K2NiF4-type Structure Research Articles

Effect of calcium and copper/iron co-doping on defect-induced properties of La2NiO4-based materials

The La2-xCaxNi1-y(Fe/Cu)yO4+δ solid solutions with х = 0.2, 0.4, 0.5 and y = 0, 0.1, 0.2, 0.3 were synthesized by decomposition of citrate–nitrate precursors. The single-phase La2-xCaxNi1-y(Fe/Cu)yO4+δ materials crystallized in the K2NiF4-type structure, sp. gr. I4/mmm. The crystal structure parameters of La2-xCaxNi1-y(Fe/Cu)yO4+δ were refined by the Rietveld method. The unit cell volume of La2-xCaxNi1-y(Fe/Cu)yO4+δ decreased with x and increased with y. All studied samples exhibited oxygen excess compared to the stoichiometric composition (δ > 0) in the temperature range 30–1100 °C except for La1.6Ca0.4Ni1-yCuyO4+δ (y = 0.2 and 0.3). Calcium and copper doping decreased the total oxygen content while iron doping increased it in La2-xCaxNi1-y(Fe/Cu)yO4+δ. Thermal expansion coefficients (TECs) of La2-xCaxNi1-y(Fe/Cu)yO4+δ slightly increased with dopant concentration and varied in the range (14.5–15.1) × 10−6 K−1. The contribution of chemical expansion was shown to be negligible. The total conductivity of La2-xCaxNi1-y(Fe/Cu)yO4+δ was thermally activated within the whole studied temperature range 30–1000 °C and could be described in terms of small polaron conduction mechanism. The values of total conductivity increased with x and decreased with y in La2-xCaxNi1-y(Fe/Cu)yO4+δ. The analysis of the Seebeck coefficient vs temperature for La2-xCaxNi1-y(Fe/Cu)yO4+δ showed that the main charge carriers are electron holes localized on Ni2+ cations represented by low-spin Ni3+ cations which had a tendency to change the spin configuration gradually to high-spin when the temperature and dopant content increased.

Synthesis and characterization of gold-containing oxides of K2NiF4 or Nd2CuO4 structure type

In this work, different materials of Nd2CuO4 and K2NiF4 Ruddlesden-Popper structure have been prepared by solid-state reaction in air and high temperatures. The powders of mixed oxides, of general formula La2Li0.5M0.5 − xAu x O4 (M = Cu, Ni), were prepared from [Au(NH3)4](NO3)3 as gold source. The stoichiometric proportions of the all reagents (including La2O3, NiCO3, CuO) were grounded with a LiOH excess as mineralizer, pressed to form into pellets and heated in static air at temperatures between 750 and 850 °C for several hours. Structural characterization of the synthesized oxides using X-ray diffraction (XRD) are presented. La2Li0.5Au0.5O4, La2Li0.5Cu0.5O4, and La2Li0.5Ni0.5O4, three previously known oxides, were obtained. As expected, La2Li0.5Ni0.5O4 and La2Li0.5Cu0.5O4 consist of an ordered K2NiF4 Ruddlesden-Popper structure, while the La2Li0.5Au0.5O4 compound exhibits a Nd2CuO4 type structure. On the other hand, the preparation of the new proposed group La2Li0.5M0.5 − xAuxO4 (x = 0.0125–0.5) containing gold and nickel/copper simultaneously, under the studied conditions, was not possible. The experimental results revealed that the host structures are intolerant with respect to M (Cu or Ni) and Au cation double substitution. The difficulty to obtain the new phases is discussed regarding the exceptional instability displayed by the trivalent state of gold ion (Au3+) and the soft interactions of this specie with the lithium cation. Finally, the monophasic oxides, more interesting in terms of catalytic activity, were analyzed by temperature-programmed reduction (TPR-H2).

