Phase Equilibria and Crystal Structure of Complex Oxides in the La–Sr–Co–Ni– System

Phase equilibria in the La–Sr–Co–Ni–O system were studied in air at 1100°. The samples for the study were synthesized by the standard ceramic and citrate processes. The limiting solubility and structure of La_1-xSr_xCo_1-yNi_yO_3- and (La_1-xSr_x)₂Co_1-yNi_yO₄ solid solutions were determined by Xray powder diffraction analysis. La_1-xSr_xCo_1-yNi_yO₃- solid solutions with 0 x 0.5 have a distorted rhombohedral perovskitelike structure (R c space group). An increase in the strontium concentration reduces the rhombohedral distortions, and the compounds with x < 0.5 have an ideal cubic structure (Pm3m space group). (La_1-xSr_x)₂Co_1-yNi_yO₄ crystals have a tetragonal K₂NiF₄ type unit cell (I4/mmm space group). The relationships between unit cell parameters and compositions were obtained for singlephase La_1-xSr_xCo_1-yNi_yO_3- and (La_1-xSr_x)₂Co_1-yNi_yO₄ samples. The existence regions of La_1-xSr_xCo_1-yNi_yO₃- and La_1-xSr_x)₂Co_1-yNi_yO₄ solid solutions were distinguished on P–T phase diagrams.     

Crystal Structure and Property of Perovskite-Type Oxides Containing Ion Vacancy

Studies on the role of oxygen vacancy in structural change of nonstoichiometric perovskites and a property of oxygen-deficient perovskite-related K₂NiF₄ compounds are reviewed.The structural changes on which the authors focused are cation ordering and lattice distortion. The relationship between the distortion and oxygen vacancy was investigated by comparing the structures of Sr₂(Sr_1-xM_x)TaO_6-d (M = Ca²⁺ and Nd³⁺) solid solutions. It was found that distortion of a perovskite-type lattice decreased with an increasing amount of oxygen vacancies. In order to investigate the relationship between the cation ordering on octahedral sites and oxygen vacancy, structures of stoichiometric Sr_2-xLa_xCo_1-yTa_1+yO₆ and oxygen-deficient Sr_2-xLa_xMg_1-yTa_1+yO_6-d solid solutions were compared. The authors' work reveals that the cation ordering affects the amount of oxygen vacancies in addition to cation charge and size.     

Symmetry and Intervalent Charge Transfer in Copper-Oxide Superconducting Precursors

A vibrationally coupled intervalent charge transfer theory is proposed for the copper oxide-superconductors. The proposed B_1g-vibrational distortion exactly interchanges the chemical environment of the neighboring copper atoms. A quadruple cell is constructed to accommodate the non-stoichiometry and a double copper-oxide-conducting chain with distorted symmetry is postulated. The distorted structure, prone to switching of the copper sites by vibration, is postulated to be the superconducting precursor. The left over-oxygen, due to oxygen deficiency, is allowed to serve as a bridge between the chains and 10 oscillate with the vibration. Symmetry of the system, before and after distortion, is used to explain the spin-angular momentum change, needed to arrive at Cooper-paired electron/holes. The O^? and O⁼ atoms are postulated to be the bridge for superexchange and charge flow between the Cu⁺¹/Cu⁺³ and Cu⁺¹/Cu⁺² pairs. The same theory is used to interpret the mechanisms for the following three types of copper oxide-superconductors: YBa₂Cu₃O_T-x; La_2-xSr_xCuO₄ and the newly discovered electron superconductor Nd_2-xCe_xCuO_4-y. The bridge oxygen oscillation may be chosen not to come from the breathing mode but from the bending deformation mode that involves the relative motion of the Cu atoms (to the oxygen) giving rise to smaller oxygen isotope effect.     

