LaxSr2−xFeyRu1−yO4±δ: a new family of K2NiF4 type oxides

The phases La_xSr_2−xFe_yRu_1−yO_4±δ (x=0.2-0.8; y=0.6-0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4/mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4/mmm structure at relatively low temperatures of only ∼500°C.     

Neutron powder diffraction study of TbBaCo2−xFexO5+γ layered oxides

The crystal and magnetic structures of layered perovskites TbBaCo_2−xFe_xO_5+γ (0.08?x?0.24) were studied by neutron powder diffraction. Increasing iron concentration up to the x values higher than 0.10 leads to the orthorhombic→tetragonal phase transition resulting from the transformation of 2×1×1-type superstructure, formed due to ordering of extra oxygen incorporated into the vacant sites in [TbO_γ] layers, into 3×3×1 superlattice. The concentration ranges, where the orthorhombic and tetragonal lattices exist, are separated with a narrow two-phase domain. For the tetragonal phases with 3×3×1 superstructure (space group P4/mmm), the Co/Fe ions are antiferromagnetically coupled, forming G-type spin-ordered configuration. The Co³⁺ cations located in square-pyramidal sites adopt intermediate spin state, whilst a relatively small magnetic moment of Co³⁺ ions in the octahedral sublattice indicates that a minor fraction of cobalt is in the low-spin state.     

Phase stability of La1−xCaxCrO3−δ in oxidizing atmosphere

The chemical stability of perovskite-type La_1−xCa_xCrO_3−δ (x=0.1, 0.2, 0.3) in high oxygen partial pressure, P_O2, was investigated with three methods: thermogravimetry, XRD analysis, and thermodynamic calculation. The second phase, CaCrO₄ was observed by XRD analysis on the powder equilibrated in high P_O2. Thermogravimetry under fixed temperatures sensitively detected the segregation of the second phase in the form of oxygen incorporation, because oxidation of chromium ion accompanies the segregation. The second phase tended to appear in high P_O2 and at low temperature. The single-phase regions of La_1−xCa_xCrO_3−δ obtained from the two experimental methods well agreed with each other. The results of thermodynamic calculation on the assumption of ideality of the solid solution also agreed with the experimental results. These results suggested the sufficient chemical stability of La_1−xCa_xCrO_3−δ in high P_O2 concerning the application to an interconnector of high-temperature solid oxide fuel cells; for example, La_0.7Ca_0.3CrO_3−δ is stable at 1273 K in air.     

Electron/hole and ion transport in La1−xSrxFeO3−δ

The conductivity of the entire solid solution La_1−xSr_xFeO_3−δ, where x=0.2, 0.4, 0.5, 0.7 and 0.9, in the oxygen partial pressure range 10⁻¹⁹-0.5 atm and temperatures between 750°C and 950°C is reported. The partial contributions from different charge carriers and the energetic parameters governing transport of charged species reveal that the lanthanum-strontium ferrites can be characterized as mixed, ion-electron conductors in the low oxygen pressure/high oxygen deficiency limit. The partial contributions to conductivity from oxygen ions, electrons and holes increase with strontium content and attain maximal values at x=0.5. Further increase in doping results in development of oxygen vacancy ordering phenomena and deterioration of conducting properties.     

Conductivity and carrier traps in La1−xSrxCo1−zMnzO3−δ (x=0.3; z=0 and 0.25)

Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La_0.7Sr_0.3Co_1-zMn_zO_3−δ (z=0 and 0.25) as a function of the temperature and oxygen partial pressure are used to determine the temperature dependence of the conductivity and thermopower at different values of the oxygen deficiency. A model for a hopping conductor with screened charge disproportionation is applied for the data analysis in combination with trapping reactions of n- and p-type carriers on local oxygen vacancy clusters and manganese cations, respectively. Changes in the ratio of n-type to p-type mobility are due to variations in oxygen vacancy concentration and manganese content, while the energetic parameters governing charge disproportionation of the trivalent cobalt cations and formation of vacancy associates are shown to be essentially invariable. These calculated charge carrier site occupancies are used to model temperature variations of the electrical properties in La_0.7Sr_0.3Co_1−zMn_zO_3−δ in favorable correspondence with experimental observations.     

