Hexaferrites and phase relations in the iron-rich part of the system Sr-La-Co-Fe-O

The iron rich part of the system was examined in the temperature range of 1200-1380 °C in air, with focus on the solid solutions of M-type hexaferrites. Samples of suitable compositions were studied by electronprobe microanalysis (EPMA). Substituted Sr-hexaferrites in the system Sr-La-Co-Fe-O do not follow the 1:1 substitution mechanism of La/Co in M-type ferrites. Due to the presence and limited Co²⁺-incorporation Fe³⁺-ions are reduced to Fe²⁺ within the crystal lattice to obtain charge balance. In all examined M-type ferrites divalent iron is formed, even at 1200 °C. The substitution principle Sr²⁺+Fe³⁺↔La³⁺+(Fe²⁺, Co²⁺) yields to the general substitution formula for the M-type hexaferrite Sr²⁺_1-xLa³⁺_xFe²⁺_x-yCo²⁺_yFe³⁺_12-xO¹⁹ (0≤x≤1 and 0≤y≤x). In addition Sr/La-perovskite_SS (_SS=solid solution), Co/Fe-spinel_SS, hematite and magnetite are formed. Sr-hexaferrite exhibits at 1200 °C a limited solid solution with small amounts of Fe²⁺ (SrFe₁₂O₁₉↔Sr_0.3La_0.7Co_0.5Fe²⁺_0.2Fe_11.3O₁₉). At 1300 and 1380 °C a continuous solid solution series of the M-type hexaferrite is stable. SrFe₁₂O₁₉ and LaCo_0.4Fe²⁺_0.6Fe₁₁O₁₉ are the end members at 1300 °C. The maximum Fe²⁺O content is about 13 mol% in the M-type ferrite at 1380 °C (LaCo_0.1Fe²⁺_0.9Fe₁₁O₁₉).     

Photomagnetism in CxCo4[Fe(CN)6](8+x)/3·n H2O Prussian blue analogues: looking for the maximum photo-efficiency

A family of CoFe Prussian blue analogues C_xCo₄[Fe(CN)₆]_(8+x/3)□_(4–x)3 (x = amount of alkali cation inserted per conventional cell, C = Na, K, Rb, Cs; □ = [Fe(CN)₆] vacancy) have been synthesized and characterized. Their photomagnetic properties have been investigated by magnetic measurements before and after irradiation and X-ray diffraction under continuous irradiation. We show that the photo-induced magnetism depends on several parameters: (i) the amount of Co^III–Fe^II diamagnetic excitable pairs per cell; (ii) the amount of [Fe(CN)₆] vacancies, and (iii) the amount and nature of the alkali cations per cell. We evidence a discontinuity in the properties' change when the amount of alkali cation x varies, around x = 1. For x < 1, there is an excitation of diluted Co^III–Fe^II diamagnetic pairs in a phase mainly composed of magnetic Co^II–Fe^III entities within the same structural phase through a second-order continuous transformation. For x ≥ 1, the formation of domains mainly composed of Co^II–Fe^III* metastable magnetic pairs in a phase mainly composed of Co^III–Fe^II diamagnetic ones through a first-order discontinuous transition is observed. The study points out that sodium derivatives are more efficient than the others. Among them, Na₁Co₄[Fe(CN)₆]₃□₁ is predicted to be the most efficient one. To cite this article: A. Bleuzen et al., C. R. Chimie 6 (2003).     

Electrical properties and initial permeability of Cu–Mg ferrites

A series of polycrystalline spinel ferrites with composition Cu_1−xMg_xFe₂O₄ where 0.0 ≤ x ≤ 1 are prepared by the standard ceramic method. The single-phase cubic spinel structure of all the samples has been confirmed from X-ray diffraction analysis. The lattice constant increases linearly with increasing magnesium content obeying Vegard's law. The electrical properties (ɛ′, and σ) of the prepared samples are measured at different temperatures as a function of applied frequency ranging from 100 kHz up to 5 MHz. The general trend of ɛ′, and σ is decreased with increasing Mg²⁺ and increases with increasing temperature. The observed variation of dielectric properties is explained on the basis of Cu²⁺/Cu¹⁺ ionic concentration as well as the electronic hopping frequency between Fe²⁺ and Fe³⁺ ions in the present samples. The data of initial permeability is also discussed.     