A-site nonstoichiometry and B-site doping with selected M3 + cations in La2-xCu1-y-zNiyMzO4-δ layered oxides

In this work studies on a modification of Cu-based, layered oxides with K2NiF4-type structure are presented concerning formation of cationic nonstoichiometry in the A-sublattice of the parent La2CuO4, as well as an introduction of selected M3+ cations (Sc, Ga, In) at the B-site into La1.9Cu1-xMxO4 and Ni-containing La1.9Cu0.45Ni0.45M0.1O4. It is shown that synthesis of phase-pure La2-xCuO4 can be done with values of x≤0.1, and in the A-site nonstoichiometric materials further doping with M3+ is also achievable. Formation of solid solutions for M3+-doped samples follows the expected trend, with the widest range observed for the smallest Sc3+ cation, z≤0.1. Apart from a different structural behavior of the doped oxides at high temperatures, it is documented that the discussed substitution affects strongly the oxygen content (e.g. high concentration of the oxygen vacancies is present in La1.9CuO4-δ), as well as electrical conductivity. This is documented by the high-temperature X-ray diffraction, thermogravimetric, iodometic titration and electrical measurements. Since the compounds possess good transport properties, are stable at elevated temperatures, exhibit moderate thermal expansion and are Co-, Sr- and Ba-free, the proposed doping strategy seems interesting concerning development of the oxygen-transporting membranes based on Cu-containing oxides. For example, 1mm thick La1.9Cu0.9Sc0.1O4-δ membrane was found to deliver 0.37mLcm−2min−1 oxygen flux at 950°C, a very high value for the K2NiF4-type materials.

XPS Studies of Sr doped La2CuO4: A potential cathode material for IT-SOFCs

The cathode material in a SOFC plays a major role in determining the efficiency of energy conversion and performance of the fuel cell. Until recently, most of the studies concerned with cathode material for SOFCs were devoted to perovskite-type oxides. However, presently, a family of oxides with the K2NiF4 type structure is attracting much attention. In this present work, one such K2NiF4 type oxide, La2-xSrxCuO4-δ has been synthesized and evaluated as a potential cathode material for IT-SOFCs. The material was synthesized by traditional solid state technique. The characterizations of the prepared sample were done by XRD, SEM, and XPS.

Crystal structure and properties of novel oxide Sm0.9Ca1.1Fe0.7Co0.3O4-d

Sm0.9Ca1.1Fe0.7Co0.3O4-δ oxide with the K2NiF4-type structure was prepared using a glycerin-nitrate technique. The XRD pattern of Sm0.9Ca1.1Fe0.7Co0.3O4-δ was refined by the Rietveld method within an orthorhombic structure (space group Bmab). The electrical conductivity, Seebeck coefficient, and thermal expansion of Sm0.9Ca1.1Fe0.7Co0.3O4-δ were measured depending on temperature in air. The change of oxygen nonstoichiometry determined by TGA in air does not exceed 0.01. The oxygen content in Sm0.9Ca1.1Fe0.7Co0.3O4-δ determined by the reduction in a hydrogen flux is equal to 3.96 ± 0.01. The positive value of Seebeck coefficient indicates that the predominant charge carriers in the oxide studied are electron holes.

Crystal structure, electrical and electrochemical properties of Cu co-doped Pr1.3Sr0.7NiO4+δ mixed ionic-electronic conductors (MIECs)

Room temperature crystal structure, electrical properties and electrochemical properties in the temperature range 25–700 °C of Cu co-doped Pr1.3Sr0.7NiO4+δ prepared by acetate combustion is investigated from intermediate temperature solid oxide fuel cell cathode viewpoint. The Pr1.3Sr0.7Ni1−xCuxO4+δ (PSNCO) solid solutions have a tetragonal I4/mmm K2NiF4-type structure which consists of a (Pr1.3Sr0.7) (Ni1.xCux)O3 perovskite unit and a (Pr1.3Sr0.7)O rock salt unit in the whole compositional range 0 ≤ x ≤ 0.4. A reduction in bond-length of Ni/Cu-O resulting from PSNCO lattice contraction eases hop of small polaron from Ni3+ to Ni2+/Cu2+ in (Ni1−xCux)-O layer with low activation energy, which increases electron conductivity. The maximum electronic conductivity (σ = Ω cm−1) with minimum activation energy (Ea = eV) is observed at x = 0.3. Lattice expansion along c-direction owing to Cu2+ doping facilitates hop of O2− from its occupied interstitial site (O3) to nearby equivalent site assisted by anisotropic thermal motion of apical oxygen O2 resulting in increase in ionic conductivity. The minimum polarization resistance value (Rp = 0.13 (2) Ω cm2) and activation energy (Ea = 1.321 (5) eV) at x = 0.3 is attributed to high electronic and ionic conductivities compared to other compositions. Complex impedance spectroscopy studies suggest that the ORR is co-limited by O2− diffusion and O2 surface exchange.