Structural properties of LaxSr2−xFeO4±δ at high temperature and under reducing conditions

A series of materials, La_xSr_2−xFeO_4±δ (x=0.6-1.0), have been investigated structurally using neutron and X-ray diffraction techniques as well as Mössbauer spectroscopy as a function of temperature and composition. These materials adopt the I4/mmm K₂NiF₄-type structure over a wide range of temperatures (25-1200°C) and oxygen stoichiometries. In particular, it was observed that at a critical composition of x>0.8 there was a significant shift in the lattice parameters. This was attributed to changes in the Fe³⁺ content and the resultant effect of this on the d_z² orbitals giving a lengthening of the Fe-O bonds. On heating, completely linear behavior in both the a and c cell constants was observed, masking underlying bond length changes.     

Systematic investigation on new SrCo1−yNbyO3−δ ceramic membranes with high oxygen semi-permeability

SrCo_1−yNb_yO_3−δ (y = 0.025–0.4) were synthesized for oxygen separation application. The crystal structure, phase stability, oxygen nonstoichiometry, electrical conductivity, and oxygen permeability of the oxides were systematically investigated. Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y = 0.025–0.2. However, the further increase in niobium concentration led to the formation of impurity phase. The niobium doping concentration also had a significant effect on electrical conductivity and oxygen permeability of the membranes. SrCo_0.9Nb_0.1O_3−δ exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of ∼2.80 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C for a 1.0-mm membrane under an air/helium oxygen gradient. The further investigation demonstrated the oxygen permeation process was mainly rate-limited by the oxygen bulk diffusion process.     

The synthesis, crystal chemistry and structures of Y-doped brannerite (U1−xYxTi2O6) and thorutite (Th1−xYxTi2O6−δ) phases

Yttrium-doped uranium brannerite (U_1−xY_xTi₂O₆) and thorutite (Th_1−xY_xTi₂O_6−δ) phases were synthesized in air at 1400°C. Powder X-ray diffraction revealed that these phases crystallized to form monoclinic (C2/m) structures. Crystal structures of U_0.54Y_0.46Ti₂O₆ (1) (a=9.8008(2); b=3.7276(1); c=6.8745(1); β=118.38(1); V=220.97(1); Z=2; R_P=7.3%; R_B=4.6%) and Th_0.91Y_0.09Ti₂O_6−δ (2) (a=9.8002(7); b=3.7510(3); c=6.9990(5); β=118.37(4); V=226.40(3); Z=2; R_P=4.5%; R_B=2.9%) were refined from powder neutron diffraction data. These two phases were isostructural, revealing planes of corner and edge-sharing TiO₆ octahedra separated by irregular eight-fold coordinate U/Y or Th/Y atoms. The oxygen sites within the structure of 1 were found to be fully occupied, confirming that the doping of lower valence Y atoms occurs in conjunction with the oxidation of U(IV) to U(V). Y doping of the thorutite phase 2 does not lead to oxidation but rather the formation of oxygen vacancies within the structure.     

Order-Disorder Enhanced Oxygen Conductivity and Electron Transport in Ruddlesden-Popper Ferrite-Titanate Sr3Fe2−xTixO6+δ

The Ruddlesden-Popper ferrite Sr₃Fe₂O_6+δ and its titania-doped derivatives Sr₃Fe_2−xTi_xO_6+δ, where 0<x≤2, have been characterized by X-ray powder diffraction and thermogravimetry. The changes in oxygen content and crystal lattice parameters are consistent with titanium ions entering the solid solution in 4+ oxidation state with octahedral oxygen coordination. Electronic conductivity measurements on polycrystalline Sr₃Fe₂O_6+δ and Sr₃Fe_0.8Ti_1.2O_6+δ in the temperature range 750-1000°C and oxygen partial pressures (pO₂) varying between 10⁻²⁰ and 0.5 atm revealed that the predominant partial conductivity of electrons is proportional to pO₂^−1/4 in the low pO₂ region, while the predominant partial contribution of holes to the conductivity is proportional to pO₂^+1/4 in the high pO₂ range. The pressure-independent oxygen ion conductivity is found to decrease with the increase in titanium content. A possible pathway for oxygen ion migration is discussed in relation to disorder in the oxygen sublattice and titanium doping.     