Effects of the partial replacement of La by M (M=Ce, Ca and Sr) in La2-xMxCuO4 perovskites on catalysis of the water-gas shift reaction

The performance of La2-xMxCuO4 perovskites (where M = Ce, Ca or Sr) as catalysts for the water-gas shift reaction was investigated at 290 ℃ and 360 ℃. The catalysts were characterized by EDS, XRD, N2 adsorption-desorption, XPS and XANES. The XRD results showed that all the perovskites exhibited a single phase (the presence of perovskite structure), suggesting the incorporation of metals in the perovskite structure. The XPS and XANES results showed the presence of Cu²⁺ on the surface. The perovskites that exhibited the best catalytic performance were La2-xCexCuO4 perovskites, with CO conversions of 85%-90%. Moreover, these perovskites have higher surface areas and larger amounts of Cu on the surface. And Ce has a higher filled energy level than the other metals, increasing the energy of the valence band of Ce and providing more electrons for the reaction. Besides, the La1.80Ca0.20CuO4 perovskite showed a good catalytic performance.     

中温固体氧化物燃料电池La1.6Sr0.4Ni1-xCuxO4阴极材料的制备及电化学性能

采用甘氨酸-硝酸盐法合成了中温固体氧化物燃料电池阴极材料La_1.6Sr_0.4Ni_1-xCu_xO₄ (x=0.2, 0.4, 0.6,0.8), 利用X射线衍射(XRD)和扫描电子显微镜(SEM)对其结构和微观形貌进行了表征. 结果表明, 该阴极材料与固体电解质Ce_0.9Gd_0.1O_1.95(CGO)在1000 °C烧结时不发生化学反应, 且烧结4 h 后, 二者之间可形成良好的接触界面. 利用电化学交流阻抗谱技术对阴极材料的电化学性能进行研究, 结果显示, 当Cu离子掺杂量(x)为0.6 时, La_1.6Sr_0.4Ni_0.4Cu_0.6O₄阴极具有最小的极化电阻, 在空气中当测试温度为750 °C时, 极化电阻为0.35 Ω·cm². 在不同氧分压条件下电化学阻抗谱分析结果表明, 电极上的两个氧还原反应主要包含氧离子从三相界面向电解质CGO 转移的过程和电荷的迁移过程, 其中电荷的迁移过程为电极反应的速率控制步骤.La_1.6Sr_0.4Ni_0.4Cu_0.6O₄电极在空气中700 °C和阴极电流密度为45 mA·cm^-2时, 阴极过电位为45 mV. 本研究的初步结果表明La_1.6Sr_0.4Ni_1-xCu_xO₄材料是一种电化学性能较为优良的新型中温固体氧化物燃料电池(IT-SOFC)阴极材料.     

La2−xCexCu1−yZnyO4 perovskites for high temperature water-gas shift reaction

The performance of La_2−xCe_xCu_1−yZn_yO₄ perovskites as catalysts for the high temperature water-gas shift reaction (HT-WGSR) was investigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites exhibited the La₂CuO₄ orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the La_1.90Ce_0.10CuO₄ perovskite alone, when calcined at 350/700 °C, also showed a (La_0.935Ce_0.065)₂CuO₄ perovskite with tetragonal structure, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those calcined at 350/700 °C and, among these, La_1.90Ce_0.10CuO₄ was outstanding, probably because of the high surface area associated with the presence of the (La_0.935Ce_0.065)₂CuO₄ perovskite with tetragonal structure and orthorhombic La₂CuO₄ phase.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Electrical Resistivity, Magnetic Susceptibility, X-Ray Photoelectron Spectroscopy, and Electronic Band Structure Studies of Cu2.33−xV4O11

The electronic and physical properties of Cu_2.33V₄O₁₁ were characterized by electrical resistivity, magnetic susceptibility and X-ray photoelectron spectroscopy (XPS) measurements and by tight-binding electronic band structure calculations. Attempts to prepare Cu_2.33−xV₄O₁₁ outside its narrow homogeneity range led to a mixture of Cu_2.33V₄O₁₁, CuVO₃ and β-Cu_xV₂O₅. The magnetic susceptibility data show no evidence for a magnetic/structural transition around 300 K. The XPS spectra of Cu_2.33V₄O₁₁ reveal the presence of mixed valence in both Cu and V. The [Cu⁺]/[Cu²⁺] ratio is estimated to be 1.11 from the Cu 2p_3/2 peak areas, so [V⁴⁺]/[V⁵⁺]=0.56 by the charge balance. Our electronic structure calculations suggest that the oxidation state of the Cu ions is +2 in the channels of CuO₄ tetrahedra, and +1 in the channels of linear CuO₂ and trigonal planar CuO₃ units. This predicts that [Cu⁺]/[Cu²⁺]=1.33 and [V⁴⁺]/[V⁵⁺]=0.50, in good agreement with those deduced from the XPS study.     

