For the development of proton-based electrolytes, high proton conductivity at intermediate temperatures (300−600 °C) is crucial, but the available materials have been confined to a limited number of the structure families, such as cubic perovskites. Herein, we report Ba 5 Er 2 Al 2 ZrO 13 , a hexagonal perovskite-related oxide, as a new class of proton conductors exhibiting higher conductivities than 10 −3 S cm −1 between 300 and 1200 °C. The protons as charge carriers are found to exist in the inherently oxygen-deficient h′ layer of Ba 5 Er 2 Al 2 ZrO 13 , which are supported by Rietveld analysis of neutron-diffraction data, bond-valencebased energy calculations, and thermogravimetric analysis. Our discovery of a new structure family of proton conductors with the inherently oxygen-deficient h′ layer offers a strategy in designing superior proton conductors based on hexagonal perovskite-related oxides.

While cation order-disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order-disorder transition from the anion-ordered fluorite structure ( P4/ nmm) for larger Ln ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln (Sm-Er). Structural analysis reveals that with the increase of Ln radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn and larger HLn tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.

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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.

We successfully synthesized the ternary bismuthides LaMBi (M = Ti, Zr, Hf). These compounds crystallize in the hexagonal HfSnCu-anti type structure (space group: P6/mcm) consisting of face-sharing MBi octahedral chains and hypervalent Bi linear chains, both separated by La atoms. Magnetic susceptibility and electrical resistivity measurements revealed that all of the compounds, including the solid solution LaTi(BiSb), exhibit a Pauli paramagnetic behavior without any trace of superconductivity down to 1.85 K, as opposed to a recently reported 4 K superconductivity in LaTiSb. The absence of superconductivity is supported by first-principles band calculations of LaTiBi and LaTiSb that demonstrate similar electronic structures with three-dimensional Fermi surfaces.