Thermodynamic quantities and defect equilibrium in La2−xSrxNiO4+δ

In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La_2−xSr_xNiO_4+δ, statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ-P(O₂)-T relation by using Gibbs-Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La_2−xSr_xNiO_4+δ. Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry of La_2−xSr_xNiO_4+δ.     

单室固体氧化物燃料电池阴极材料La0.75Sr0.25Cr0.5Mn0.5O3的电化学性能研究

采用高温固相法制备了La0.75Sr0.25Cr0.5Mn0.5O3(LSCM)并利用XRD,SEM以及电化学阻抗谱(EIS)分别对粉体及电极进行研究。结果发现LSCM在C3H8-O2-N2混合气氛下能够保持很好的高温化学稳定性,且与电解质材料YSZ在1400℃空气气氛下不发生化学反应。电化学测试结果表明,阳极支撑型单室固体氧化物燃料电池Ni-YSZ|YSZ|LSCM在700℃、C3H8-O2-N2混合气氛下的短路电流密度达317 mA·cm-2,最大功率密度73 mW·cm-2。将LSCM与CGO形成梯度阴极,相同测试条件下,单室电池的短路电流密度为560 mA·cm-2,功率密度达到110 mW·cm-2,电池输出性能提高约50%。     

Effect of oxygen non-stoichiometry on the structural and magnetotransport properties of LaMn0.85Cr0.15O3+δ

Thermogravimetry, X-ray diffraction (XRD), d.c. magnetization, high-temperature susceptibility and electrical resistivity measurements were performed for LaMn_0.85Cr_0.15O_3+δ perovskites with accurate control of the oxygen content (0?δ?0.11). For 0?δ<0.09, three orthorhombic structures (Pnma) are found: for 0?δ<0.045, the O′ phase (b/√2<c<a), for 0.045?δ<0.06, the O″ (b/√2<a<c) and for 0.06?δ<0.09, the O^? (a<b/√2<c). For 0.09?δ?0.11, a rhombohedral symmetry coexists with O^? in a biphasic field. Magnetic measurements revealed the ferromagnetic interactions (FM) character of the Mn³⁺-O-Cr³⁺ interaction, but also the intricate magnetic phase diagram due to the presence of multiple interactions (Mn^3+,4+-O-Mn^3+,4+, Cr³⁺-O-Mn³⁺, etc.). The comparison of the results for LaMn_0.85Cr_0.15O_3+δ with those of LaMn_0.9Cr_0.1O_3+δ allows discuss the role of Cr³⁺ on the structural, magnetic and magnetotransport properties of the LaMn_1−xCr_xO_3+δ perovskites.     

Influence of electrospraying parameters on the microstructure of La0.6Sr0.4Co0.2F0.8O3−δ films for SOFCs

Ceramics can play a remarkable role in the engineering of intermediate temperature solid oxide fuel cells (IT-SOFCs) capable of meeting the ambitious targets of reduced cost and improved lifetime. While mixed ionic-electronic conductors such as La_xSr_1−xCo_yFe_1−yO_3−δ are being used as volumic cathodes to increase the catalytic performance of these components, adequate microstructures are also an important requirement for optimal performance, particularly at lower operating temperatures. This work is devoted to the fabrication of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ films on Ce_0.9Gd_0.1O_2−δ substrates by electrostatic spray deposition (ESD) and to the characterization of the microstructural dependence on the deposition conditions. A wide variety of microstructures ranging from dense to porous, with particular features such as reticulation and micro-porosity, were obtained by varying the ESD deposition parameters: nozzle-to-substrate distance (15, 30, 43, 45, and 58 mm), solution flow rate (0.34 and 1.5 mL/h), and substrate temperature (300, 350, 400 and 450 °C). The correlation between deposition parameters and resulting microstructures was systematically studied and put into evidence.     