Ethylbenzene to chemicals: Catalytic conversion of ethylbenzene into styrene over metal-containing MCM-41

Isomorphously substituted (MeDM) and impregnated metal-containing MCM-41 (MeO_x/IM) catalysts, in which Me = Co, Cu, Cr, Fe or Ni, have been prepared. Structural and textural characterizations of the catalysts were performed by means of X-ray diffraction (XRD), chemical analysis, Raman spectroscopy, electron paramagnetic resonance (EPR), N₂ adsorption isotherms and temperature programmed reduction (TPR). Cu²⁺, Co²⁺, and Cr⁴⁺/Cr³⁺ species were found over the catalysts as cations incorporated in the MCM-41 structure (MeDM) or highly dispersed oxides on the surface (MeO_x/IM). The MeDM catalysts exhibited a good performance in the dehydrogenation of ethylbenzene with CO₂. However, MeO_x/IM catalysts had a low performance in styrene production (activity less than 15 × 10^?3 mmol h^?1 and selectivity for styrene less than 80%) due to the high reducibility of the metals species. However, Ni²⁺ or Fe³⁺ coordinated with the MCM-41 framework, as well as NiO_x and Fe₂O₃ extra-framework species, is continuously oxidized by the CO₂ to maintain the active sites for dehydrogenating ethylbenzene. Deactivation studies on the FeDM sample showed that Fe³⁺ species produced active sp² carbon compounds, which are removed by CO₂; the referred sample is catalytically selective for styrene and stable over 24 h of reaction. In contrast, highly active Ni²⁺ and Ni⁰ species produced a large amount of polyaromatic carbonaceous deposits from styrene, as identified by TPO, TG and Raman spectroscopy. An acid–base mechanism is proposed to operate to adsorb ethylbenzene and abstract the β-hydrogen. CO₂ plays a role in furnishing the lattice oxygen to maintain the Fe³⁺ active sites in the dehydrogenation of ethylbenzene to form styrene.     

X-ray photoelectron (XPS) and Diffuse Reflectance Infra Fourier Transformation (DRIFT) study of Ba0.5Sr0.5CoxFe1−xO3−δ (BSCF: x=0-0.8) ceramics

The X-ray photoelectron spectra (XPS) of sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1-xO_3-δ,0≤x≤0.8) were measured at room temperature (RT). Peak areas of Fe_2p1, Fe_2p3, Fe_3p and Co_3p increased systematically with increasing cobalt concentration, while their binding energies (BEs) remained the same (723.3, 710.0, 55.0 and 60.9 eV, respectively). However, the BEs of lattice oxygen in O_1s (528.1 eV) and Ba_4d for the BaO bond (87.9V and 90.2 eV) increased with increasing cobalt concentration. The shoulder peak of Ba_3d/Co_2p increased from 778.0 to 778.7 eV, which implies that this peak can be attributed to another Ba XPS peak (described as Ba_2nd in this study) due to the overlapping area between barium cations and oxygen anions. The overall peak areas of Ba_4d increased up to x=0.4, and then decreased, which coincides with the behavior of the Diffuse Reflectance Infrared Fourier Transform (DRIFT) bands representing adsorbed CO₃²⁻ (νCO₃²⁻) and structurally bonded CO₃²⁻ (ν₂, ν₃) (800-1200 and 862/1433 cm⁻¹, respectively).     

Electrochemical and mechanochemical formation of solid solutions of potassium copper(II)/zinc(II) hexacyanocobaltate(III)/hexacyanoferrate(III) KCu x Zn1-x [hcc] x [hcf]1-x

Electrochemical reduction/oxidation cycles of immobilised powder mixtures of KCu[hcc] and KZn[hcf] as well as their mechanical milling lead to the formation of a quaternary solid solution (mixed crystals) with Cu²⁺ and Zn²⁺ on the nitrogen coordinated sites and Fe³⁺ and Co³⁺ on the carbon coordinated sites. The reaction products were studied by the X-ray diffractometry and voltammetric techniques. The formation of solid solutions of the general formula KCu_xZn_1-x[hcc]_x[hcf]_1-x is the first example of an electrochemical and mechanochemical reaction leading to mixed hexacyanometalates.Dedicated to Professor Dr. G. Horányi on the occasion of his 70th birthday.     

