Correlation of Local Structure and Diffusion Pathways in the Modulated Anisotropic Oxide Ion Conductor CeNbO4.25

Computational discovery of fast interstitial oxygen conductors

New highly oxygen-active materials may enhance many energy-related technologies by enabling efficient oxygen-ion transport at lower temperatures, for example, below ~400 °C. Interstitial oxygen conductors have the potential to realize such performance but have received far less attention than vacancy-mediated conductors. Here we combine physically motivated structure and property descriptors, ab initio simulations and experiments to demonstrate an approach to discover new fast interstitial oxygen conductors. Multiple new families were found, which adopt completely different structures from known oxygen conductors. From these families, we synthesized and studied oxygen kinetics in La4Mn5Si4O22+δ, a representative member of the perrierite/chevkinite family. We found that La4Mn5Si4O22+δ has higher oxygen-ion conductivity than the widely used yttria-stabilized ZrO2, and among the highest surface oxygen exchange rates at the intermediate temperature of known materials. The fast oxygen kinetics is the result of simultaneously active interstitial and interstitialcy diffusion pathways. We propose that the essential features for forming an effective interstitial oxygen conductor are the availability of electrons and structural flexibility, enabling a sufficient accessible volume. This work provides a powerful approach for understanding and discovering new interstitial oxygen conductors.

High-Entropy CeNbO4+δ-Based Ceramics with Ultrahigh Comprehensive Thermosensitive Performances

Next-generation advanced high-temperature sensors rely heavily on negative temperature coefficient thermosensitive ceramics with low cost, small volume, high sensitivity, and fast response. However, thus far, the enormous challenge of achieving ultrahigh stability and accuracy has become a critical bottleneck restricting the development of thermosensitive ceramics in high-temperature sensor applications. Here, we propose a high-entropy strategy to design a "cation valence self-equilibrium" system in CeNbO4+δ-based ceramics introducing redox couple compensation and ultrahigh density dislocations to solve the problem of temperature-dependent oxygen nonstoichiometry that restricts the performances of high-temperature thermosensitive ceramics. Ferroelastic domains are generated by enhancing the configurational entropy at both A and B sites, resulting in a dramatic increase of dislocation density to >1010 mm-2, which ultimately optimizes the thermosensitive performances. Extreme temperature measurement accuracy with R2 as high as 999.98‰ and RSS as low as 0.011 and high-temperature stability with ΔR/R0 as low as 0.23% after aging at 873 K for 1000 h are realized in high-entropy CeNbO4+δ-based ceramics, indicating a breakthrough in the comprehensive performances of thermosensitive ceramics. This work opens up an effective way to design thermosensitive materials with ultrahigh comprehensive performance to meet the requirements of advanced high-temperature sensors.

Enhanced Thermal Stability and Broad Temperature Range in High-Entropy (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)NbO4 Ceramics

Next-generation high-temperature applications increasingly rely heavily on advanced thermistor materials with enhanced thermal stability and electrical performance. However, thus far, the great challenge of realizing high thermal stability and precision in a wide temperature range has become a key bottleneck restricting the high-temperature application. Here, we propose a high-entropy strategy to design novel high-temperature thermistor ceramics (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)NbO4. Differences in atomic size, mass, and electronegativity in this high-entropy system cause high lattice distortion, substantial grain boundaries, and high dislocation density. These enhance the charge carrier transport and reduce the grain boundary resistance, thus synergistically broadening the temperature range. Our samples maintain high precision and thermal stability over a wide temperature range from room temperature to 1523 K (ΔT = 1250 K) with an aging value as low as 0.42% after 1000 h at 1173 K, showing breakthrough progress in high-temperature thermistor ceramics. This study establishes an effective approach to enhancing the performance of high-temperature thermistor materials through high-entropy strategies.

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.