铁和锰的化学状态对 LaFexMnyAl12-x-yO19 催化剂上 N2O 分解的影响

 以共沉淀法制备了 LaFexMnyAl12-x-yO19 六铝酸盐催化剂, 并用 X 射线衍射、扫描电镜、N2 吸附-脱附、紫外-可见漫反射光谱和穆斯堡尔谱对催化剂进行了表征, 考察了催化剂上高浓度 N2O 分解反应的性能. 结果表明, 在所考察的条件下, Mn 比 Fe 更有利于促进六铝酸盐晶相的形成. LaFexAl12-xO19 (x = 0.5, 1) 中 Fe 以 Fe3+位于六铝酸盐尖晶石结构中的四面体位和镜面层结构中的三角双锥位, 其中后者为 N2O 分解的主要活性中心. LaMnyAl12-yO19 (y = 0.5, 1) 中 Mn 优先以 Mn2+进入四面体位, 然后以 Mn3+进入尖晶石结构中的八面体位, 并成为 N2O 分解的主要活性中心.     

Solid-state nuclear magnetic resonance investigation of synthetic phlogopite and lepidolite samples

In the present work, our aim is to decipher the cationic ordering in the octahedral and tetrahedral sheets of two Al-rich synthetic materials, namely, phlogopites of nominal composition K(Mg_3-xAl_x)[Al_1+xSi_3-xO₁₀](OH)_yF_2-y and lepidolites in the system trilithionite–polylithionite with composition K (Li_xAl_3-x)[Al_4-2xSi_2xO₁₀](OH)_yF_2-y, by directly probing the aluminium distribution through ²⁷Al and ¹⁷O magic-angle spinning, multiple-quantum magic-angle spinning, and ²⁷Al-²⁷Al double-quantum single-quantum nuclear magnetic resonance (NMR) experiments. Notably, ²⁷Al-²⁷Al double-quantum single-quantum magic-angle spinning NMR spectra, recorded at 9.34 and/or 20.00 T, show the spatial proximity or avoidance of the Al species inside or between the sheets. In both studied minerals, the ensemble of NMR data suggests a preference for ^[4]Al in the tetrahedral sheet to occupy position close to the ^[6]Al of the octahedral sheets.     

Preparation and electrical and magnetic properties of Sr2−xNaxCuO3 (0 ≤ x ≤ 1) solid solutions

Mixed-valence copper(II/III) oxide solid solutions Sr_2?xNa_xCuO₃ (0 ≤ x ≤ 1) have been prepared by solid-state reactions in oxygen atmosphere. All solid solutions exhibit the structure of Sr₂CuO₃ (S.G. Immm). Electrical conductivity and thermoelectric power measurements show a semiconducting behavior in the whole composition range. The electronic structure of Sr₂CuO₃ is compared with that of La₂CuO₄ on the basis of an iono-covalent model. Interpretation of transport properties suggests the formation of small polarons. Magnetic susceptibility and EPR measurements show that the antiferromagnetic ordering of Sr₂CuO₃ tends to vanish as x increases, however magnetic interactions are still strong for a concentration of Cu²⁺ ions corresponding to x = 0.8.     

Oxidation of isobutane catalyzed by vanadyl, copper and cesium substituted H3PMo12O40

Effects of Cs⁺, H⁺ and Cu²⁺ counterions in the vanadium containing heteropoly compounds Cs_xH_1-xVO[PMo₁₂O₄₀] and Cs_yH_0.5-yCu_0.25VO[PMo₁₂O₄₀] on the catalytic oxidation of isobutane and characterization by TGA, IR and ESR spectroscopies are reported. A high selectivity of 76% for methacrylic acid and methacrolein together has been obtained with Cs_0.75H_0.25VO[PMo₁₂O₄₀] catalysts at a reactivity of 5.3x10^-1 mmol/h cm³.     