Synthesis and electrochemical characteristics of xLi2MnO3–(1−x)Li(Mn0.375Ni0.375Co0.25)O2 (0.0 ≤ x ≤ 1.0) thin film cathode for lithium rechargeable micro batteries

We have compared the structure, microstructure, and electrochemical characteristics of xLi₂MnO₃–(1−x)Li(Mn_0.375Ni_0.375Co_0.25)O₂ (0.0 ≤ x ≤ 1.0) thin films with their bulk cathode laminate counterparts of identical compositions. Pure Li(Mn_0.375Ni_0.375Co_0.25)O₂ as well as the synthesized composite films partially transform into cubic spinel structure during charge–discharge cycling. In contrast, such layered to spinel phase transformation has only been identified in bulk cathode laminates with x ≥ 0.75. At a current density 0.05 mAcm⁻², the discharge capacity of Li(Mn_0.375Ni_0.375Co_0.25)O₂ thin film was measured to be ∼60 μAhcm⁻². The discharge capacity (∼217 μAhcm⁻²) was markedly improved in x∼0.5 composite thin film. The capacity retention after 20 charge discharge cycles are improved in composite films; however, their capacity fading could not be eliminated completely.     

Enzyme mimics of spinel-type CoxNi1−xFe2O4 magnetic nanomaterial for eletroctrocatalytic oxidation of hydrogen peroxide

A series of spinel-type Co_xNi_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) magnetic nanomaterials were solvothermally synthesized as enzyme mimics for the eletroctrocatalytic oxidation of H₂O₂. X-ray diffraction and scanning electron microscope were employed to characterize the composition, structure and morphology of the material. The electrochemical properties of spinel-type Co_xNi_1−xFe₂O₄ with different (Co/Ni) molar ratio toward H₂O₂ oxidation were investigated, and the results demonstrated that Co_0.5Ni_0.5Fe₂O₄ modified carbon paste electrode (Co_0.5Ni_0.5Fe₂O₄/CPE) possessed the best electrocatalytic activity for H₂O₂ oxidation. Under optimum conditions, the calibration curve for H₂O₂ determination on Co_0.5Ni_0.5Fe₂O₄/CPE was linear in a wide range of 1.0 × 10⁻⁸–1.0 × 10⁻³ M with low detection limit of 3.0 × 10⁻⁹ M (S/N = 3). The proposed Co_0.5Ni_0.5Fe₂O₄/CPE was also applied to the determination of H₂O₂ in commercial toothpastes with satisfactory results, indicating that Co_xNi_1−xFe₂O₄ is a promising hydrogen peroxidase mimics for the detection of H₂O₂.     

Solid state reaction synthesis of filled skutterudite compounds (Ce or Y)yFexCo4-xSb12 and the effect of filling atoms Ce or Y on lattice thermal conductivity

The synthesis of filled skutterudite compounds (Ce or Y)yFexCo₄-xSb12, through a solid state reaction using chloride of Ce or Y, high purity powder of Co, Fe, and Sb as starting materials, was investigated. (Ce or Y)yFexCo₄-xSb12 (x = 0 1.0,y = 0 0.15) compounds were obtained at 850 1 123 K. The results of Rietveld analysis demonstrate that (Ce or Y)yFexCo₄-xSb12 synthesized by a solid state reaction possesses a filled skutterudite structure. The filling fraction of Ce or Y obtained by Rietveld analysis agrees well with the composition obtained by chemical analysis. The lattice constant of CeyFexCo₄-xSb12 increases with increasing substitution of Fe at Co sites, and with an increasing Ce filling fraction in the Sb-dodecahedron voids. The lattice thermal conductivity of (Ce or Y)yFexCo₄-xSb12 decreases significantly with an increasing Ce or Y filling fraction in the voids and with substitution of Fe at Co sites.     

Calorimetric investigation of mixing enthalpy of liquid (Co + Cu + Zr) alloys at T = 1873 K

The enthalpies of mixing of liquid (Co + Cu + Zr) alloys have been determined using the high-temperature isoperibolic calorimeter. The measurements have been performed along three sections (x_Co/x_Cu = 3/1, 1/1, 1/3) with x_Zr = 0 to 0.55 at T = 1873 K. Over the investigated composition range, the partial mixing enthalpies of zirconium are negative. The limiting partial enthalpies of mixing of undercooled liquid zirconium in liquid (Co + Cu) alloys are (−138 ± 18) kJ · mol⁻¹ (the section x_Co/x_Cu = 3/1), (−155 ± 10) kJ · mol⁻¹ (the section x_Co/x_Cu = 1/1), and (−130 ± 22) kJ · mol⁻¹ (the section x_Co/x_Cu = 1/3). The integral mixing enthalpies are sign-changing. The isenthalpic curves have been plotted on the Gibbs triangle. The main features of the composition dependence of the integral mixing enthalpy of liquid ternary alloys are defined by the pair (Co + Zr) and (Cu + Zr) interactions.     