EFFECTS OF Sr SUBSTITUTION ON CRYSTALLINE STRUCTURE AND RAMAN SPECTRA OF La2-xSrxNiO4+δ

Effects of Sr2+ substitution for La3+ on oxygen concentration and structure of La2-xSrxNiO4+δ have been observed by X-ray diffraction and Raman scattering. Oxygen concentration and unit cell volume of the material decrease monotonously in dependence of x. The vibration modes were calculated for La2NiO4 in K2NiF4 type structure with the group symmetry of I4/mmm. The Raman active modes for this symmetry were also theoretically identified. Raman back scattering spectra have been carried out for all the ceramic polycrystalline samples at room temperature. Two Raman lines at 224.6 cm-1 and 433 cm-1 , assigned to Eg and A1g vibration modes of oxygen O(II) in bonding La/Sr-O were observed for all the samples. The lower mode 224.6 cm-1 shifts down to 208 cm-1 , whereas the upper mode 433 cm-1 shifts up when concentration of the Sr substituting for La increases.

Synthesis, structural and electrical characterizations of LaSrCu0.4Al0.6O4\u2212\u03b4

A new compound LaSrCu0.4Al0.6O4−δ has been prepared by sol gel method, annealed at 1623 K in oxygen gas flow and examined by X-ray diffraction. Rietveld refinement shows that the sample adopts the K2NiF4-type structure, space group I4/mmm (a = 3.7694(2) and c = 12.8248(5) Ǻ, Z = 2). Vibrational properties were investigated using Raman scattering and the most characteristic vibrations are discussed with reference to the available structural data. Differential thermal analysis shows two endothermic effects at 329 and 593 K and one exothermic at 693 K. Dielectric study as a function of temperature in the frequency range of 1 kHz–1 MHz has confirmed the observed phase transitions. The value of the measured dielectric constant depends on the frequency and undergoes a large increase near the phase transition temperatures indicating a diffuse behavior of these transitions. Variation of conductivity as a function of temperature shows that the compound has a semiconducting behavior. Oxygen vacancies could be the possible ionic charge carriers. The electrical transport mechanism agrees with Adiabatic Small Polaron Hopping model.

In-depth study of the Ruddlesden-Popper LaxSr2−xMnO4±δ family as possible electrode materials for symmetrical SOFC

The Ruddlesden Popper (RP) manganites LaxSr2−xMnO4±δ with compositions 0.25 ≤ x ≤ 0.6 have been successfully synthesized as single phases by solid-state reaction in air. All those materials are not only stable in reducing atmosphere but they also maintain the K2NiF4-type structure with I4/mmm symmetry under redox cycling conditions with limited volume changes. The x = 0.5 phase was analyzed by in situ high temperature neutron powder diffraction (HTNPD), under flowing hydrogen, showing the formation of oxide-ion vacancies on the equatorial sites of the perovskite planes, during reduction process. The total electrical conductivity was optimized and found maximum for x = 0.5 with values of 35.6 S cm−1 and 1.9 S cm−1 at 800 °C in air and 3% H2/Ar, respectively, what is judged to be sufficient for an active layer of symmetrical SOFC electrode. First Electrochemical Impedance Spectroscopy (EIS) measurements in both oxidizing and reducing conditions, using an YSZ electrolyte and a GDC buffer layer, are presented giving rise to promising values.

Preparation of Structural Phase Diagram of Nd2Ni1-X Cu x O4+δ As New Cathode Materials – Clarification of Existence of Miscibility Gap