Temperature-driven order-disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7

Structural order-disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure-property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order-disorder phase transition of the polar oxides (Sr_1-xCa_x)SrZn₂Ga₂O₇. The low-temperature (LT) structure crystallizes in Pna2₁ with partial Zn/Ga ordering. Upon heating, (Sr_1-xCa_x)SrZn₂Ga₂O₇ undergoes an irreversible phase transition from orthorhombic Pna2₁ to hexagonal P6₃. Interestingly, the high-temperature (HT) P6₃ structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna2₁ structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO₃] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO₄-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn₂Ga₂O₇ and Sr₂Zn₂Ga₂O₇ representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.

Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba3-x Sr x YGa2O7.5 0 ≤ x ≤ 3 Materials

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba_3-x Sr _x YGa₂O_7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Praseodymium Orthoniobate and Praseodymium Substituted Lanthanum Orthoniobate: Electrical and Structural Properties

In this paper, the structural properties and the electrical conductivity of La_1−xPr_xNbO_4+δ (x = 0.00, 0.05, 0.1, 0.15, 0.2, 0.3) and PrNbO_4+δ are presented and discussed. All synthesized samples crystallized in a monoclinic structure with similar thermal expansion coefficients. The phase transition temperature between the monoclinic and tetragonal structure increases with increasing praseodymium content from 500 °C for undoped LaNbO_4+δ to 700 °C for PrNbO_4+δ. Thermogravimetry, along with X-ray photoelectron spectroscopy, confirmed a mixed 3⁺/4⁺ oxidation state of praseodymium. All studied materials, in humid air, exhibited mixed protonic, oxygen ionic and hole conductivity. The highest total conductivity was measured in dry air at 700 °C for PrNbO_4+δ, and its value was 1.4 × 10⁻³ S/cm.

Oxide-ion Conductivity Optimization in BiVO4 Scheelite by Acceptor Doping Strategy

BiVO4 scheelite is one of the few tetrahedra-based structures able to display vacancy-mediated oxide ion conduction upon acceptor doping strategy, leading to oxide ionic migration. In order to modulate the...

Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A₃OhTd₂O_7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO_2.5 compositions relative to that in the perovskite ABO₃ or brownmillerite A₂B₂O₅ structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O^2- migration. The low- and high-temperature average and local structures of Ba₃YGa₂O₇ (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P112₁/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O^2- ions was identified. Ionic conductivities of Ba₃YGa₂O_7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba₃YGa₂O_7.5 retaining proton conductivity at high temperatures. Ba_2.9La_0.1YGa₂O_7.55 and Ba₃YGa_1.9Ti_0.1O_7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.

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.

Experimental and Theoretical Solid-State 29Si NMR Studies on Defect Structures in La9.33+x(SiO4)6O2+1.5x Apatite Oxide Ion Conductors

Oxide ion conductors can be used as electrolytes in solid oxide fuel cells, a promising energy-conversion technology. Local structures around the defects in oxide ion conductors are key for understanding the defect stabilization and migration mechanisms. As the defect contents are generally low, it is rather difficult to characterize the defect structure and therefore elucidate how oxide ions migrate. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for probing the local structures. However, the interpretation of NMR signals mainly based on the empirical knowledge could lead to unprecise local structures. There is still controversy regarding the defect structures in the apatite-type interstitial oxide ion conductors containing isolated tetrahedral units. Here, we combine the experimental solid-state ²⁹Si NMR spectroscopy with theoretical density functional theory calculations to investigate the defect structures in La_9.33+x(SiO₄)₆O_2+1.5x apatites. The results indicate that the ²⁹Si resonance signals on the high field side of the main peak corresponding to the Si atoms in the bulk structure are related to La vacancies and there is no steady-state SiO₅ in the defect structures. This finding provides new atomic-level understanding to the stabilization and migration of interstitial oxide ions in silicate apatites, which could guide the design and discovery of new solid oxide fuel cell electrolyte materials.

Insights into the structural variations in SmNb1-xTaxO4 and HoNb1-xTaxO4 combined experimental and computational studies

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1-xTaxO4 (A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm-O distance and volume of the SmO8 polyhedra were invariant of the Ta content across the SmNb1-xTaxO4 solid solution whereas the average M-O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1-xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M'-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb-O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M-O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm-1 due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra.