Control of Co content and SOFC cathode performance in Y1−ySr2+yCu3−xCoxO7+δ

The electrochemical performance of the layered perovskite YSr₂Cu_3−xCo_xO_7+δ, a potential solid oxide fuel cell (SOFC) cathode, is improved by increasing the Co content from x = 1.00 to a maximum of x = 1.30. Single phase samples with x > 1.00 are obtained by tuning the Y/Sr ratio, yielding the composition Y_1−ySr_2+yCu_3−xCo_xO_7+δ (where y ≤ 0.05). The high temperature structure of Y_0.95Sr_2.05Cu_1.7Co_1.3O_7+δ at 740 °C is characterised by powder neutron diffraction and the potential of this Co-enriched material as a SOFC cathode is investigated by combining AC impedance spectroscopy, four-probe DC conductivity and powder XRD measurements to determine its electrochemical properties along with its thermal stability and compatibility with a range of commercially available electrolytes. The material is shown to be compatible with doped ceria electrolytes at 900 °C.     

Magnetostructural correlations in the antiferromagnetic Co2−x Cux(OH)AsO4 (x=0 and 0.3) phases

The Co_2−xCu_x(OH)AsO₄ (x=0 and 0.3) compounds have been synthesized under mild hydrothermal conditions and characterized by X-ray single-crystal diffraction and spectroscopic data. The hydroxi-arsenate phases crystallize in the Pnnm orthorhombic space group with Z=4 and the unit-cell parameters are a=8.277(2) Å, b=8.559(2) Å, c=6.039(1) Å and a=8.316(1) Å, b=8.523(2) Å, c=6.047(1) Å for x=0 and 0.3, respectively. The crystal structure consists of a three-dimensional framework in which M(1)O₅-trigonal bipyramid dimers and M(2)O₆-octahedral chains (M=Co and Cu) are present. Co₂(OH)AsO₄ shows an anomalous three-dimensional antiferromagnetic ordering influenced by the magnetic field below 21 K within the presence of a ferromagnetic component below the ordering temperature. When Co²⁺ is partially substituted by Cu²⁺ions, Co_1.7Cu_0.3(OH)AsO₄, the ferromagnetic component observed in Co₂(OH)AsO₄ disappears and the antiferromagnetic order is maintained in the entire temperature range. Heat capacity measurements show an unusual magnetic field dependence of the antiferromagnetic transitions. This λ-type anomaly associated to the three-dimensional antiferromagnetic ordering grows with the magnetic field and becomes better defined as observed in the non-substituted phase. These results are attributed to the presence of the unpaired electron in the dx²−y² orbital and the absence of overlap between neighbour ions.     

Behavior of Co4+ ion in K2NiF4-type (Ca1+xSm1−x)CoO4

Orthorhombic K₂NiF₄-type (Ca_1+xSm_1−x)CoO₄ (0.00 ≤ x ≤0.15) with space group Bmab has been synthesized by the polymerized complex route. The cell parameters (a and b) decrease, while the cell parameter (c) increases with increasing Co⁴⁺ ion content. The global instability index (GII) indicates that the crystal stability of (Ca_1+xSm_1−x)CoO₄ is not influenced by the Co⁴⁺ ion content. (Ca_1+xSm_1−x)CoO₄ is a p-type semiconductor and exhibits hopping conductivity in the small-polaron model at low temperatures. The magnetic measurement indicates that (Ca_1+xSm_1−x)CoO₄ shows paramagnetic behavior above 5 K, and that the spin state of both the Co³⁺ and Co⁴⁺ ions is low. The Co⁴⁺ ion acts as an acceptor, and the electron transfer becomes active through the Co³⁺–O–Co⁴⁺ path as the Co⁴⁺ ions increase.     

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe_1−xCo_xO_3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO_3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe³⁺OCo⁴⁺↔Fe⁴⁺OCo³⁺. This model is strongly supported by ⁵⁷Fe Mössbauer measurements which show the existence of two sites at room temperature, high spin localized Fe⁴⁺ and delocalized Fe^3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO₃ structure.     

Structural variations and cation distributions in Mn3−xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn_3?xCo_xO₄ were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn²⁺ and/or Co²⁺ occupy the tetrahedral site while Mn³⁺, Co²⁺, Co^III (cobalt in low-spin state) and Mn⁴⁺ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn³⁺/Mn⁴⁺ and Co²⁺/Co^III on the octahedral sites.     

