Hexagonal perovskite related oxide ion conductor Ba₃NbMoO_8.5: phase transition, temperature evolution of the local structure and properties

Abstract

Ba3NbMoO8.5 has recently been demonstrated to exhibit competitive oxide ion conductivity and to be stable under reducing conditions, making it an excellent potential electrolyte for solid oxide fuel cells.

The relationship between oxide-ion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba3VWO8.5

“…Large positional oxygen disorder along the O2-O3 layers leads to local disruption of the crystallographic oxygen site congurations and generates 4-, 5-, and 6-fold coordination geometries on the local scale. 23,24 The barium atoms occupy the 6c and 3a Wyckoff positions (Ba1 and Ba2 respectively), while the transition metal cations are at Wyckoff position 6c (M1) and at Wyckoff position 3b (M2). 21,27 Renement of the fractional occupancy of the transition metals in Ba 3 VWO 8.5 from neutron diffraction data is difficult because vanadium has a low scattering cross section for neutrons.…”

“…It is expected that the occurrence of neighbouring tetrahedral and octahedral units, in conjunction with the large positional oxygen disorder, results in disruption of the local oxygen conguration and in the formation of local intermediate 5-fold geometries as reported for Ba 3 NbMoO 8.5 . 23,24 Bond-valence site energy (BVSE) calculations were employed to analyse the migration pathway and energy landscape of the oxide ion conduction in the average crystal structure of Ba 3 VWO 8.5 . Bond-valence based methods have been shown to reproduce well the ionic conduction pathway in several ionic conductors 17,[45][46][47][48][49][50] and have successfully been employed for the screening and discovery of new oxide ion conducting materials.…”

“…Average octahedral and tetrahedral units arise from partial occupation of two crystallographic oxygen positions (O2 and O3), 21,22 with the existence of 5-fold geometries on the localscale. 23,24 The random distribution of intrinsic oxygen vacancies and available oxygen sites results in a continuous 2-dimensional ionic diffusion pathway for oxide migration along the palmierite-like layers. 25 The cationic vacancies are disordered on two mutually exclusive metal sites (M1 and M2), leading to the formation of complex disordered stacking congurations of M1O x and M2O 6 polyhedral units.…”

High oxide ion and proton conductivity in a disordered hexagonal perovskite

“…This disorder creates variable and dynamic MOx configurations accommodating the intrinsic anion vacancies and the protonic defects in Ba7Nb4MoO20, also assisting the ionic migration. The ionic conductivity in these hybrid systems is correlated with the relative average concentration of lower and higher coordination environments along the palmierite-like layers 30,33,48 , indicating that the ionic conductivity properties might be tailored by insertion of cations with preference for lower coordination geometries 49 . Other derivatives able to support ionic conduction could potentially be found among the members of the pseudo-ternary phase diagram of the Ba-Nb-Mo-O system (Supplementary Figure S4).…”

“…The presence of a flexible average tetrahedral environment facilitates proton mobility in Ba7Nb4MoO20, as previously reported for La0.8Ba1.2GaO3.9 10 , γ-Ba4Nb2O9 37 and the solid-acid protonic conductor CsH2PO4 38 . In contrast the related phase Ba3NbMoO8.5 which contains predominantly higher coordination geometries in the palmierite layer does not exhibit proton conductivity 29,33 (Supplementary Figure S21). In addition, the change in crystal structure results in a considerable reduction in the shortest O1-O1 distance above 300 °C (from  3.12 Å to  2.85 Å, Supplementary Figure S29), which may further enhance proton migration.…”