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. Ba₇Nb_3.9Mo_1.1O_20.05 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 Ba₇Nb_3.9Mo_1.1O_20.05, 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 Ba₇Nb_3.9Mo_1.1O_20.05 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.

Powder X-ray diffraction (XRD) has been exploited to establish the structural properties of a porous interpenetrated mixed-ligand metal–organic framework material prepared by solid-state grinding, recognizing that product phases from mechanochemical synthesis are typically microcrystalline powders. The importance of subjecting the powder XRD data to rigorous scrutiny in such applications is emphasized.

Most solid-state materials are composed of p-block anions, only in recent years the introduction of hydride anions (1s²) in oxides (e.g., SrVO₂H, BaTi(O,H)₃) has allowed the discovery of various interesting properties. Here we exploit the large polarizability of hydride anions (H^–) together with chalcogenide (Ch^2–) anions to construct a family of antiperovskites with soft anionic sublattices. The M₃HCh antiperovskites (M = Li, Na) adopt the ideal cubic structure except orthorhombic Na₃HS, despite the large variation in sizes of M and Ch. This unconventional robustness of cubic phase mainly originates from the large size-flexibility of the H^– anion. Theoretical and experimental studies reveal low migration barriers for Li⁺/Na⁺ transport and high ionic conductivity, possibly promoted by a soft phonon mode associated with the rotational motion of HM₆ octahedra in their cubic forms. Aliovalent substitution to create vacancies has further enhanced ionic conductivities of this series of antiperovskites, resulting in Na_2.9H(Se_0.9I_0.1) achieving a high conductivity of ~1 × 10^–4 S/cm (100 °C).

Oxide-ion conductors have found applications in various electrochemical devices, such as solid-oxide fuel cells, gas sensors, and separation membranes. Dion–Jacobson phases are known for their rich magnetic and electrical properties; however, there have been no reports on oxide-ion conduction in this family of materials. Here, for the first time to the best of our knowledge, we show the observation of fast oxygen anionic conducting behavior in CsBi₂Ti₂NbO_10−δ. The bulk ionic conductivity of this Dion–Jacobson phase is 8.9 × 10⁻² S cm⁻¹ at 1073 K, a level that is higher than that of the conventional yttria-stabilized zirconia. The oxygen ion transport is attributable to the large anisotropic thermal motions of oxygen atoms, the presence of oxygen vacancies, and the formation of oxide-ion conducting layers in the crystal structure. The present finding of high oxide-ion conductivity in rare-earth-free CsBi₂Ti₂NbO_10−δ suggests the potential of Dion–Jacobson phases as a platform to identify superior oxide-ion conductors.