Oxide Ion Conduction in Ca-Doped Yb3Ga5O12 Garnet.

Developing oxide ion conductors with new structural families is important for many energy conversion and storage techniques. Herein, a series of Ca-doped Yb3Ga5O12 garnet-type materials are prepared through a traditional solid-state reaction method, with their oxide ion conduction properties being reported for the first time. The results revealed that Ca substitution for Yb would significantly improve the conductivity of Yb3Ga5O12 from 3.57 × 10-7 S/cm at 900 °C under air to 1.66 × 10-4 S/cm, with an oxide ion transporting number of ∼0.52. The oxygen vacancy defect formation energy (∼0.127 eV) and the local structure around an oxygen vacancy were studied by atomic-level static lattice simulations based on the interatomic potential method. The oxide ion conducting mechanism was studied by the bond-valence-based method, which revealed three-dimensional pathways for oxide ion migration in both the parent and Ca-doped structures. The simulated activation energy of oxide ion migration decreased slightly from ∼0.358 eV in the parent structure to 0.346 eV in the doped one. These discoveries in the Ca-doped Yb3Ga5O12 will stimulate extensive exploitation and fundamental research on garnet-type materials.

Thermally Induced Oxygen Vacancies and High Oxide Ion Conduction in K2ZnV2O7 with a Melilite-Related Structure.

Donor-doped melilite materials with interstitial oxygen defects in the structure are good oxide ion conductors with negligible electronic conduction and show great potential in the ceramic electrolyte of intermediate-temperature solid oxide fuel cells (IT-SOFC). However, the parent melilite-structured materials with stoichiometric oxygen are usually insulators. Herein, we reported high and pure oxide ion conduction in the parent K2ZnV2O7 material with a melilite-related structure, e.g., ∼1.14 × 10-3 S/cm at 600 °C, which is comparable to that of the state-of-the-art yttrial-stabilized ZrO2 applied in practical fuel cells. Neutron diffraction data revealed the interesting thermally induced formation of oxygen vacancies at elevated temperatures, which triggered the transformation of the material from electronically conducting to purely and highly oxide ion-conducting. The VO4 tetrahedron with non-bridging terminal oxygen in K2ZnV2O7 was proved to be the key structural factor for transporting oxygen vacancies. The molecular dynamic simulation based on the interatomic potential approach revealed that long-range oxide ion diffusion was achieved by breaking and re-forming the 5-fold MO4 (M = Zn and V) tetrahedral rings. These findings enriched our knowledge of melilite and melilite-related materials, and creating oxygen vacancies in a melilite-related material may be a new strategy for developing novel oxide ion conductors.

Tetrahedral Tilting and Oxygen Vacancy Stabilization and Migration in La1-xSr2+x(GaO4)O1-0.5x Mixed Electronic/Oxide Ionic Conductors.

The La2O3/SrO/Ga2O3 ternary system contains several compounds with remarkable oxide or proton ionic conduction. Among them, the layered LaSr2(GaO4)O compound is a less commonly studied material. Here, the crystal structure, electrical conduction properties, and ionic migration mechanism of the La1-xSr2+x(GaO4) O1-0.5x (0 ≤ x ≤ 0.3) system are thoroughly analyzed. Diffraction methods indicate that the system crystallizes in the tetragonal space group P4/ncc, which is compatible with the presence of a subtle GaO4 tetrahedral tilting along the c axis, leading to a slight deviation of the body-centered tetragonal structure previously reported. This feature is essential to further understanding the mixed p electronic/oxide ion-conducting behavior of the system. Upon La3+ for Sr2+ substitution, oxygen vacancies arise at the loosely bound oxide sublattice, which at high temperature go through the GaO4 tetrahedral layer, leading to the formation of intermediate corner-sharing Ga2O7 tetrahedral dimers, and migrate via the continuous breaking and re-formation of the dimers, assisted by the synergic rotation and deformation of neighboring GaO4 tetrahedra. The unique structural and electrical features of La1-xSr2+x(GaO4)O1-0.5x materials within the La2O3/SrO/Ga2O3 ternary system emphasize their potential application as cathode materials in LaGaO3-based fuel cells.

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 Ca12Al14-xGaxO33 (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.

Development of Melilite-Type Oxide Ion Conductors.

Lowering the operating temperature of solid oxide fuel cells (SOFCs) requires high performance oxide ion conductor electrolytes. Recently tetrahedra-based structures have been attracting considerable attention for oxide ion conductor development, among which the layered tetrahedral network melilite structure appears particularly interesting owing to its remarkable capability to accommodate and transport interstitial oxide ions, compared with isolated tetrahedral anion structures. Stabilization and migration mechanisms of interstitial oxide ions in melilites have been systematically investigated using local structural relaxation from both electrostatic Coulomb interaction and chemical bonding aspects based on atomic and electronic structures respectively using experimental and theoretical approaches. These reveal cationic size and chemical bonding effects on stabilization and migration mechanisms of interstitial oxide ions. Lately, full crystallization from glass, an innovative synthesis method, was employed to produce new metastable melilite oxide ion conductors which are inaccessible using classic solid state reaction owing to cationic size effect. Finally, the thermal and chemical stability at low temperature and the high oxide ion conductivity of the best melilite oxide ion conductors based on LaSrGa3 O7 are likely to provide real possibilities of applications of melilite-type electrolytes in SOFCs and other related devices.

Electrical properties of Ca12Al14O33 synthesised through polymer assisted deposition and glass crystallization routes.

