Oxide-Ion Occupational Disorder, Diffusion Path, and Conductivity in Hexagonal Perovskite Derivatives Ba₃WNbO_8.5 and Ba₃MoNbO_8.5

Abstract

Hexagonal perovskite derivatives Ba 3 MNbO 8.5 (M: W and Mo) are attracting much interest due to high oxide-ion conductivity and potential use for many applications. This work shows the electrical conductivities of Ba 3 WNbO 8.5 (3.7 × 10 −2 S cm −1 ) and Ba 3 MoNbO 8.5 (8.8 × 10 −2 S cm −1 ) at 900 °C and confirms higher activation energy for conductivity of Ba 3 WNbO 8.5 than that of Ba 3 MoNbO 8.5 . Key factors governing the conductivity and activation energy are the ratio of tetrahedral O3 to octahedral O2 oxide ions and diffusion pathways in Ba 3 MNbO 8.5 . However, the O2/O3 disorders and oxide-ion diffusion paths are unresolved important issues in Ba 3 MNbO 8.5 . Here, Rietveld and maximumentropy method (MEM) analyses of in situ neutron-diffraction data up to 800 °C were performed to obtain the crystal structure and neutron scattering length densities (NSLDs) of Ba 3 WNbO 8.5 . MEM NSLDs show two-dimensional oxide-ion migration through the octahedral O2 and tetrahedral O3 sites in the intrinsically oxygen-deficient layer. Numbers of the interstitial O3 and lattice O2 atoms n(O3) and n(O2) increase and decrease, respectively, with increasing temperature, which indicates that the O2/O3 disorder is more prominent at high temperatures. The O2/O3 disordering makes the minimum NSLD on the O2−O3 path higher, which enhances oxide-ion conductivity, leading to higher activation energies of Ba 3 WNbO 8.5 compared with Ba 3 MoNbO 8.5 .

Simultaneous Reduction of Proton Conductivity and Enhancement of Oxide-Ion Conductivity by Aliovalent Doping in Ba7Nb4MoO20

“…Neutron diffraction experiments clarified unique two-dimensional (2D) oxide-ion diffusion pathways in this material . Subsequent material explorations have led to the discovery of structurally related oxide-ion conductors such as Ba 3 Nb 1– x V x MoO 8.5 , Ba 3 NbWO 8.5 , and Ba 3 VWO 8.5 . − …”

Different Local Structures of Mo and Nb Polyhedra in the Oxide-Ion-Conducting Hexagonal Perovskite-Related Oxide Ba3MoNbO8.5 Revealed by 95Mo and 93Nb NMR Measurements

“…The development of solid oxide-ion (O 2– ) conductors has led to a range of energy and environmental technologies, including gas sensors, solid oxide fuel cells (SOFCs), and oxygen separation membranes. Research on the high energy efficiency of SOFCs at low temperatures has led to the discovery of new oxide-ion conductors and an improved understanding of the diffusion mechanisms of oxide ions. − Recently, it has been reported that hexagonal perovskite-related materials such as Ba 3 MoNbO 8.5 and the related materials, − which are oxide-ion conductors, as well as Ba 7 Nb 4 MoO 20 , and Ba 7 Ta 3.7 Mo 1.3 O 20.15 , which are mixed conductors of oxide ions and protons, show high ion conductivity. For example, Ba 3 MoNbO 8.5 exhibits oxide-ion conduction over a p (O 2 ) range of 10 –20 –1 atm with a bulk conductivity of 2.2 × 10 –3 S cm –1 at 600 °C.…”

“… , Oxide ions were also found to migrate two-dimensionally through the mixed oxygen sites of O2 octahedra and O3 tetrahedra on the O2–O2–O2 faces of the M1O 5−ε polyhedra, where ε is the oxygen vacancy concentration . The O2/O3 disordering makes the minimum neutron scattering length densities on the O2–O3 path higher, which enhances oxide-ion conductivity, leading to higher activation energies of Ba 3 WNbO 8.5 compared to that of Ba 3 MoNbO 8.5 . Recently, modeling using a combination of Bragg, D ( r ), and F ( q ) data from neutron scattering experiments suggested a preference for M1O 5 polyhedra in Ba 3 MoNbO 8.5 .…”

“…In this study, the coordination numbers of the M cations were determined by using a cutoff length based on the M–O distance histograms obtained from reverse Monte Carlo simulations. The variable coordination environment and coordination dynamics of the M1 site in Ba 3 MoNbO 8.5 are believed to be important for creating a low-energy transfer pathway for oxide ions; however, these structures are usually studied by using X-ray diffraction, neutron scattering, and electron diffraction, assuming an even distribution of Mo and Nb at each site and analyzed as if there are averaged Mo and Nb atoms, although there are no averaged Mo/Nb atoms. − This is because X-rays, neutrons, and electrons all have a close scattering ability, thereby making the distinction between Nb and Mo difficult. In addition, the reverse Monte Carlo simulation is model-dependent.…”

“…15 The O2/O3 disordering makes the minimum neutron scattering length densities on the O2−O3 path higher, which enhances oxide-ion conductivity, leading to higher activation energies of Ba 3 WNbO 8.5 compared to that of Ba 3 MoNbO 8.5 . 12 Recently, modeling using a combination of Bragg, D(r), and F(q) data from neutron scattering experiments suggested a preference for M1O 5 polyhedra in Ba 3 MoNbO 8.5 . 16 In this study, the coordination numbers of the M cations were determined by using a cutoff length based on the M−O distance histograms obtained from reverse Monte Carlo simulations.…”