Ca3−xLaxCo4O9+δ (x=0, 0.3): New cobaltite materials as cathodes for proton conducting solid oxide fuel cell

Misfit-type Ca_3−xLa_xCo₄O_9+δ (x=0, 0.3) oxides were synthesised to be evaluated as possible cathode materials for proton conducting fuel cells (PCFCs) based on BaCe_0.9Y_0.1O_3−δ (BCY10) dense ceramic electrolyte. The electrical conductivity value of Ca_2.7La_0.3Co₄O_9+δ (σ≈53 S cm^-1 at 600 °C) is in the range of usually required value for a cathode application (about 50-100 S cm^-1). In order to test the performance of each compound as cathode material, impedance measurements were carried out on Ca_3−xLa_xCo₄O_9+δ/BaCe_0.9Y_0.1O_3−δ/Ca_3−xLa_xCo₄O_9+δ symmetrical half cells over the temperature range 400-800 °C under wet air. A promising electrocatalytic activity has been observed with both compounds Ca₃Co₄O_9+δ and Ca_2.7La_0.3Co₄O_9+δ. Factually, the area specific resistance obtained was about 2.2 Ω cm² at 600 °C.     

Energetics of La1−xAxCrO3−δ perovskites (A=Ca or Sr)

A series of perovskites with the general formula La_1−xA_xCrO_3−δ (A=Ca or Sr) have been synthesized in the solid solution range 0.0<x?0.3 and 0.0?δ?0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B₂O₃ at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from −620±260 to −280±80 kJ/mol O₂ when x=0.1-0.3 for Ca-doped samples, and from −440±150 to −290±50 kJ/mol O₂ for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of ) for the reaction (A=Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO₃ is more stable than its Ca-doped counterpart, which is consistent with basicity arguments.     

Sol-gel synthesis, structural and superconducting properties of (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ

A new series of Sr-based Hg-1212 superconducting cuprate (Hg_1−ySe_y)Sr₂(Y_1−xCa_x)Cu₂O_6+δ (y=0.25; 0.0?x?0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr₂(Y_1−xCa_x)Cu₂O_z prepared by the citrate sol-gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg_1−ySe_y)Sr₂YCu₂O_6+δ; y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg_0.75Se_0.25)Sr₂(Y_1−xCa_x)Cu₂O_6+δ samples. Superconductivity is observed only for samples with x?0.3 and T_c increases with increasing Ca content as well as O₂-annealing. A maximum T_c(onset) of 85 K is found in the (Hg_0.75Se_0.25)Sr₂(Y_0.3Ca_0.7)Cu₂O_6.84 sample annealed in an oxygen atmosphere. The structure of O₂-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Structure and magnetic properties of Ca2Fe1−xMnxAlO5+δ

Ca₂Fe_1−xMn_xAlO₅ (0?x?1) compounds were prepared by a self-combustion method under air (x=0, 0.1, 0.2 and 0.3) and nitrogen (x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Mössbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x?0.3 and increases in the case of the x?0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments’ orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Mössbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn³⁺ over Fe³⁺, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions.     

High pressure synthesis, crystal, magnetic structure and magnetotransport of SrFe0.5Co0.5O3−δ

A polycrystalline sample with SrFe_0.5Co_0.5O_3−δ stoichiometry has been prepared under moderate pressures of 2 GPa in the presence of KClO₄ as oxidizing agent. The crystallographic and magnetic structures have been studied from neutron powder diffraction (NPD), complemented with magnetotransport and magnetic susceptibility data. The study of the crystallographic structure confirms that this compound, with the actual stoichiometry SrFe_0.5Co_0.5O_2.88(3), is a simple cubic perovskite at 2 and 295 K, defined in the Pm-3m space group, where Fe and Co atoms are distributed at random over the B positions. The magnetic measurements show that SrFe_0.5Co_0.5O_2.88 is a ferromagnet with , which is also confirmed by neutron diffraction: the magnetic structure is collinear, characterized by a propagation vector k=0; the ordered magnetic moment values for the (Fe⁴⁺,Co⁴⁺) cations are 1.54(9) and 0.65(15) μ_B at T=2 and 295 K, respectively. Transport measurements show a semiconducting behaviour, and a negative magnetoresistance (MR) of −6.5% is observed at for .     

