Oxide-ion conduction in the Dion-Jacobson phase CsBi2Ti2NbO10-δ

Defect Structure, Oxygen Ion Conduction, and Conducting Mechanism in Ruddlesden-Popper Sr3Zr2-xMxO7-0.5x (M = Ga, Y, In)

Ruddlesden-Popper (RP)-structured materials based on transition metals with a variable valence, such as Fe, Mn, Ni, and so on, have been well documented for their potential of being used as electrodes in solid-oxide fuel cells. However, RP materials with pure or dominant ionic conduction are rare. Here, a series of Zr-based RP materials Sr3Zr2-xMxO7-0.5x (M = Ga, Y, In) with electrical conductivity as high as 3.25 × 10-3 S cm-1 at 900 °C in air was reported, which represents the highest conductivity for the Zr-based RP materials and is comparable to that of the recently reported In-based RP oxide-ion conductors, such as NdBaInO4-based and La2BaIn2O7-based materials. Under low oxygen partial pressure (pO2), the doped samples show pure ionic conducting behaviors without n-type electronic conductivity. The defect formation energies, local structure around the oxygen vacancies, and oxide-ion-conducting mechanism of the acceptor-doped Sr3Zr2O7-based materials were studied for the first time. The results revealed a two-dimensional oxide ion migration characteristic within the perovskite slabs. This work therefore provides a good reference for developing new oxide-ion conductors in the Zr-based RP-structured materials.

Atomic Layer Engineering of Ferroelectricity in Dion-Jacobson Perovskites

Recent advances in "hybrid-improper" ferroelectricity in Dion-Jacobson (DJ)-type layered perovskites have caused renewed interest in the search for new ferroelectrics. Here, we present an approach for the tailored synthesis of a new homologous series of DJ-type layered perovskites Cs(Bi2Srn-3)(Tin-1Nb)O3n+1. Starting from CsBi2Ti2NbO10 (n = 3), higher-order homologous phases with n = 4 and 5 were successfully synthesized by repeated solid-state calcination with SrTiO3. Characterizations by X-ray diffraction, electron diffraction, transmission electron microscopy, Raman scattering, and second harmonic generation showed the detailed structural features in Cs(Bi2Srn-3)(Tin-1Nb)O3n+1, and the polar structures could be stabilized by proper or hybrid-improper ferroelectricity, depending on the odd or even number of the perovskite layers. Our results provide important insights into the competition between the different mechanisms and the consequences of the ferroelectric properties in homologous layered perovskites.

Computational Insights into Dion-Jacobson Type Oxide Ion Conductors

Dion-Jacobson type materials have recently emerged as a new structural family of oxide ion conductors, materials important for applications in a variety of electrochemical devices. While some attempts to improve their ionic conductivity have been reported, a detailed understanding of the underlying oxide ion diffusion mechanisms in these materials is still missing. To explore the structure-property relationships leading to the favorable properties, we carried out ab initio molecular dynamics simulations of oxide ion diffusion in CsBi2Ti2NbO10-δ. Our computational study reveals significant out-of-plane dynamics, indicating that the dominant pathway for oxide ion migration is via jumps into and out of the (ab) crystallographic plane. This suggests that further improvement of oxide ion conductivity relative to CsBi2Ti2NbO10-δ could be achieved by enhancing the rotational flexibility of the coordination polyhedra located in the inner perovskite layer, thereby facilitating faster out-of-plane motions.

High Conductivity and Diffusion Mechanism of Oxide Ions in Triple Fluorite-Like Layers of Oxyhalides

