Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

Hexagonal perovskite-related oxides have garnered a great deal of research interest because of their high oxide-ion conductivity at intermediate temperatures, with Ba7Nb4MoO20 being a notable example. However, concomitant proton conduction in Ba7Nb4MoO20 may cause a decrease in power efficiency when used as the electrolyte in conventional solid oxide fuel cells. Here, through investigations of the transport and structural properties of Ba7Nb4–x W x MoO20+x/2 (x = 0–0.25), we show that the aliovalent substitution of Nb5+ by W6+ not only increases the oxide-ion conductivity but also dramatically lowers proton conductivity. The highest conductivity is achieved for x = 0.15 composition, with 2.2 × 10–2 S cm–1 at 600 °C, 2.2 times higher than that of pristine Ba7Nb4MoO20. The proton transport number of Ba7Nb3.85W0.15MoO20.075 is smaller compared with Ba7Nb4MoO20, Ba7Nb3.9Mo1.1O20.05, and Ba7Ta3.7Mo1.3O20.15. The structure analyses of neutron diffraction data of Ba7Nb3.85W0.15MoO20.075 at 25 and 800 °C reveal that the aliovalent W6+ doping introduces interstitial oxide ions in the intrinsically oxygen-deficient c′ layers, thereby simultaneously increasing the carrier concentration for oxide-ion conduction and decreasing oxygen vacancies responsible for dissociative absorption of water. Neutron scattering length density distribution was examined using the maximum-entropy method and neutron diffraction data at 800 °C, which indicates the interstitialcy oxide-ion diffusion in the c′ layers of Ba7Nb3.85W0.15MoO20.075. Ba7Nb3.85W0.15MoO20.075 exhibits extremely high chemical and electrical stability in the wide oxygen partial pressure P(O2) region [ex. 10–23 ≤ P(O2) ≤ 1 atm at 903 °C]. The present results offer a strategy for developing pure oxide-ion conducting hexagonal perovskite-related oxides for possible industrial applications.

In this work, we have discovered Ca 3 Ga 4 O 9 as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (E b ) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca 3 Ga 4 O 9 as a candidate of oxide-ion conductors, because Ca 3 Ga 4 O 9 had a relatively low E b . Ca 3 Ga 4 O 9 was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca 3 Ga 4 O 9 indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca 18 and (Ga 4 O 9 ) 6 units where the (Ga 4 O 9 ) 6 units form the two-dimensional (2D) corner-sharing GaO 4 tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca 3 Ga 4 O 9 , which indicates that the major carrier of Ca 3 Ga 4 O 9 is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10 −5 S cm −1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca 3 Ga 4 O 9 sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca 3 Ga 4 O 9 indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga 4 O 9 ) 6 unit]. It was found that the structural and transport properties of Ca 3 Ga 4 O 9 are similar to those of LaSrGa 3 O 7 melilite.

The high efficiency and fuel flexibility of SOFCs make them a promising technology for clean energy conversion. The development of SOFC devices requires materials with high oxide-ion conductivity and chemical and electrical stability. [2,3,8,9] However, commercially available oxide-ion electrolytes such as yttria-stabilized zirconia show sufficient conductivity only at temperatures above 700 °C, limiting the practical application of SOFCs in a wide range of fields. Therefore, there is a strong need for alternative materials with high ion conductivity at intermediate temperatures (300-600 °C).Discovery of oxide-ion conductors with new structures has been a challenging task. Since ion conduction in solid oxides is fundamentally related to their underlying crystal structures, high oxide-ion conductivity has been reported in a limited number of structure families. Examples include perovskites, [10][11][12][13][14] apatites, [15,16] fluorites, [17,18] and melilites, [19,20] among which perovskite-type oxides are one of the most well-studied structural families to date. The perovskite-type and related oxides can be broadly classified into four structural groups: a) AMO 3 perovskite-type, b) AMO 3 -related, c) hexagonal perovskite-related, and d) modular structures. [21] Here, A and M represent, respectively, large and small cations. Many AMO 3 perovskites have been reported to be excellent oxide-ion conductors, such as LaGaO 3 -based and Na 0.5 Bi 0.5 TiO 3 -based materials. [11,12] Several compounds crystallizing in the AMX 3 -related (e.g., a double perovskite PrBaCo 2 O 6−δ ; δ is the amount of oxygen deficiency) and modular structures (e.g., Dion-Jacobson phase CsBi 2 Ti 2 NbO 10−δ ) have also been reported as promising oxide-ion conductors. [22][23][24][25][26][27][28][29] Hexagonal perovskite-related oxides are composed of hexagonal close-packed AO 3 (h) layers or sequences of hexagonal and cubic close-packed AO 3 (c) layers. [21,[30][31][32] Oxygendeficient AO 3−δ layers can be formed for both hexagonal and cubic layers (labeled h′ and c′, respectively). Despite the variety of crystal structures, pure oxide-ion conduction in hexagonal perovskite-related oxides has been limited so far, although electronic, proton, and mixed (oxide-ion and electronic) conductors have been reported in several compounds. [33][34][35][36] In 2016, significant oxide-ion conductivity was reported in Ba 3 NbMoO 8.5 , Solid oxide-ion conductors are crucial for enabling clean and efficient energy devices such as solid oxide fuel cells. Hexagonal perovskite-related oxides have been placed at the forefront of high-performance oxide-ion conductors, with Ba 7 Nb 4−x Mo 1+x O 20+x/2 (x = 0−0.1) being an archetypal example. Herein, high oxide-ion conductivity and stability under reducing conditions in Ba 7 Ta 3.7 Mo 1.3 O 20.15 are reported by investigating the solid solutions Ba 7 Ta 4-x Mo 1+x O 20+x/2 (x = 0.2−0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba 7 Ta 3.7 Mo 1.3 O 20.15 , leading to a high level...

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 .