Structural features, nonstoichiometry and high-temperature transport in SrFe1−xMoxO3−δ

The oxide solid solutions SrFe_1−xMo_xO_3−δ, where x=0.05, 0.1 and 0.2, are studied in this work. It is shown that substitution of iron for molybdenum results in stabilization of a cubic quasi-perovskite locally inhomogeneous structure, which is evidenced by HREM and Mössbauer spectroscopy. The coulometric titration is used in order to determine changes of oxygen nonstoichiometry in the obtained solutions with temperature and ambient oxygen partial pressure. Partial molar thermodynamic functions of the labile oxygen are calculated from the measured data. The variations of partial molar entropy with oxygen content follow the ideal gas model reasonably well thus demonstrating approximately random distribution of oxygen vacancies in the doped ferrite at high temperatures. The partial molar enthalpy is found to increase with doping, which is indicative of a progressive decrease in average values of the bonding energy of labile oxygen ions. The measurements of total conductivity are used in order to determine partial contributions of charge carriers. The oxygen ion component is shown to increase at small level of doping, x=0.05 while further increase in molybdenum content is accompanied with the decline in the ion conductivity. The electron contribution in reducing conditions tends to increase with molybdenum content, which is interpreted as a manifestation of involvement of Mo⁵⁺ cations in electron transport. Concentration and mobility of electron carriers are calculated. Some increase in mobility of electron holes at small doping is explained as related to the filling of oxygen vacancies.     

Conditions for superconductivity in the electron-doped copper-oxide system, (Nd1−xCex)2CuO4+δ

We report systematic studies on the relations among the Ce^IV-for-Nd^III substitution level (x), oxygen-partial pressure (P_O2), oxygen content (4+δ), lattice parameters (a, c) and superconductivity characteristics (T_c, volume fraction) in the (Nd_1−xCe_x)₂Cu_1−yO_4+δ system which includes electron-doped superconductors. Independent of the Ce-doping level x, samples synthesized in air are found oxygen deficient, i.e. δ<0. Nevertheless, reductive annealing is needed to induce superconductivity in the air-synthesized samples. At the same time, the amount of oxygen removed upon the annealing is found very small (e.g. 0.004 oxygen atoms per formula unit at x=0.075), and consequently the effect of the annealing on the valence of copper (and thereby also on the electron doping level) is insignificant. Rather, the main function of the reductive annealing is likely to repair the Cu vacancies believed to exist in tiny concentrations (y) in the air-synthesized samples.     

Experimental and theoretical studies of Sn3−δPbδBi2Se6 (δ=0.0-0.7)

New ternary and quaternary chalcogenides, Sn_3−δPb_δBi₂Se₆ (δ=0.0-0.7), were synthesized from pure elements using the solid-state method. Their crystal structures, determined using single crystal X-ray diffraction, belong to the orthorhombic space group Pnma (No. 62). The structure is related to Pb₃Bi₂S₆, which contains NaCl [311] layer units and zigzag arrays of metal atoms along the c-axis. A correlation between the Pb composition and the shifted position of a metal site was observed. Band structure calculations confirmed that the structure is stabilized when the position of the M5 site is farther from the mirror plane. Thermopower and conductivity measurements indicated that all of the compounds are n-type semiconductors with small band gaps.     