Oxygen-permeable membranes based on partially B-site substituted BaFe1−yMyO3−δ (M=Cu or Ni)

We carried out the partial substitution of the B-site in BaFeO_3−δ perovskite with divalent cations to develop novel oxygen-permeable materials. We demonstrated that the partial substitution of Cu or Ni by more than 10% resulted in the stabilization of the cubic perovskite structure even at room temperature in a highly oxygen-permeable phase, as revealed by the X-ray diffraction (XRD) analysis. The Cu substitution was more effective for the stabilization, because the introduction of Cu in the lattice more effectively made the Goldschmidt tolerance factor (t) close to 1.0. Ni- and Cu-substituted BaFeO_3−δ membranes showed higher oxygen permeabilities than their parent BaFeO_3−δ membranes particularly at lower temperatures around 600-700 °C owing to the stabilization of the cubic phase. Among the fabricated membranes, a BaFe_0.85Cu_0.15O_3−δ membrane (1.0 mm thickness) showed the highest oxygen permeation flux (1.8 cm³ min⁻¹ cm⁻² at 930 °C) under an air/He gradient. The results indicated that Cu-substituted BaFeO_3-δ is promising as a material for Co-free membranes with high oxygen permeabilities.     

The defect chemistry of La1−ΔMnO3+δ

The effects of oxygen reduction treatments on the magnetic properties of La-deficient manganites, La_1−ΔMnO_3+δ and Sr- and Ca-doped manganites, La_1−xM_xMnO_3+δ (M: Sr, Ca) have been investigated to confirm the contrasting oxygen reduction effects on the magnetization properties. It is found that oxygen reduction treatments in reduced oxygen pressures of 10³- for La_1−ΔMnO_3+δ result in a continuous change in the magnetization but the reduction treatments for La_1−xM_xMnO_3+δ result in a negligible change under the same reduction conditions. To interpret the contrasting behavior of the La-deficient manganites, several possible models have been discussed. Among the models, the most probable model is that vacancies generated by the La deficiency Δ are partially replaced by Δ₂(=Δ−Δ₁?Δ₁) Mn ions to give both La and Mn site vacancies according to the formula La_1−ΔV_ΔMnO_3+δ→{La_1−ΔMn_Δ2V_Δ1}{Mn_1−Δ2V_Δ2}O_3+δ. Details of thermodynamic basis of this model have been presented.     

Stability and oxygen ionic conductivity of zircon-type Ce1−xAxVO4+δ (A=Ca, Sr)

Zircon-type Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0-0.2) are stable in air up to approximately 1300 K, whilst further heating or reducing oxygen partial pressure leads to formation of A-site deficient zircon and CeO_2−δ phases. The stability boundaries of Ce_1−xA_xVO_4+δ are comparable to those of vanadium dioxide and calcium orthovanadate. At oxygen pressures lower than 10⁻¹⁵ atm, perovskite-type CeVO_3−δ is formed. The oxygen ion transference numbers of Ce_1−xA_xVO_4+δ, determined by faradaic efficiency measurements in air, vary in the range from 2×10⁻⁴ to 6×10⁻³ at 973-1223 K, increasing with temperature. The oxygen ionic conductivity has activation energy of 87-112 kJ/mol and is essentially independent of A-site dopant content. Contrary to the ionic transport, p-type electronic conductivity and Seebeck coefficient of Ce_1−xA_xVO_4+δ are influenced by the divalent cation concentration. The average thermal expansion coefficients of Ce_1−xA_xVO_4+δ, calculated from high-temperature XRD and dilatometric data in air, are (4.7-6.1)×10⁻⁶ K⁻¹.     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.     

Effects of temperature and Nd composition on non-linear transport properties in substituted Ce1−xNdxO2−δ cerium dioxides

Cerium dioxides doped or substituted by neodymium have been prepared using low- (320°C) and high-temperature (1600°C) processes. The Nd substituted ceria phase obtained at high temperature is a solid solution Ce_1−xNd_xO_2−δ 0?x?0.30. Electrical impedance spectroscopy analyses have been performed in the temperature range 40-700°C. At temperatures above 400°C, Nyquist representations allow to separate three signals corresponding to bulk, grain boundary and electrode responses. Non-linear variations of the resistance and the capacitance as functions of temperature and composition x are observed. In the case of grain boundary and electrode interface signals, constant phase elements with non-integer exponent n have been used to represent the equivalent circuits. For each contribution, the conductance strongly increases then reaches a limit value, above x=0.10. When composition x increases, the condensation of Nd-vacancy defect clusters might be at the origin of the non-linear evolution of the conductance. Bulk and grain boundary conductions present different activation energies (0.7 and 1.3 eV).     