Oxide ion conductors are attractive materials because of their wide range of applications, such as solid oxide fuel cells. Oxide ion conduction in oxyhalides (compounds containing both oxide ions and halide ions) is rare. In the present work, we found that Sillén oxychlorides, Bi2-xTexLuO4+x/2Cl (x = 0, 0.1, and 0.2), show high oxide ion conductivity. The bulk conductivity of Bi1.9Te0.1LuO4.05Cl reaches 10-2 S cm-1 at 431 °C, which is much lower than 644 °C of yttria-stabilized zirconia (YSZ) and 534 °C of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM). Thanks to the low activation energy, Bi1.9Te0.1LuO4.05Cl exhibits a high bulk conductivity of 1.5 × 10-3 S cm-1 even at a low temperature of 310 °C, which is 204 times higher than that of YSZ. The low activation energy is attributed to the interstitialcy oxide ion diffusion in the triple fluorite-like layer, as evidenced by neutron diffraction experiments (Rietveld and neutron scattering length density analyses), bond valence-based energy calculations, static DFT calculations, and ab initio molecular dynamics simulations. The electrical conductivity of Bi1.9Te0.1LuO4.05Cl is almost independent of the oxygen partial pressure from 10-18 to 10-4 atm at 431 °C, indicating the electrolyte domain. Bi1.9Te0.1LuO4.05Cl also exhibits high chemical stability under a CO2 flow and ambient air at 400 °C. The oxide ion conduction due to the two-dimensional interstitialcy diffusion is considered to be common in Sillén oxyhalides with triple fluorite-like layers, such as Bi1.9Te0.1RO4.05Cl (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) and Bi6-2xTe2xO8+xBr2 (x = 0.1, 0.5). The present study opens a new field of materials chemistry: oxide ion-conducting Sillén oxyhalides with triple fluorite-like layers.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH₅ (PO₄ )₃ (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10^-2  S cm^-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Recent developments in oxide ion conductors: focusing on Dion-Jacobson phases

Oxide-ion conductors, also known as "oxygen ion conductors," have garnered significant attention in recent years due to their extensive applications in a variety of electrochemical devices, including oxygen concentrators, solid-oxide fuel cells (SOFCs), and solid oxide electrolysis cells. The key to improving the performance of these devices is the creation of novel oxide-ion conductors. In this feature article, we discuss the recent developments of new structural families of oxide-ion conductors and of the Dion-Jacobson-type layered oxide-ion conductors with a particular emphasis on CsM₂Ti₂NbO_10-δ (M = Bi and lanthanoids; δ represents oxygen-vacancy content) and their solid solutions. CsBi₂Ti₂NbO_10-δ is the first example of an oxide-ion conductor with a Dion-Jacobson-type layered perovskite structure, and the structural characteristics of these materials are extracted here. We have proposed an original concept that the large sized Cs⁺ cations and M³⁺ displacements yield the large bottlenecks for oxide-ion migration, which would facilitate the discovery of novel oxide-ion conductors. This article presents evidence that Dion-Jacobson-type layered perovskites are superior oxide-ion conductors. We also demonstrate how the information gleaned from these studies can be applied to the design of novel oxide-ion conductors.

Proton and Oxide Ion Conductivity in Palmierite Oxides

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A₃V₂O₈ (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO₄ units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t _H ∼ 0.6-0.8) and high protonic mobility. In particular, the proton conductivity of Sr₃V₂O₈ is 1.0 × 10^-4 S cm^-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO₄ units, which can stabilize oxygen defects via formation of V₂O₇ dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

Ge-Containing Oxide-Ion Conductors with CaEu2Ge3O10-Type Structure Discovered by the Bond-Valence Method and Experiments

In the present work, we have discovered the first example of a CaEu₂Ge₃O₁₀-type oxide-ion conductor, Ca_1.05Sm_1.95Ge₃O_9.975. The CaEu₂Ge₃O₁₀-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu₂Ge₃O₁₀-type CaEu₂Ge₃O₁₀, CaGd₂Ge₃O₁₀, and a new material CaSm₂Ge₃O₁₀ were synthesized. CaSm₂Ge₃O₁₀ showed the highest electrical conductivity among these three materials. Ca_1+xSm_2-xGe₃O_10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca_1.05Sm_1.95Ge₃O_9.975 exhibited the highest conductivity of 1.2 × 10^-5 S cm^-1 at 1373 K. Oxygen transport numbers in Ca_1.05Sm_1.95Ge₃O_9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu₂Ge₃O₁₀-type Ca_1.05Sm_1.95Ge₃O_9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm₂Ge₃O₁₀ and Ca_1.05Sm_1.95Ge₃O_9.975 were successfully analyzed by the CaEu₂Ge₃O₁₀-type structure (space group P2₁/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca_1.05Sm_1.95Ge₃O_9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca_1.05Sm_1.95Ge₃O_9.975.