Synthesis and electrochemical properties of nonstoichiometric LiAlxMn2−xO4−δ as cathode materials for rechargeable lithium ion battery

Nonstoichiometric spinel oxides, LiAl_xMn_2−xO_4−δ (x=0.1,0.2), were synthesized under controlled partial pressure of oxygen, and their elecrochemical performances were investigated. As an Al content increases, solubility limit of the oxygen nonstoichiometry, δ, increased, while partial molar enthalpy of the formation of oxygen nonstoichiometry decreased.Cycle performance of LiAl_xMn_2−xO₄ showed significant improvement comparing with that of LiMn₂O₄ cathode. However, the decrease of theoretical capacity was accompanied with Al doping. Nonstoichiometric LiAl_xMn_2−xO_4−δ showed the increase in capacity with keeping good cycle performances as well as stoichiometric LiAl_xMn_2−xO₄. Although the introduction of oxygen nonstoichiometry leads to the increase of Mn³⁺ which is known as Jahn-Teller ion, DSC curves for LiAl_xMn_2−xO_4−δ showed no exothermic peak due to phase transition arising from Jahn-Teller distortion around room temperature.     

Incommensurately modulated LT″-Ni1+δSn (δ=0.60, 0.63): Rietveld refinement, line-broadening analysis and structural relation with LT- and LT′-Ni1+δSn

X-ray powder diffraction data of NiAs/Ni₂In-type Ni_1.60Sn and Ni_1.63Sn alloys annealed at or below about 573 K reveal the development of an incommensurately ordered phase called LT″. In this phase Ni(2) atoms occupy partially the trigonal-bipyramidal interstices formed by five Sn within an NiAs-type arrangement Ni(1)Sn. The modulated occupational ordering of Ni(2) in the LT″ phase can be described in the superspace group Cmcm(α00)0 s 0, and the parameters describing this occupational modulation were refined together with atomic displacement modulations using the Rietveld method. The structure parameters revealed close structural analogies of the LT″ phase with the previously reported commensurate LT-Ni_1+δSn and incommensurate LT′-Ni_1+δSn phases (A. Leineweber, J. Solid State Chem. 177 (2004) 1197-1212), which both occur for lower Ni contents than the LT″ phase. The 1st-order satellite reflections visible in the powder-diffraction patterns exhibit, with respect to the fundamental reflections, a considerable diffraction-line broadening, caused by a small size of the particularly ordered domains. This small-domain-size broadening was successfully described by a recently developed reflection-index (hklm) dependent (anisotropic) line-broadening model (A. Leineweber, V. Petricek, J. Appl. Crystallogr. 40 (2007) 1027-1034) designed to consider the effect of fluctuations of the lattice metrics on the peak widths in powder diffraction patterns of incommensurately modulated crystal structures. The small domain sizes encountered for the LT″ phase indicate that domain coarsening is much more difficult than for the LT and LT′ phases. This special feature of the LT″ phase goes along with a compared to the LT and LT′ phases absent orthorhombic distortion and the low ordering temperature, which are discussed as a consequence of the ordering patterns due to the Ni(2) atoms.     

Topotactic synthesis, structure and magnetic properties of a new hexagonal polytype of silver cobaltate(III) AgCoO2+δ

A new form of delafossite-type AgCoO_2+δ was prepared using ion exchange from Na_0.75CoO₂ in molten AgNO₃-NH₄NO₃ at 175 °C. Its structure was determined by the Rietveld refinement from X-ray powder diffraction measurements (XRD) data; it is hexagonal, space group P6₃/mmc, a=2.871 and c=12.222 Å. Its structure differs from previously reported AgCoO₂ (, 3R polytype) by the stacking of Co-O layers; in the new phase, the 2H stacking of the precursor Na_0.75CoO₂ is consistent with a topotactic ion exchange of Na by Ag. The new phase is found to contain a slight oxygen excess (δ=0.06). Magnetic susceptibility measurements show the absence of magnetic transition and a weak Curie term, consistent with the non-magnetic character of Co³⁺ ions.     