Nd2NiO4+δ with K2NiF4 type structure (T-phase) composed of rock salt layer and perovskite layer has attracted much interest as new cathode material for solid oxide fuel cell. It was reported that partial substitution of Cu for Ni-site in Nd2NiO4+δ improved ionic and hole conductivity [1]. However, crystal structure must change depending on Cu content, resulting in possible variation of electrical property, since crystal structure of Nd2CuO4 is T’-phase composed of fluorite layer and [CuO2] sheet, which is completely different from T-phase. For optimization of Cu content in Nd2Ni1-x Cu x O4+δ as cathode material, structural phase diagram between Nd2NiO4+δ and Nd2CuO4+δshould be established. In this study, preparation of Nd2Ni1-x Cu x O4+δ with various compositions was attempted and stability at high temperature was evaluated to establish structural phase diagram. It has been established that there exists miscibility gap in Nd2Ni1-x Cu x O4+δ for 0.4≤x≤0.9 for the first time. Nd2Ni1-x Cu x O4+δ (0.0≤x≤1.0) were prepared by the Pechini method by which preparation of homogeneous specimens can be expected. Nd2O3, Ni(NO3)2·6H2O and CuO powders were dissolved with mixture of dilute HNO3 and H2O2, distilled H2O and dilute HNO3, respectively. They were mixed with nominal composition. After mixing ethylene glycol and citric acid, the mixture was heated at about 450 °C, resulting in precursor. The precursor obtained was calcined at 700 °C for 24 h in air, followed by pressing into pellet and heat-treatment at 1000 – 1200 °C in air for 10 h. For identification of the crystal structure and evaluation of the lattice constants, X-ray diffraction measurement (XRD) was performed. Some XRD patterns were analyzed using the Rietveld analysis employing program RIETAN-FP [2]. Homogeneity of the specimens was evaluated by SEM-EDX analysis (JEOL: JCM-5700 equipped with JED-2300). Temperature dependence of lattice constants and molar volume and existence of the structural phase transition were evaluated using high-temperature X-ray diffraction measurements in the air. XRD patterns at room temperature of Nd2Ni1-x Cu x O4+δ prepared at 1200 °C could be indexed as single orthorhombic T-phase with space group of Fmmm (No. 69) and single tetragonal T-phase with I4/mmm (No.131) for 0.0≤x≤0.1 and 0.2≤x≤0.3, respectively. For diffraction patterns of 0.4≤x≤0.9, peaks assigned as T’-phase were observed in addition to ones indexed as T-phase. With increasing Cu content, intensity of the peaks indexed as T’-phase increased and ones assigned as T-phase decreased. All the peaks of XRD pattern for the specimen with x=1.0, i, e., Nd2CuO4+δ was successfully indexed as tetragonal T'-phase with space group of I4/mmm (No.131). Rietveld analysis revealed that lattice constants and molar volume of both T-phase and T’-phase in the specimens with 0.4≤x≤0.9 were independent on Cu content, indicating existence of miscibility gap for 0.4≤x≤0.9. Also revealed by Rietveld analysis was linear relationship between Cu content and molar ratio of T’-phase which could be ascribed to satisfaction of lever rule in the miscibility gap. EDX image and spectra of Nd2Ni0.4Cu0.6O4+δ depicted in Fig. 1 indicate that two phases with Ni rich and Ni poor composition coexist in the specimen, showing agreement with proposed miscibility gap by Rietveld analysis. High temperature XRD measurements revealed that the miscibility gap for 0.4≤x≤0.9 was maintained up to 1000 °C. For the specimens prepared at 1000 °C, the miscibility gap was observed for 0.3≤x≤0.9, suggesting expansion of the miscibility gap region by decreasing temperature.

Relation between the Co-O bond lengths and the spin state of Co in layered Cobaltates: a high-pressure study

The pressure-response of the Co-O bond lengths and the spin state of Co ions in a hybrid 3d-5d solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4-type structure was studied by using hard X-ray absorption and emission spectroscopies. The Co-K and the Ir-L3 X-ray absorption spectra demonstrate that the Ir5+ and the Co3+ valence states at ambient conditions are not affected by pressure. The Co Kβ emission spectra, on the other hand, revealed a gradual spin state transition of Co3+ ions from a high-spin (S = 2) state at ambient pressure to a complete low-spin state (S = 0) at 40 GPa without crossing the intermediate spin state (S = 1). This can be well understood from our calculated phase diagram in which we consider the energies of the low spin, intermediate spin and high spin states of Co3+ ions as a function of the anisotropic distortion of the octahedral local coordination in the layered oxide. We infer that a short in-plane Co-O bond length (<1.90 Å) as well as a very large ratio of Co-Oapex/Co-Oin-plane is needed to stabilize the IS Co3+, a situation which is rarely met in reality.

