In particular, the proton conductivity of Sr3V2O8 is 1.0 x 10-4 S cm-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy driven and is facilitated by rotational motion of the VO4 units, which can stabilize oxygen defects via formation of V2O7 dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.