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

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 Ba₇Nb_4-xW_xMoO_20+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 Ba₇Nb_3.85W_0.15MoO_20.075 is smaller compared with Ba₇Nb₄MoO₂₀, Ba₇Nb_3.9Mo_1.1O_20.05, and Ba₇Ta_3.7Mo_1.3O_20.15. The structure analyses of neutron diffraction data of Ba₇Nb_3.85W_0.15MoO_20.075 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 Ba₇Nb_3.85W_0.15MoO_20.075. Ba₇Nb_3.85W_0.15MoO_20.075 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.

High Oxide-Ion Conductivity in a Hexagonal Perovskite-Related Oxide Ba7 Ta3.7 Mo1.3 O20.15 with Cation Site Preference and Interstitial Oxide Ions

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 Ba₇ 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 Ba₇ Ta_3.7 Mo_1.3 O_20.15 are reported by investigating the solid solutions Ba₇ 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 Ba₇ Ta_3.7 Mo_1.3 O_20.15 , leading to a high level of oxide-ion conductivity (e.g., 1.08 × 10^-3 S cm^-1 at 377 °C). The conductivity of Ba₇ Ta_3.7 Mo_1.3 O_20.15 is higher than that of Ba₇ Nb₄ MoO₂₀ and conventional yttria-stabilized zirconia. In contrast to Ba₇ Nb_{4- x} Mo_{1+ x} O_{20+ x /2} (x = 0-0.1), the oxide-ion conduction in Ba₇ Ta_3.7 Mo_1.3 O_20.15 is dominant even in highly reducing atmospheres (e.g., oxygen partial pressure of 1.6 × 10^-24 atm at 909 °C). From structural analyses of the synchrotron X-ray diffraction data for Ba₇ Ta_3.7 Mo_1.3 O_20.15 , contrasting X-ray scattering powers of Ta⁵⁺ and Mo⁶⁺ allow identification of the preferential occupation of Mo⁶⁺ adjacent to the intrinsically oxygen-deficient layers, as supported by DFT calculations. The high conductivity and chemical and electrical stability in Ba₇ Ta_3.7 Mo_1.3 O_20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite-related oxides.

Structure, electrical properties, and conduction mechanism of new germanate mixed Zn-doped In2Ge2O7 conductors

A new germanate mixed electronic and oxide ionic conductor In1.8Zn0.2Ge2O6.9 was developed, which exhibits two-dimensional anisotropic transport nature by oxygen exchange between adjacent Ge2O7 units, with a conductivity of 1.62 × 10–2 S cm−1 at 1000 °C.

Synthesis and Ion-Transport Properties of EuKGe2O6-, Ca3Fe2Ge3O12-, and BaCu2Ge2O7-Type Oxide-Ion Conductors

EuKGe₂O₆-, Ca₃Fe₂Ge₃O₁₂-, and BaCu₂Ge₂O₇-type germanates are synthesized by a conventional solid-state method and characterized to reveal their oxide-ion-conducting properties. Materials of the EuKGe₂O₆ group exhibit oxide-ion conductivity (e.g., 4.6 × 10^-3 S/cm at 973 K for Eu_0.8Ca_0.2KGe₂O_6-δ) and transport numbers above 96%, whereas materials of the Ca₃Fe₂Ge₃O₁₂ and BaCu₂Ge₂O₇ groups exhibit mixed electron-/oxide-ion conduction. Conduction involves oxide-ion vacancies in the EuKGe₂O₆ group, interstitial oxide ions in the Ca₃Fe₂Ge₃O₁₂ group, and both oxide-ion vacancies and interstitial oxide ions in the BaCu₂Ge₂O₇ group. The doping-induced formation of impurity phases decreases the amount of oxide-ion carriers relative to the expected values.

Theoretical Investigation of Proton Diffusion in Dion-Jacobson Layered Perovskite RbBiNb2O7