Structural observations on La2(Ni,Co)O4±δ phases determined from in situ neutron powder diffraction

In situ neutron powder diffraction data have been analysed for a number of compositions in the solid solution series La₂Ni_1−xCo_xO_4±δ over the temperature range 150-650 °C. High-quality Rietveld refinements indicate no major structural transitions have occurred in any of the compositions, although close examination of the Ni/CoO bond distances highlight changes in the coordination environment with increasing temperature. Further, the changes in the coordination can be correlated to changes in both the oxide ion and electronic conductivity indicating a direct link between transport properties, bonding and transition metal valence state in these Co containing layered perovskites.     

固体氧化物燃料电池La0.7Sr0.3Cr1-xMnxO3-δ阳极性能研究

采用溶胶凝胶法制备了La0.7Sr0.3Cr1-xMnxO3-δ(x=0.3,0.4,0.5,0.6)系列阳极粉体。在1000℃下焙烧后,XRD结果显示粉体物相为单一的钙钛矿相。制备以La0.7Sr0.3Cr1-xMnxO3-δ为阳极,Ce0.8Sm0.2O1.9(SDC)为电解质,Pr0.6Sr0.4Co0.8Fe0.2O3-δ-SDC复合阴极的电解质支撑型固体氧化物燃料单电池。由扫描电子显微镜(SEM)观察表明单电池电解质致密,阳极孔径分布均匀,厚度约为20μm,多孔阴极厚度为10μm。采用直流四电极法测试以La0.7Sr0.3Cr0.5Mn0.5O3-δ为阳极用湿氢气作燃料时在800℃下获得最大输出功率为232.84 mW.cm-2,短路电流为0.92 A.cm-2。     

Mixed conductivity, oxygen permeability and redox behavior of K2NiF4-type La2Ni0.9Fe0.1O4+δ

The total conductivity and Seebeck coefficient of La₂Ni_0.9Fe_0.1O_4+δ with K₂NiF₄-type structure, studied in the oxygen partial pressure range from 10⁻⁵ to 0.5 atm at 973-1223 K, were analyzed in combination with the steady-state oxygen permeability, oxygen non-stoichiometry and Mössbauer spectroscopy data in order to examine the electronic and ionic transport mechanisms. Doping of La₂NiO_4+δ with iron was found to promote hole localization on nickel cations due to the formation of stable Fe³⁺ states, although the electrical properties dominated by p-type electronic conduction under oxidizing conditions exhibit trends typical for both itinerant and localized behavior of the electronic sublattice. The segregation of metallic Ni on reduction, which occurs at oxygen chemical potentials close to the low-p(O₂) stability boundary of undoped lanthanum nickelate, is responsible for the high catalytic activity towards partial oxidation of methane by the lattice oxygen of La₂Ni_0.9Fe_0.1O_4+δ as revealed by thermogravimetry and temperature-programmed reduction in dry CH₄-He flow at 573-1173 K. A model for the oxygen permeation fluxes through dense La₂Ni_0.9Fe_0.1O_4+δ ceramics, limited by both bulk ionic conduction and surface exchange kinetics, was proposed and validated.     

The Sr2.75Ce0.25Co2O7−δ oxide, n=2 member of the Ruddlesden-Popper series: Structural and magnetic evolution depending on oxygen stoichiometry

The second member of the Ruddlesden-Popper series, n=2 in Sr_n+1Co_nO_3n+1, has been stabilized by substituting cerium for strontium leading to the pure compound Sr_2.75Ce_0.25Co₂O_7−δ. The oxygen vacancies of this phase can be partially filled by a post-annealing oxidizing treatment with δ decreasing from 1.1 to 0.3 for the as-prepared and oxidized phases, respectively. As the samples are oxidized from δ≈1.1 to 0.3, the a and b unit cell parameters decrease from 3.836 to 3.815 Å and from 20.453 to 20.047 Å, respectively. Despite the average value of the cobalt valence state, V_Co≈+3.5, obtained in the oxidized Sr_2.75Ce⁺⁴_0.25Co₂O_6.7 phase, a clear ferromagnetic state wit T_C=175 K and M_S=0.8 μB/Co is reached.