纳米CexFe1-xO2固溶体的模板法制备及其乙醇水蒸气重整催化性能

以超高比表面炭材料为模板,硝酸盐为氧化物前体,通过改进的模板路线制备了具有较高比表面积的纳米CexFe1-xO2固溶体.采用X射线衍射、拉曼光谱、物理吸附和透射电镜对制备的样品进行了表征.结果表明,α-Fe2O3,CexFe1-xO2固溶体和CeO2的粒子尺寸为5~15nm,CeO2中部分Ce4 离子被Fe3 离子取代,从而形成了CexFe1-xO2固溶体.乙醇水蒸气重整反应结果显示,CexFe1-xO2固溶体比相应的α-Fe2O3和CeO2具有更高的催化活性和对氢气的选择性.     

Investigation of the LiCo1−xMgxPO4 (0 ≤ x ≤ 0.1)–graphitic carbon foam composites

LiCo_1−xMg_xPO₄–graphitic carbon foam (LCMP–GCF with 0 ≤ x ≤ 0.1) composites are prepared by Pechini-assisted sol-gel method and annealed with the 2-steps annealing process (T = 300 °C for 5 min in flowing air, then at T = 730 °C for t = 12 h in flowing nitrogen). The XRD analysis, performed on powders reveals LiCoPO₄ as major crystalline phase, Co₂P and Co₂P₂O₇ as secondary phases. The morphological investigation revealed the formation and growth of microcrystalline “islands” which consist of acicular crystallites with different dimensions (typically 5–50 μm). By addition of Mg-ions, CV-curves of LCMP–GCF composites show a decrease of the surface between anodic and cathodic sweeps by cycling and a stark contribution of faradaic processes due to the graphitic structured foam. The electrochemical measurements, at a discharge rate of C/10 at room temperature, show the decrease of the discharge specific capacity from 100 mAh g⁻¹ for x = 0.0 to ∼35 mAh g⁻¹ for 0.025 ≤ x ≤ 0.05, then an increase to 69 mAh g⁻¹ for x = 0.1. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity at high Mg-ions content into the LiCoPO₄ phase (x ≥ 0.025).     

Charge dynamics of 57Fe probe atoms in La2Li0.5Cu0.5O4

The objective of this study is to characterize the electronic state and local surrounding of ⁵⁷Fe Mössbauer probe atoms within iron-doped layered perovskite La₂Li_0.5Cu_0.5O₄ containing transition metal in unusual formal oxidation states “+3”. An approach based on the qualitative energy diagrams analysis and the calculations within the cluster configuration interaction method have been developed. It was shown that a large amount of charge is transferred via Cu-O bonds from the O: 2p bands to the Cu: 3d orbitals and the ground state is dominated by the d⁹L configuration (“Cu²⁺-O^-” state). The dominant d⁹L ground state for the (CuO₆) sublattice induces in the environment of the ⁵⁷Fe probe cations a charge transfer Fe³⁺ + O⁻(L) → Fe⁴⁺ + O²⁻, which transforms “Fe³⁺” into “Fe⁴⁺” state. The experimental spectra in the entire temperature range 77–300 K were described with the use of the stochastic two-level model based on the assumption of dynamic equilibrium between two Fe³⁺↔Fe⁴⁺ valence states related to the iron atom in the [Fe(1)O₄]^4- center. The relaxation frequencies and activation energies of the corresponding charge fluctuations were estimated based on Mössbauer data. The results are discussed assuming a temperature-induced change in the electronic state of the [CuO₄]^5- clusters in the layered perovskite.     

Crystal structure of the Fe6−xGayGe5−y (x ∼0.5, y = 1.3) ternary compound

The single crystal of Fe_6−xGa_yGe_5−y (x ∼0.5, y = 1.3(1)) has been obtained from arc-melting of the elements and its crystal structure has been investigated by single-crystal X-ray diffraction analysis. The compound crystallizes in the hexagonal space group P6₃/mmc (No. 194) with a = 8.0346(2) Å, c = 5.007(1) Å, Z = 1 and adopts the Ti₆Sn₅ structure type with a refined composition of Fe_5.52(12)(Ga/Ge)₅. By a mean of scanning electron microscopy the composition of the single crystal has been determined as Fe_4.9(2)Ga_1.3(1)Ge_3.7(1). The crystal structures of the Fe_6−xGa_yGe_5−y (x ∼0.5, y = 1.3(1)) and all compounds in the Fe–Ge binary system have been crystallographically analyzed and structural relationship has been established.