Chemical and physical characterizations of the n = 1 Ruddlesden–Popper phases: Nd2 − y Sr y Ni1 − x Co x O4 ± δ (y = 1 and 0.1 ≤ x ≤ 0.9)

The structural stability and physical properties of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) mixed oxides, elaborated by conventional sol–gel process, have been investigated and obtained results show that substitution of nickel by cobalt at x = 0.5 enhances conductivity at room temperature; σ = 17.24 Ω−1 cm−1 coinciding with minimum activation energy (Ea = 0.05 eV). Rietveld refinements of X-ray powder diffraction patterns at room temperature indicate that all compositions crystallize in a tetragonal system with I4/mmm space group and exhibit K2NiF4-type structure. Variations of a and c parameters display various behavior with increasing cobalt content. Changes in cell parameters are discussed in terms of crystal field theory. In addition, transition metal oxidation state is investigated on the basis of the Brown bond valence calculation. The deduced Global Instability Index (GII) value decreases when cobalt substance increases, indicating that the structure becomes more stable once cobalt is introduced. Oxygen stoichiometry of these compounds was determined from thermogravimetric analyses (TGA) followed by reduction in 5% H2 in N2 gas. Conductivity of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) oxides was measured by an ac four-probe method. Oxygen vacancies are the possible ionic charge carriers. Specimens exhibit a semiconducting behavior in the whole range of temperature. The electrical transport mechanism agrees with an adiabatic small polaron hopping (ASPH) model. [Figure not available: see fulltext.]. © 2017, Springer-Verlag Berlin Heidelberg.

LaSrMnO4: Reversible Electrochemical Intercalation of Fluoride Ions in the Context of Fluoride Ion Batteries

This article reports on the investigation of LaSrMnO4 with K2NiF4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO4F and LaSrMnO4F2–x. Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20–25 mAh/g was found, which is ∼4–5 times higher compared to what was reported previously on the discharging of BaFeO2.5/BaFeO2.5F0.5, approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO4 → LaSrMnO4F) a...

Phase equilibria, crystal structure and properties of complex oxides in the Nd2O3–SrO–CoO system

The phase equilibria in the ½Nd2O3–SrO–CoO system were systematically studied at 1373K in air. The intermediate phases formed in the ½Nd2O3–SrO–CoO system at 1373K in air are: Nd1-xSrxCoO3-δ (0.0≤x≤0.5 with orthorhombic structure, sp. gr. Pbnm and 0.6≤x≤0.95 whose structure was detected as cubic according to XRD sp. gr. Pm3m, but shown to be tetragonal by TEM due to the oxygen vacancy ordering), Nd2-ySryCoO4-δ (0.6≤y≤1.1 with tetragonal K2NiF4-type structure, sp. gr. I4/mmm) and Nd2-zSrzO3 (0.0≤z≤0.15 with hexagonal structure, sp. gr. P-3m1). The unit cell parameters for the single phase samples were refined by the Rietveld analysis. The changes of oxygen content in Nd1-xSrxCoO3-δ (0.6≤x≤0.95) and Ruddlesden-Popper oxide Nd2-ySryCoO4-δ were examined by TGA. All were found to be oxygen deficient phases. High-temperature dilatometry allows calculating the thermal expansion coefficient and evaluating the chemical expansion coefficient at high temperature. The projection of isothermal-isobaric phase diagram for the Nd–Sr–Co–O system at 1373K in air to the compositional triangle of metallic components has been constructed. The phase equilibria in the studied Nd–Sr–Co–O system were compared to La–Sr–Co–O and Nd–M–Co–O (M=Ca and Ba).

Charge Transfer and Structural Anomaly in Stoichiometric Layered Perovskite Sr2Co0.5Ir0.5O4

A layered Sr2Co0.5Ir0.5O4 (space group I4/mmm) perovskite of the K2NiF4 structure type was synthesized as a single‐phase sample. Neutron powder diffraction measurements revealed full site occupancy of oxygen atoms. The Co3+/Ir5+ valence states were evaluated by a combination of X‐ray absorption and X‐ray photoemission spectroscopy as well as by magnetization measurements, and thus the differences to the parent compounds Sr2CoO4 and Sr2IrO4 with similar structures were confirmed. Co‐K edge EXAFS studies indicated a long average Co–O distance of 1.967 Å suggesting a high‐spin state of the Co3+ ion in Sr2Co0.5Ir0.5O4. No long‐range magnetic ordering down to 4 K was detected by neutron powder diffraction, probably hindered by the random distribution of Co and Ir cations in the Sr2Co0.5Ir0.5O4 structure. High‐pressure synchrotron powder diffraction studies showed stability of the Sr2Co0.5Ir0.5O4 structure up to at least 27 GPa at room temperature. A maximum in the c/a ratio at 10 GPa together with an anomaly in the pressure dependence of the unit‐cell volume could be a sign of a change in the spin state of the Co ions under pressure.