In this paper, two new methods, polymer assisted deposition (PAD) and glass crystallization (GC) methods were reported for the first time to prepare oxide ion conductor Ca12Al14O33, with their microstructures and electrical properties compared to that of a sample prepared by the traditional solid-state reaction (SSR) method. The results showed that these two new methods are effective for improving ceramic densities and oxide ion conductivities compared to the SSR method, and the ceramic prepared from the GC method in the present work possessed the highest level of ∼94% of the theoretical density and ∼1.0 × 10-3 S cm-1 at 900 °C for the bulk conductivity. A new bond valence method was applied to study the oxide ion conducting mechanism, which revealed an exchange process between the free oxide ion and the framework ion, and the wide windows connecting the Ca-Al-O framework cages were shown to be the key factor limiting oxide ion transport.

Ga-doped Ca12Al14O33 mayenite oxide ion conductors: synthesis, defects, and electrical properties.

Although mayenite Ca12Al14O33 has been known as an oxygen ion conductor for several decades, its relatively low oxide ion conductivity limits its applications in electrochemical devices. Thus, many efforts have been made by researchers, employing a doping strategy, in order to further improve its ionic conductivity, but with little success. In this work, a series of pure phase Ca12Al14-x Ga x O33+δ (0 ≤ x ≤ 1.2) materials were synthesized by a traditional solid state reaction method. Scanning electron microscopy (SEM) combined with energy dispersion spectrum (EDS) analyses disclosed well-sintered ceramics with uniform Ga distributions. The defect formation energies for Ga replacing the two distinguishable Al1 and Al2 sites in Ca12Al14O33 calculated by static lattice atomistic simulation are nearly identical, ∼3.03 and ∼3.04 eV, respectively, consistent with the results of Rietveld refinements based on the XRD data, from which no preferred distribution of Ga on Al1 or Al2 site was observed. The electrical properties investigated by alternating current (AC) impedance spectroscopy show increased bulk conductivities for 0 ≤ x ≤ 0.4. Thus, here we present the first work that successfully improves the bulk oxide ion conductivity of Ca12Al14O33 by Ga-doping.

La1+xBa1-xGa3O7+0.5x Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites.

Substitution of La3+ for Ba2+ in LaBaGa3O7 melilite yields a new interstitial-oxide-ion conducting La1+xBa1-xGa3O7+0.5x solid solution, which only extends up to x = 0.35, giving a maximum interstitial oxygen content allowed in La1+xBa1-xGa3O7+0.5x as about half of those allowed in La1+x(Sr/Ca)1-xGa3O7+0.5x. La1.35Ba0.65Ga3O7.175 ceramic displays bulk conductivity ∼1.9 × 10-3 S/cm at 600 °C, which is lower than those of La1.35(Sr/Ca)0.65Ga3O7.175, showing the reduced mobility for the oxygen interstitials in La1+xBa1-xGa3O7+0.5x than in La1+x(Sr/Ca)1-xGa3O7+0.5x. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La1.35Ba0.65Ga3O7.175 are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the c-axis and stabilized via incorporating into the bonding environment of a three-linked GaO4 among the five GaO4 tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa3O7 has the largest formation energy for oxygen interstitial defects among La1+xM1-xGa3O7+0.5x (M = Ba, Sr, Ca), consistent with the large Ba2+ cations favoring interstitial oxygen defects in melilite less than the small cations Sr2+ and Ca2+. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La1+xM1-xGa3O7+0.5x (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba2+ along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba2+-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels.

Localization of oxygen interstitials in CeSrGa3O(7+δ) melilite.

The solubility of Ce in the La(1-x)Ce(x)SrGa3O(7+δ) and La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce(3+) into Ce(4+). Under CO reducing atmosphere, the La in LaSrGa3O7 may be completely substituted by Ce to form the La(1-x)Ce(x)SrGa3O(7+δ) solid solution, which is stable in air to ∼600 °C when x ≥ 0.6. On the other side, the La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) compositions displayed much lower Ce solubility (x ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La(1-x)CexSrGa3O(7+δ), the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce(3+) cations. At 600 °C, CeSrGa3O(7+δ) showed a conductivity of ∼10(-4) S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa3O7. The oxidation of Ce(3+) into Ce(4+) in CeSrGa3O(7+δ) slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa3O(7+δ). The Ce doping in air also reduced the interstitial oxide ion conductivity of La1.54Sr0.46Ga3O7.27. Neutron powder diffraction study on CeSrGa3O7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO4 polyhedral environment, leading to distorted GaO5 trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa3O(7+δ), in contrast with those in La(1+x)Sr(1-x)Ga3O(7+0.5x), may be correlated with the cationic size contraction from the oxidation of Ce(3+) to Ce(4+). These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure.

Solid-State (29)Si NMR and neutron-diffraction studies of Sr(0.7)K(0.3)SiO(2.85) oxide ion conductors.

K/Na-doped SrSiO3-based oxide ion conductors were recently reported as promising candidates for low-temperature solid-oxide fuel cells. Sr0.7K0.3SiO2.85, close to the solid-solution limit of Sr1-xKxSiO3-0.5x, was characterized by solid-state (29)Si NMR spectroscopy and neutron powder diffraction (NPD). Differing with the average structure containing the vacancies stabilized within the isolated Si3O9 tetrahedral rings derived from the NPD study, the (29)Si NMR data provides new insight into the local defect structure in Sr0.7K0.3SiO2.85. The Q(1)-linked tetrahedral Si signal in the (29)Si NMR data suggests that the Si3O9 tetrahedral rings in the K-doped SrSiO3 materials were broken, forming Si3O8 chains. The Si3O8 chains can be stabilized by either bonding with the oxygen atoms of the absorbed lattice water molecules, leading to the Q(1)-linked tetrahedral Si, or sharing oxygen atoms with neighboring Si3O9 units, which is consistent with the Q(3)-linked tetrahedral Si signal detected in the (29)Si NMR spectra.