Investigation of the Relationship between the Structure and Conductivity of the Novel Oxide Ionic Conductor Ba₃MoNbO_8.5

Abstract

A variable temperature neutron diffraction study of the novel oxide ion conductor Ba 3 MoNbO 8.5 has been performed between 25 °C and 600 °C. Non-monotonic behaviour of the cell parameters, bond lengths and angles are observed indicating a structural rearrangement above 300 °C. The oxygen/vacancy distribution changes as the temperature increases so that the ratio of (Mo/Nb)O 4 tetrahedra to (Mo/Nb)O 6 octahedra increases upon heating above 300 °C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)O 4 tetrahedra within the average structure of Ba 3 MoNbO 8.5 is observed. The increase in the number of (Mo/Nb)O4 tetrahedra upon heating from 300-600 °C most likely offers more low energy transition paths for transport of the O 2ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, enhancing the ionic conductivity. The second order Jahn-Teller effect most likely further enhances the distortion of the MO 4 /MO 6 polyhedra as distortions created by both electronic and structural effects are mutually supportive.

High oxide ion and proton conductivity in a disordered hexagonal perovskite

“…The unique ionic conduction properties of Ba7Nb4MoO20 are associated with the disorder within the crystal structure. As analogously observed in the hexagonal perovskite derivative Ba3NbMO8.5 34,48 , the structure exhibits considerable disorder of the oxide ions and of the cation vacancies. This disorder creates variable and dynamic MOx configurations accommodating the intrinsic anion vacancies and the protonic defects in Ba7Nb4MoO20, also assisting the ionic migration.…”

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

“…However, the presence of a continuous connectivity network formed by O1-O2 and O1-O1 pathway segments with accessible relative energy barriers implies long-range diffusion parallel to the c-axis, so that the ionic conductivity in the hybrid structure of Ba 3 M 0 M 00 O 8.5 (M 0 ¼ Nb; M 00 ¼ Mo, W) is 3-dimensional. 23 The disorder in the cation sub-lattice, together with the inherent ability of the cation defects to rearrange themselves in the hybrid structure, 22,24 allow for the formation of a complex and dynamic 3-dimensional ionic percolation network, the topology of which will depend on the average temperature-and timedependent distribution of cationic and anionic vacancies. In contrast, ordering of the cationic vacancies on the M2 sites in Ba 3 VWO 8.5 disrupts the 3-dimensional conduction/percolation network by hindering any long-range oxygen diffusion (O1-O2 + O1-O1) parallel to the c-axis 59,60 so that the conductivity mechanism is two-dimensional.…”

“…21 Variable coordination M1O x polyhedral units are generated within the palmierite-like layers due to considerable positional oxygen disorder. 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.…”

Associating and Tuning Sodium and Oxygen Mixed‐Ion Conduction in Niobium‐Based Perovskites

“…The conductivity of these state‐of‐the‐art oxygen conductors remains lower than the Na + conductivity in NASICON or β‐alumina in the same temperature range . O‐deficient perovskites (general formula ABO 3 ) stand also amongst the best O 2− ionic conductors at relatively low temperatures (NaNb 0.5 Al 0.5 O 2.5 , NaTa 0.5 Al 0.5 O 2.5 , Na 0.5 Bi 0.49 TiO 2.985 and Ba 3 MoNbO 8.5 ), with conductivity varying between 5 × 10 −5 and 5 × 10 −4 S cm −1 at T = 400 °C . As far as the activation energies are concerned, they are 0.7–1 eV for O‐deficient fluorite‐type networks, between 0.4 and 0.5 eV for the perovskite‐derived phases mentioned above.…”