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.

Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ

CeNbO_4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO₄, CeNbO_4.08, CeNbO_4.25, CeNbO_4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO_4.08 and CeNbO_4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO_4.08, and the supercell structure of CeNbO_4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO_4.08 and CeNbO_4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb₂O₉ units. However, the excess oxygen in CeNbO_4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO_4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

Disorder in La1−x Ba1+x GaO4−x/2 ionic conductor: resolving the pair distribution function through insight from first-principles modeling

Ionic conduction in dry LaBaGaO4 occurs through the vacant oxygen sites formed by the substitution of Ba for La. The resulting La1−x Ba1+x GaO4−x/2 solid solution shows significant disorder characteristics. The local structure of compositions x = 0, 0.20 and 0.30 was studied using the pair distribution function (PDF). Unfortunately, increasing peak overlap and the number of independent structural parameters make PDF modeling challenging when dealing with low-symmetry phases. To overcome this problem, density functional theory (DFT) was employed to create different structural models, each one with a different relative position for the substitutional Ba ion with respect to the oxygen vacancy. The atomic distributions generated by DFT were used as a starting point to refine experimental PDF data. All models result in the formation of Ga2O7 dimers, with their major axis oriented along the c axis. At the local scale, the most stable DFT model also provides the best fit of the PDF. This accounts for the dopant as first and second neighbors of the vacancy and of the O bridge in the dimer, suggesting that substitutional barium ions act as pinning centers for oxygen vacancies. Above 6 Å the average orthorhombic structure fits the PDF better than the DFT models, thus indicating that Ga2O7 dimers are not correlated with each other to form extended ordered structures. The combination of DFT simulations and X-ray diffraction/PDF refinements was used successfully to model the local atomic structure in La1−x Ba1+x GaO4−x/2, thus suggesting that this approach could be positively applied in general to disordered systems.

Room temperature structure and transport properties of the incommensurate modulated LaNb0.88W0.12O4.06

The crystal structure of a (3 + 2)D incommensurate modulated LaNb_0.88W_0.12O_4.06 phase, a novel oxygen ionic conductor, is refined using a combination of synchrotron X-ray diffraction and electron diffraction data. The superspace group I2/c(α₁0γ₁)00(α₂0γ₂)00 (a = 5.4131(1) Å, b = 11.6432(2) Å, c = 5.2963(1) Å, β = 91.540(1)°, q₁ = 0.2847(5)a^* + 0.1098(9)c^* and q₂=-0.1266(9)a* + 0.2953(1)c*) was chosen for the refinement. Similar to other scheelite type modulated structures, the modulation of LaNb_0.88W_0.12O_4.06 stems from the cation occupancy ordering in the xz plane. To facilitate the modulated cation sub-lattice, and to compensate for the difference in their size and charge, the B site polyhedra are distorted by stretching the B-O bond lengths. Consequently, an extension in the B site coordination number from 6 to 8 is observed in the modulated phase. An interconnected 3D network of BO_x polyhedra, similar to that of modulated CeNbO_4.25, is obtained as a result of the structure modulation, which is not available in the unmodulated parent structure. Tracer diffusivity measurements indicate that the composition is an oxygen ion conductor, which relies on an intersticalcy conduction mechanism. Oxygen tracer diffusivity of 2.46 × 10^-9 cm² s^-1, at 750 °C is reported.

Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO₄ Scheelite structure through Sr²⁺ for Bi³⁺ substitution, leading to corner-sharing V₂O₇ tetrahedral dimers, and migrate via a cooperative mechanism involving V₂O₇-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO₄ tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.

Fast oxide ion conductors are the key materials for some technological devices. Here the authors report the creation and stabilization of oxygen vacancies in BiVO₄ Scheelite with isolated tetrahedral anion structures for improved ionic conducting performance and understanding of the conduction mechanism.

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O' anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations.