Hexagonal perovskite derivatives: a new direction in the design of oxide ion conducting materials

Strategic Design and Insights into Lanthanum and Strontium Perovskite Oxides for Oxygen Reduction and Oxygen Evolution Reactions

Perovskite oxides exhibit bifunctional activity for both oxygen reduction (ORR) and oxygen evolution reactions (OER), making them prime candidates for energy conversion in applications like fuel cells and metal-air batteries. Their intrinsic catalytic prowess, combined with low-cost, abundance, and diversity, positions them as compelling alternatives to noble metal and metal oxides catalysts. This review encapsulates the nuances of perovskite oxide structures and synthesis techniques, providing insight into pivotal active sites that underscore their bifunctional behavior. The focus centers on the breakthroughs surrounding lanthanum (La) and strontium (Sr)-based perovskite oxides, specifically their roles in zinc-air batteries (ZABs). An introduction to the mechanisms of ORR and OER is provided. Moreover, the light is shed on strategies and determinants central to optimizing the bifunctional performance of La and Sr-based perovskite oxides.

In Situ Formation of Ba3CoNb2O9/Ba5Nb4O15 Heterostructure in Electrolytes for Enhancing Proton Conductivity and SOFC Performance

The in situ formation of a heterostructure delivers superior electrochemical properties as compared to the mechanical mixing, which shows great promise for developing new electrolytes for solid oxide fuel cells (SOFCs). Herein, in an SOFC constructed by the Ba5Nb4O15 electrolyte and Ni0.8Co0.15Al0.05LiO2-δ anode, an in situ formation of Ba3CoNb2O9/Ba5Nb4O15 heterostructure is designed by Co-ion diffusion from the anode to the electrolyte during cell operation, resulting in improved ion conductivity and fuel cell performance. An abnormal phenomenon is observed that the SOFC based on the Ba3CoNb2O9/Ba5Nb4O15 electrolyte delivered a peak power density of 703 mW/cm2 at 510 °C, which is higher than that at 550 °C. Characterization in terms of X-ray photoelectron spectroscopy and X-ray diffraction verifies that the operating temperature affected the Co doping concentrations, leading to different conducting behaviors of the heterostructure. Furthermore, it is found that the heterojunction of Ba3CoNb2O9 and Ba5Nb4O15 can restrict the electron migration to avoid current leakage of the cell and simultaneously enhance the proton conductivity. These findings manifest the developed in situ Ba3CoNb2O9/Ba5Nb4O15 heterostructure as a promising electrolyte for SOFCs.

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba₇Nb₄MoO₂₀ by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

Structure and Magnetic Properties of Pseudo-1D Chromium Thiolate Coordination Polymers

The synthesis, structure, and magnetic properties of two novel, pseudo-one-dimensional (1D) chromium thiolate coordination polymers (CPs), CrBTT and Cr₂BDT₃, are reported. The structures of these materials were determined using X-ray powder diffraction revealing highly symmetric 1D chains embedded within a CP framework. The magnetic coupling of this chain system was measured by SQUID magnetometry, revealing a switch from antiferromagnetic to ferromagnetic behavior dictated by the angular geometrical constraints within the CP scaffold consistent with the Goodenough-Kanamori-Anderson rules. Intrachain magnetic coupling constants J_NN of -32.0 and +5.7 K were found for CrBTT and Cr₂BDT₃, respectively, using the 1D Bonner-Fisher model of magnetism. The band structure of these materials has also been examined by optical spectroscopy and density functional theory calculations revealing semiconducting behavior. Our findings here demonstrate how CP scaffolds can support idealized low-dimensional structural motifs and dictate magnetic interactions through tuning of geometry and inter-spin couplings.

Enhanced Oxide Ion Conductivity by Ta Doping of Ba3Nb1-xTaxMoO8.5

Significant oxide ion conductivity has previously been reported for the Ba₃M'M″O_8.5 family (M' = Nb⁵⁺, V⁵⁺; M″ = Mo⁶⁺, W⁶⁺) of cation-deficient hexagonal perovskite derivatives. These systems exhibit considerable structural disorder and competitive occupation of two distinct oxygen positions (O3 site and O2 site), enabling two-dimensional (2D) ionic conductivity within the ab plane of the structure; higher occupation of the tetrahedral O3 site vs the octahedral O2 site is known to be a major factor that promotes oxide ion conductivity. Previous chemical doping studies have shown that substitution of small amounts of the M' or M″ ions can result in significant changes to both the structure and ionic conductivity. Here, we report on the electrical and structural properties of the Ba₃Nb_1-xTa_xMoO_8.5 series (x = 0.00, 0.025, 0.050, 0.100). AC impedance measurements show that substitution of Nb⁵⁺ with Ta⁵⁺ leads to a significant increase in low-temperature (<500 °C) conductivity for x = 0.1. Analysis of neutron and X-ray diffraction (XRD) data confirms that there is a decrease in the M1O₄/M1O₆ ratio upon increasing x from 0 to 0.1 in Ba₃Nb_1-xTa_xMoO_8.5, which would usually coincide with a lowering in the conductivity. However, neutron diffraction results show that Ta doping causes an increase in the oxide ion conductivity as a result of longer M1-O3 bonds and increased polyhedral distortion.

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.

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.

Variable Temperature Neutron Diffraction Study of the Oxide Ion Conductor Ba3VWO8.5

Ba₃VWO_8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10^-5 S cm^-1 at 600 °C. Ba₃VWO_8.5 is anomalous to the other Ba₃M'M″O_8.5 (M' = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba₃VWO_8.5, which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba₃M'M″O_8.5 are dictated by water absorption. The significant water uptake in Ba₃M'M″O_8.5 (M' = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V⁵⁺ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.

Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1-2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba₃M₂O_8.5 (M = Mo/W⁶⁺ and Nb⁵⁺/V⁵⁺). Herein, strong 4-fold coordination geometry preferring Ge⁴⁺ ions are doped into Ba₃Mo_1+xNb_1-2xGe_xO_8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 reveals that Ge⁴⁺-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 is about twice higher than that of the parent compound at intermediate temperatures (300-500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba₃Mo_1+xNb_1-2xGe_xO_8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge⁴⁺-substitution-induced redistribution of oxygen ions within the palmierite-like layers.

Investigation of the Crystal Structure and Ionic Pathways of the Hexagonal Perovskite Derivative Ba3-xVMoO8.5-x

The hexagonal perovskite derivatives Ba₃NbMoO_8.5, Ba₃NbWO_8.5, and Ba₃VWO_8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba_3-xVMoO_8.5-x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1O_x polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba₃M'M″O_8.5 family, Ba_3-xVMoO_8.5-x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba_3-xVMoO_8.5-x is unstable at lower temperatures, as illustrated by the formation of BaCO₃ and BaMoO₄ by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba_3-xVMoO_8.5-x.

Cation Substitution of Hexagonal Triple Perovskites: A Case in Trimetallic Tellurates A2A'BTe2O9

Three new tellurates, namely, K₂BaPb_0.62Ba_0.38Te₂O₉ (1), Rb₂BaPb_0.7Ba_0.3Te₂O₉ (2), and Ba₂KLiTe₂O₉ (3), with hexagonal triple-perovskite structures have been designed via a cation substitution strategy. All three centrosymmetric compounds crystallize in the same hexagonal space group P6₃/mmc. In their structures, the [BTe₂O₉] (B = Pb/Ba for 1 and 2 and Li for 3) groups construct the final three-dimensional framework using face-sharing [Te₂O₉] dimmers and vertex-linked [BO₆] linkages. Meanwhile, the theoretical calculations demonstrate the numerical differences of their birefringence. Thermal stability analysis and differential scanning calorimetry were performed, and UV-vis-NIR diffuse reflectance spectra and infrared spectra of the three title compounds were also characterized and analyzed.

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.

Signature of Oxide-Ion Conduction in Alkaline-Earth-Metal-Doped Y3GaO6

We have studied alkaline-earth-metal-doped Y3GaO6 as a new family of oxide-ion conductor. Solid solutions of Y3GaO6 and 2% -Ca2+-, -Sr2+-, and -Ba2+-doped Y3GaO6, i.e., Y(3-0.06)M0.06GaO6-δ (M = Ca2+, Sr2+, and Ba2+), were prepared via a conventional solid-state reaction route. X-ray Rietveld refined diffractograms of all the compositions showed the formation of an orthorhombic structure having the Cmc21 space group. Scanning electron microscopy (SEM) images revealed that the substitution of alkaline-earth metal ions promotes grain growth. Aliovalent doping of Ca2+, Sr2+, and Ba2+ enhanced the conductivity by increasing the oxygen vacancy concentration. However, among all of the studied dopants, 2% Ca2+-doped Y3GaO6 was found to be more effective in increasing the ionic conductivity as ionic radii mismatch is minimum for Y3+/Ca2+. The total conductivity of 2% Ca-doped Y3GaO6 composition calculated using the complex impedance plot was found to be ∼0.14 × 10-3 S cm-1 at 700 °C, which is comparable to many other reported solid electrolytes at the same temperature, making it a potential candidate for future electrolyte material for solid oxide fuel cells (SOFCs). Total electrical conductivity measurement as a function of oxygen partial pressure suggests dominating oxide-ion conduction in a wide range of oxygen partial pressure (ca. 10-20-10-4 atm). The oxygen-ion transport is attributed to the presence of oxygen vacancies that arise from doping and conducting oxide-ion layers of one, two-, or three-dimensional channels within the crystal structure. The oxide-ion migration pathways were analyzed by the bond valence site energy (BVSE)-based approach. Photoluminescence analysis, dilatometry, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy studies were also performed to verify the experimental findings.

Understanding the Correlation between Oxide Ion Mobility and Site Distributions in Ba3NbWO8.5

A correlation between oxygen site distributions and ionic conductivity has been established in the recently discovered family of oxide-ion conductors Ba₃M₂O_8.5±δ (M = Nb, V, Mo, W). We rationalize this observation on the basis of structural insights gained from the first single-crystal neutron diffraction data collected for a member of this family, Ba₃NbWO_8.5, and theoretical considerations of bonding and O site energies.

High oxide ion and proton conductivity in a disordered hexagonal perovskite

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba₇Nb₄MoO₂₀ supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm^-1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba₇Nb₄MoO₂₀ also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

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

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.

Oxide ion conductors are an exciting class of materials with applications in various domains. Here, the authors show that Dion–Jacobson Phases are a structure supporting high O²⁻ mobility. The bulk conductivity of CsBi₂Ti₂NbO_10−δ even exceeds that of YSZ, offering new possibilities in electrolyte discovery.

Hybrid Halide Perovskites: Discussions on Terminology and Materials

Hybrid halide perovskites (HP) have emerged in the last decade as a new class of semiconductors with superior performances in photovoltaic and electronic devices. The literature about these halide semiconductors is abundant and a lot of names/expressions are used to define networks, structures, or materials. In this context, there is a need to offer some discussions about the relevance of using the word "perovskite" and the associated expressions ("RP" (Ruddlesden-Popper), "DJ" (Dion-Jacobson), "ACI" (alternating cations in the interlayer space), "hexagonal perovskites", "hollow perovskites", "d-HP" (deficient 3D HP),…). Moreover, the description of perovskite networks through elimination/substitution processes from the ABX₃ structure will be compared to the known dimensional reduction concept.