Synthesis and characterization of mixed valent Fe containing K2NiF4-type phases

The phases LaSr1−xNaxFeO4 (x=0.00, 0.05, 0.10, 0.15, 0.20) with K2NiF4-type structure have been synthesized by standard ceramic method. The Rietveld analysis of the powder XRD data assigns tetragonal structures to all the compositions with I4/mmm symmetry. The lattice constant a almost remains constant, while there is increase in c with increasing x due to Jahn–Teller effect. The anomaly for x=0.2 is consistent with smaller ionic radii of Na+ than Sr2+. For all the studied samples the paramagnetic Curie temperature (θp) is negative, indicating the existence of antiferromagnetic interactions in them. The change in Weiss constant (θp) is attributed to the presence of Fe4+ content. All the compositions of two dimensional compounds LaSr1−xNaxFeO4 are within the semiconducting range. The electronic conduction in these materials is obtained via hopping of electrons from the occupied sites, Fe3+ ions, to the unoccupied sites, Fe4+ ions. The transport properties are dominated by the Arrhenius model with the decrease of activation energy with increasing x.

Crystal structure and electrical transport properties of single layered perovskite LaSrCoO4

We present here investigations on the influence of structure on electrical transport properties of polycrystalline LaSrCoO4 that is single layered perovskite with K2NiF4 type structure synthesized using solid state reaction route. Using Reitveld refinement of X-ray diffraction (XRD) data, it is found that the sample is in single phase with tetragonal structure (space group I4/mmm). Electrical resistivity performed in the temperature range 140-300K shows semiconducting character of the sample. Considerable contrasts in the Co-O bond length is associated with the intermediate spin (IS) state of Co ion that correlates the structural and transport properties. Detailed analysis indicates that the temperature dependent electrical resistivity follows the three-dimensional variable range hopping (VRH) model in low temperature region below 225K. The high temperature (225-300K) resistivity data has been found to follow the thermally activated behaviour.

Synthesis and H- Conductivity of Ba2LiH3-2x O1+X

Hydride ions are attractive as charge carriers because these have an ionic radius appropriate for fast ionic conduction and also show strong reducing properties with a standard redox potential of H–/H2 (–2.25 V vs. SHE), which is close to that of Mg/Mg2+ (–2.38 V vs. SHE). Therefore, H– conductors may be applied in energy storage and conversion devices with high energy densities. Recently, we have found La2-x-y Sr x +y LiH1-x+y O3-y (LSLHO) oxyhydride system possessing pure H– conductivity, and have confirmed the capability of the oxyhydride to act as an H– solid electrolyte by demonstrating the battery reaction of an all-solid-state Ti/LSLHO/TiH2 cell [1]. The LSLHO system shows H– conductivity of 10-4 S·cm–1 at 300 ºC. However, a higher H– conductivity will be required to make novel electrochemical devices using H– conduction phenomenon. In this study, we attempted to synthesize a new H– conductive oxyhydride, Ba2LiH3-2x O1+x (BLHO), in order to achieve a higher H– conductivity than that of LSLHO [2]. BLHO was designed on the basis of LSLHO with the K2NiF4-type structure. La/Sr in LSLHO was substituted by Ba to expand the bottleneck size for H– diffusion in the crystal lattice. Furthermore, H– vacancies were introduced into the structure by controlling H/O ratio in the composition of BLHO. Crystal structure of BLHO was determined by X-ray and neutron diffraction. The X-ray diffraction peak of the sample could be indexed as the K2NiF4-type structure. The crystal structure of Ba2LiH3O (BLHO: x = 0) and Sr2LiH3O (LSLHO: x = 0, y = 2) determined by the Rietveld analysis are compared in Figure 1. The anion arrangement of Ba2LiH3O is basically same with Sr2LiH3O. The two apical anion sites in Li-octahedra are preferentially occupied by oxide ions. Oxide and hydride ions randomly occupy the apical site. In contrast, the four axial anion sites of the Li-anion octahedra prefer to be occupied by only hydride ions. As a result, square-planar LiH4 is formed in the Li-octahedra. When the A-site cation in the composition of A 2LiH3O changed from Sr to Ba, the lattice volume expand depending on the difference of ionic radii between Sr2+ (1.58 Å) and Ba2+ (1.75 Å). In particular, influences of the volume expansion mainly appeared to ab plane in the crystal lattice. The details of H– conduction properties for BLHO will be discussed in our presentation.