Perovskite materials are considered to be promising electrolyte membrane candidates for electrochemical applications owing to their excellent proton- or oxide-ion-conducting properties. RbBiNb₂O₇ is a double-layered Dion–Jacobson perovskite oxide, with Pmc2₁ symmetry. In this study, the electronic structure and proton-diffusion properties of bulk RbBiNb₂O₇ were systematically investigated using first-principles calculations. The unique layered crystal structure of RbBiNb₂O₇ plays a crucial role in proton storage and proton conductivity. Different proton-diffusion steps in RbBiNb₂O₇ were considered, and the activation energies of the relevant diffusion steps were evaluated using the climbing image-nudged elastic band (CI-NEB) technique. The proton diffusion in RbBiNb₂O₇ presents a two-dimensional layered characteristic in the a-b plane, owing to its layered crystalline nature. According to the transition state calculations, our results show that the bulk RbBiNb₂O₇ exhibits good proton-transport behavior in the a-b plane, which is better than many perovskite oxides, such as CaTiO₃, CaZrO₃, and SrZrO₃. The proton diffusion in the Rb–O and Nb–O layers is isolated by a higher energy barrier of 0.86 eV. The strong octahedral tilting in RbBiNb₂O₇ would promote proton transport. Our study reveals the microscopic mechanisms of proton conductivity in Dion–Jacobson structured RbBiNb₂O₇, and provides theoretical evidence for its potential application as an electrolyte in solid oxide fuel cells (SOFCs).

Electrical Properties, Defect Structures, and Ionic Conducting Mechanisms in Alkali Tungstate Li2W2O7

Discovery of new high-conductivity solid-state ionic conductors has been a long-lasting interest in the field of solid-state ionics for their important applications in solid-state electrochemical devices. Here, we report the mixed oxide-ion and Li-ion conductions, together with their conducting mechanisms in the Li₂W₂O₇ material with triclinic symmetry. The process for the ionic identity is supported by several electrochemical measurements including electrochemical impedance spectroscopy, DC polarization, oxygen concentration cell, and theoretical analysis of neutron diffraction data and bond-valence-based energy landscape calculations. We show from electrochemical measurements strong evidences of the predominating oxide-ion conducting and minor Li-ion chemistry in Li₂W₂O₇ at high temperatures, while the bond-valence-based energy landscape analysis reveals possible multidimensional ionic migration pathways for both oxide-ions and Li-ions. Thus, the presented results provide fundamental insights into new mixed ionic conduction mechanisms in low-symmetry materials and have implications for discoveries of new ionic conductors in years to come.

Phase Evolution, Electrical Properties, and Conduction Mechanism of Ca12Al14-xGaxO33 (0 ≤ x ≤ 14) Ceramics Synthesized by a Glass Crystallization Method

Mayenite Ca₁₂Al₁₄O₃₃, as an oxide-ion conductor, has the potential of being applied in many fields, such as solid-oxide fuel cells. However, its relatively low oxide-ion conductivity hinders its wide practical applications and thus needs to be further optimized. Herein, a new recently developed glass crystallization route was used to prepare a series of Ga-doped Ca₁₂Al_14-xGa_xO₃₃ (0 ≤ x ≤ 14) materials, which is not accessible by the traditional solid-state reaction method. Phase evolution with the content of gallium, the corresponding structures, and their electrical properties were studied in detail. The X-ray diffraction data revealed that a pure mayenite phase can be obtained for 0 ≤ x ≤ 7, whereas when x > 7, the samples crystallize into a melilite-like orthorhombic Ca₅Ga₆O₁₄-based phase. The electrical conduction studies evidence no apparent enhancement in the total conductivity for compositions 0 ≤ x ≤ 7 with the mayenite phase, and therefore, the rigidity of the framework cations and the width of the windows between cages are not key factors for oxide-ion conductivity in mayenite Ca₁₂Al₁₄O₃₃-based materials, and changing the free oxygen content through aliovalent cation substitution may be the right direction. For compositions with a pure melilite-like orthorhombic phase, the conductivities also mirrored each other and are all slightly higher than those of the mayenite phases. These melilite-like Ca₅Ga₆O₁₄-based materials show mixed Ca-ion, oxide-ion, and electron conduction. Furthermore, the conduction mechanisms of Ca ions and oxide ions in this composition were studied by a bond-valence-based method. The results suggested that Ca-ion conduction is mainly due to the severely underbonded Ca3 ions and that the oxide ions are most likely transported via oxygen vacancies.

High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

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.

Theoretical insights into the diffusion mechanism of alkali ions in Ruddlesden–Popper antiperovskites

The diffusion properties of alkali ions in a series of RP antiperovskites are investigated by density functional theory, which provides a theoretical guide for enhancing the ionic conductivity of solid-state antiperovskite electrolytes.