Hydrothermal synthesis, structure and property of nano-BaTiO3-based dielectric materials

A series of Ba_1-xSr_xTi_1-yZr_yO₃ (0≤x≤0.5, 0≤y≤0.4) and BA_1?xZn_xTi_1?ySn_yO₃ (0≤x≤0.3, 0≤y≤0.3) solid solutions were synthesized by low-temperature/low-pressure hydrothermal method below 170°C, 0.8 MPa. XRD pattern and cell parameters-composition figures of these prepared powders demonstrated that they are completely miscible solid solutions based on BaTiO₃. Furthermore, TEM showed that they have a shape of uniform, substantially spherical particles with an average particle size of 70 nm in diameter. The sintered ceramics of those powders doped by Sr²⁺ and Zr⁴⁺ or Zn²⁺ and Sn⁴⁺ have dielectric constant twelve times higher than and dielectric loss 1/6 those of pure BaTiO₃ phase at room temperature.     

Determination of oxygen in ternary uranium oxides by a gravimetric alkaline earth addition method

The applicability of a gravimetric method based on alkaline earth metal addition for the determination of oxygen in ternary uranium oxides of the type M—U—O (M=La, Ce and Th) is described. The oxide sample is mixed with MgO or Ba_2.8UO_5.8 and heated in air under suitable conditions. Because uranium is completely oxidized to the hexavalent state during the reaction, oxygen can be determined from the weight change. Oxygen in La_yU_1-y O_2+x is determined up to y = 0.8 with a standard deviation for x of ±0.006 with MgO. For Th_yU_1-y O_2+x, the value of x is determined with Ba_2.8UO_5.8 with a standard deviation of ±0.01 at y = 0.8. For Ce_yU_1-y O_2+x, the method can be applied only for low cerium concentrations where y = 0–0.2; the value for x with Ba_2.8UO_5.8 at y = 0.2 showed a standard deviation of ±0.002.     

Ein sauerstoffreiches Neodymcuprat: Hochdrucksynthese und Kristallstruktur von Nd12Cu6O25 („Nd2CuO4,17”︁)

An Oxygen-rich Neodymium Cuprate: High-pressure Synthesis and Crystal Structure of Nd₁₂Cu₆O₂₅ (“Nd₂CuO_4.17”) Single crystals of Nd₁₂Cu₆O₂₅ (“Nd₂CuO_4.17”) could be obtained by reaction of Nd₂O₃ und CuO with KO₂ in a modified Belt-type apparatus at 40 kbar, 1500°C. The compound crystallizes in the monoclinic space group C2/m with a = 17.128(6), b = 3.7288(6), c = 18.364(6) Å, β = 111.22(1)º and Z = 2. The structure refinement converged at R1 = 0.0302 for 1451 reflections with F_o > 4σ(F_o) and R1 = 0.0903, wR2 = 0.0767 for all 2571 data. The structure-determining feature are Cu(1,2)? O octahedral chains, interrupted by Cu(3) atoms in square-planar coordination. Each of this CuO₄ groups is connected to another one, resulting in short copper-copper distances (d_{Cu? Cu} = 3.012 Å). A comparison of the structure of “Nd₂CuO_4.17” with those of La₂CuO₄ and Nd₂CuO₄ makes the structural relations of the former with La₂CuO₄ and, hence, the aristotype K₂NiF₄ evident, in spite of the close stoichiometric similarity between Nd₂CuO₄ and “Nd₂CuO_4.17”.     