Effect of Heterointerface on Cathode Performance of Solid Oxide Fuel Cells: (Sm,Sr)CoO3-Based Oxide

Solid oxide fuel cells (SOFCs) offer many advantages such as high energy conversion efficiency and high fuel flexibility. However, their high operating temperatures (700-900 °C) sometimes accelerate the degradation of components and result in a long time to start-up and shut- down time of the systems. Thus, many researchers have tried to reduce the operating temperature of SOFCs. The lowering of operating temperatures leads to the decrease in the reaction rate over electrodes. One possible approach to overcome this issue is the development of cathode materials with high activity for oxygen reduction reaction (ORR). Nowadays, mixed ionic-electronic conductors, a series of ABO3 perovskite-type materials such as (Ln,Sr)CoO3-δ (Ln: Lanthanoid), have attracted much attention as promising cathode materials due to their higher performance in comparison to a conventional cathode of (La,Sr)MnO3+δ. The previous studies on electrode kinetics of (La,Sr)CoO3-δ cathode revealed that the rate-determining step of the electrode reaction was the oxygen exchange process over the surface [1,2]. Recently, the oxygen surface exchange property at the heterointerface between (La,Sr)CoO3-δ and (La,Sr)2CoO4-δ (K2NiF4-type structure) has been studied. In particular, Sase et al. have reported that the rate of oxygen surface exchange was much faster at the (La,Sr)CoO3-δ/(La,Sr)2CoO4-δ heterointerface than at the surface of (La,Sr)CoO3-δ [3]. Yashiro et al. demonstrated high electrochemical performance of the heterointerface-introduced (La,Sr)CoO3-δ-based composite cathode [4]. In this study, then, we focused on (Sm,Sr)CoO3-δ, which has higher performance than (La,Sr)CoO3-δ, and examined the effect of Sm0.5Sr0.5CoO3-δ(SSC113)/SmSrCoO4(SSC214) heterointerface on the cathode performance. The oxide materials with different weight ratios of SSC113 and SSC214 were synthesized by mixing starting materials of metal nitrates in the desired ratios. The resulting composite oxide was mixed with Ce0.9Gd0.1O1.95 (GDC) in a weight ratio of 70:30. The symmetric cells consist of the obtained material as electrodes and GDC as an electrolyte were fabricated. The electrode performance was evaluated in air at 550-800 °C by the electrochemical impedance spectroscopy. The composite material with SSC113:SSC214 = 80:20 wt.% exhibited higher performance than SSC113. This was attributed to the promotion of electrode reaction. The 18O isotope exchange reaction was also carried out for dense pellets of SSC-based materials. The composite pellets of SSC113 and SS214 provided higher surface exchange coefficient than SSC113. These results indicate that the introduction of (Sm,Sr)CoO3-δ /SmSrCoO4 heterointerface is one of the promising ways to design the high performance cathode for SOFCs operative at low temperatures. [1]K. Masuda, A.Kaimai, K. Kawamura, Y. Nigara, T. Kawada, J. Misusaki, H. Yugami, H, Yugami, H. Arashi, Solid State Fuel Cells V, U. Stimming, S. C. Singhal, H. Togawa, W. Lehnert, Editors, PV 97-40, p473, The Electrochemical Society Proceedings Series, Pennington, NJ (1997) [2]T. Kawada, K. Masuda, H. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, et al., Solid State Ionics, 121, E252 (2002). [3] M. Sase, F. Hermes, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, H. Yokokawa, J. Electrochem. Soc., 155, 793 (2008). [4] K. Yashiro, T. Nakamura, M. Sase, F. Hermes, K. Sato, T. Kawada, J. Mizusaki, Electrochem. Solid-State Lett. , 12, 135 (2009).

Influence of Crystal Orientation and Annealing on the Oxygen Diffusion and Surface Exchange of La2NiO4+δ

La2NiO4+δ is a mixed ionic–electronic conducting material with the 2D K2NiF4-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k*) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La2NiO4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450–600 °C. The differences in k* have been correlated w...