Structure, bonding, and magnetic response in two complex borides: Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2

Polycrystalline samples of two complex intermetallic borides Zr₂Fe_1−δRu_5+δB₂ and Zr₂Fe_1−δ(Ru_1−xRh_x)_5+δB₂ (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc₂Fe(Ru_1−xRh_x)₅B₂ and crystallize in the space group P4/mbm (no. 127) with the Ti₃Co₅B₂-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr₂FeRu₅B₂” using LDA yielded a narrow peak at the Fermi level assigned to Fe-Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.     

Synthesis and characterization of La4BaCu5O13+δ and La4BaCu5−xMxO13+δ: M=Fe, Co, Ni, Zn

La₄BaCu_5−xM_xO_13+δ: M=Fe, Co, Ni, Zn were prepared by the solid-state route at 1000°C. Solid solution limits of x=1.0(1) [Fe], x=1.1(1) [Co], x=1.56(7) [Ni] and x=0.47(1) [Zn] were determined from XRD and EPMA results. Rietveld refinement of combined XRD/neutron powder diffraction data was carried out on undoped La₄BaCu₅O_13+δ and x=1 for M=Fe, Co, Ni. For La₄BaCu₅O_13+δ, which is an oxygen-deficient perovskite, the presence of square planar CuO₄ groups, disordered over the Cu(2) sites with CuO₅ square pyramids, is indicated, together with, for δ<0, either square planar CuO₄ or square pyramidal CuO₅ and octahedral CuO₆ groups disordered over the Cu(1) sites. For M=Fe, Ni, there was preferential substitution onto the one-fold octahedral site; for M=Co, substitution took place on both the one-fold octahedral and four-fold square pyramidal sites.     

Using CBED and crystallographic image processing to evidence a structural distortion in a new family of ionic conductor Sr1−xLa1+xAl1−xMgxO4 (0?x?0.7)

A previous study by Raman scattering of the Sr_1−xLa_1+xAl_1−xMg_xO₄ solid solution evidenced a distortion from ideal K₂NiF₄ structure. X-ray powder diffraction and selected area electron diffraction studies were carried out and no lowering of symmetry was observed. All the reflections could be indexed in the space group I4/mmm with a=b=0.38 nm and c=1.27 nm. A coupled study by convergent beam electron diffraction and crystallographic image processing was performed. These techniques have been used to determine a crystal distortion due to small atom displacements from mirrors or axes, which lower the structure symmetry. The mm2 point group symmetry was determined. A microdiffraction study leads to the Imm2 space group. This orthorhombic distortion allows a better understanding of the ionic conductivity behavior of these compounds.     

Enthalpies of Formation of La1−xAxMnO3±δ (A=Ca and Sr) Measured by High-Temperature Solution Calorimetry

High-temperature reaction calorimetry using molten lead borate as a solvent has been used to study the thermochemistry of La_1−xA_xMnO₃ (A=Ca and Sr, 0≤x≤1). The structural properties and the oxygen stoichiometry of the calcined powders were characterized by powder X-ray diffraction, thermogravimetry, and iodometry. The enthalpy of formation of the materials from their binary constituents has been calculated from measured enthalpy of solutions. The enthalpy of formation of La_1−xCa_xMnO₃ from the component oxides becomes more exothermic with increasing substitution level x due to the increasing valence state of Mn, and demonstrates a nearly linear dependence of the average valence state of manganese. La_1−xSr_xMnO₃ has a significantly more exothermic enthalpy of formation from the binary constituents than the Ca-substituted materials, and demonstrates a negative deviation from ideal behavior. The present thermodynamic data are discussed in relation to the Goldschmidt tolerance factor for the perovskite structure. Finally, the phase stability of Sr- and Ca-substituted LaMnO₃ is addressed in relation to the application of these materials in solid oxide fuel cells.