Oxide-ion conductivity of the oxygen deficient perovskite solid-solution system, (Ba0.5-xSrxLa0.5)2(In1-yMy)2O5.5 (M=Y or Ga)

In order to elucidate the mechanism of oxide-ion conductivity for (Ba_0.5-xSr_xLa_0.5)(In_1-yM_y)₂O_5.5 (M=Y or Ga, 0<x<0.2, 0<y<0.2) solid-solution system, the electrical conductivity was measured as a function of oxygen partial pressure and temperature, and the results were investigated in terms of a dopant content and unit cell free volume. The system was confirmed to be an oxide-ion conductor from the oxygen partial pressure dependence on electrical conductivity. The oxide-ion conductivity increased with increasing the unit cell free volume at first. However, it showed a maximum at a value of free volume, and then decreased. The decreasing conductivity vs. the volume would be related to the crystal symmetry change. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization and Activity of Iron-Manganese Catalysts for Carbon Monoxide Hydrogenation

Studies on the structural changes and catalytic behavior of iron-manganese catalysts for CO hydrogenation were conducted using Mossbauer spectroscopy, X-ray diffraction, temperature programmed reduction and kinetic measurements. It was observed that the reduction of the mixed oxide catalyst precursors proceeds via the formation of Fe_3-xMn_xO₄,Mn_3-xFe_xO₄ mixed spinel and Fe_1-zMn₂O mixed oxide to α-iron and MnO. After use for CO hydrogenation, catalysts are oxidized as well as carburized. The Mn_3-yFe_yO₄ mixed spinel and Fe_1-2Mn_zO mixed oxide are the most powerful phases for olefin production. The highest attainable 2–4 low carbon olefin selectivity is 41% with an 86% conversion level. Higher manganese content or lower reduction temperatures may change the carbide formed from χ-Fe₅C₂ to the more unstable ?′-Fe₂₂C. Carbide formation is greatly dependent on manganese content and activation procedure used.     

Influence of partial substitution of zinc for copper on electronic structures of YBa2Cu3O y

Summary By use of an approximate band-structure treatment based on the EHMO approach, the energy band structures for the Zn-doped superconductor YBa₂Cu_3–x Zn _x O _y were calculated in the present paper and the influence of partial substitution of zinc for copper on the electronic structures for orthorhombic YBa₂Cu₃O_y was studied. From analysis of the band structures and the densities of states for YBa₂Cu_3–x Zn _x O _y , it was demonstrated that the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system have a direct and dominant influence on superconductivity, whereas the role of the 1D Cu-O ribbons and the O(4) atoms is also of some importance.     

Synthesis and Crystal Structure of Na7Cu3O8, Containing a New Type of Oxocuprate(III) Oligoanion

Na₇Cu₃O₈ was prepared through oxidation of a Na₃CuO₂/NaCuO mixture (2:1) in dried oxygen at 450 °C. Single crystals have been grown by annealing of Na₇Cu₃O₈, in the presence of Na₂O₂, at 450 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure ( , Z = 1, a = 5.5891(2), b = 6.0945(2), c = 7.8890(3) Å, α = 110.059(2), β = 108.669(2), γ = 90.237(2)°) a new Cu₃O₈^7- oxocuprate anion, consisting of three edge sharing CuO₄ squares, is the prominent structural feature. These anions are aligned parallel to the space diagonal of the unit cell and can be regarded as infinite chains from which every fourth copper atom has been removed. This new representative of an oxocuprate(III) anion gives support to the expectation that the gap between dimeric and infinite edge sharing units of square planar cuprate anions can be closed, in principle.     

Hydrothermal synthesis, structure and property of nano-BaTiO3-based dielectric materials

A series of Ba_1-xSr_xTi_1-yZr_yO₃ (0≤x≤0.5, 0≤y≤0.4) and BA_1?xZn_xTi_1?ySn_yO₃ (0≤x≤0.3, 0≤y≤0.3) solid solutions were synthesized by low-temperature/low-pressure hydrothermal method below 170°C, 0.8 MPa. XRD pattern and cell parameters-composition figures of these prepared powders demonstrated that they are completely miscible solid solutions based on BaTiO₃. Furthermore, TEM showed that they have a shape of uniform, substantially spherical particles with an average particle size of 70 nm in diameter. The sintered ceramics of those powders doped by Sr²⁺ and Zr⁴⁺ or Zn²⁺ and Sn⁴⁺ have dielectric constant twelve times higher than and dielectric loss 1/6 those of pure BaTiO₃ phase at room temperature.