Nonstoichiometry,point defects and magnetic properties in Sr2FeMoO6−δ double perovskites

The phase stability, nonstoichiometry and point defect chemistry of polycrystalline Sr₂FeMoO_6?δ (SFMO) was studied by thermogravimety at 1000, 1100, and 1200 °C. Single-phase SFMO exists between ?10.2≤log pO₂≤?13.7 at 1200 °C. At lower oxygen partial pressure a mass loss signals reductive decomposition. At higher pO₂ a mass gain indicates oxidative decomposition into SrMoO₄ and SrFeO_3?x. The nonstoichiometry δ at 1000, 1100, and 1200 °C was determined as function of pO₂. SFMO is almost stoichiometric at the upper phase boundary (e.g. δ=0.006 at 1200 °C and log pO₂=?10.2) and becomes more defective with decreasing oxygen partial pressure (e.g. δ=0.085 at 1200 °C and log pO₂=?13.5). Oxygen vacancies are shown to represent majority defects. From the temperature dependence of the oxygen vacancy concentration the defect formation enthalpy was estimated (ΔH_OV=253±8 kJ/mol). Samples of different nonstoichiometry δ were prepared by quenching from 1200 °C at various pO₂. An increase of the unit cell volume with increasing defect concentration δ was found. The saturation magnetization is reduced with increasing nonstoichiometry δ. This demonstrates that in addition to Fe/Mo site disorder, oxygen nonstoichiometry is another source of reduced magnetization values.     

High-temperature transport and stability of SrFe1−xNbxO3 − δ

The conductivity is measured in the series of solid solutions SrFe_1 ? xNb_xO_3 ? δ, where x = 0.05, 0.1, 0.2, 0.3, 0.4, within the oxygen partial pressure limits 10^?18–0.5 atm and temperature range 650–950 °C. The contributions to the total conductivity from oxygen ions, electrons and electron holes are obtained based on their different pressure dependences. The doped derivative with x = 0.1 is found to be a singular composition where ion conductivity attains a maximal value while activation energy for ion transport is minimal. This peculiar behavior is attributed to formation of favorable microstructure in the oxide. The deeper doping results in deterioration of ion transport, which is explained by oxygen vacancy filling. It is shown that replacement of iron for niobium favors enhanced thermodynamic stability towards reduction. The oxygen permeability is evaluated from the conductivity data, and it achieves rather high values in the doped derivatives. These oxides can, therefore, be recommended for further evaluation as oxygen separating membrane materials for partial oxidation of natural gas.     

The role of oxygen stoichiometry on phase stability, structure, and magnetic properties of Sr2CoIrO(6-δ)

The phase stability, crystal structure, and magnetic properties of perovskite-like nonstoichiometric Sr(2)CoIrO(6-δ) were studied. Oxygen deficiency can be well controlled and reversibly varied up to δ = 0.33. A single phase exists at least for partial oxygen pressures between 10(-5) and 1 bar at 1273 K, followed by phase decomposition at higher temperature with the elimination of metallic Ir and the formation of a new phase with approximately Sr(3)CoIrO(6) composition crystallizing in K(4)CdCl(6) structure type. The structural features of Sr(2)CoIrO(6-δ) are dependent on both temperature and oxygen content and were determined by synchrotron and neutron powder diffraction. Both the increasing amount of oxygen vacancies at constant temperature and increasing temperature at constant oxygen content result in the same higher crystal symmetry of Sr(2)CoIrO(6-δ): (1) The oxygen-stoichiometric phase Sr(2)CoIrO(6.00) is monoclinic (I2/m or P2(1)/n) at room temperature but cubic (Fm-3m) for Sr(2)CoIrO(5.67). (2) A sequence of phase transitions [Formula: see text] was observed for Sr(2)CoIrO(6.00) in air. All Sr(2)CoIrO(6-δ) compositions show weak ferromagnetism at low temperature with a canted but predominantly antiferromagnetic ground state. The magnetic ordering temperature decreases monotonously with increasing oxygen deficiency, while pronounced extrema are observed for the paramagnetic moment and the Curie-Weiss temperature at an oxygen deficiency δ ≈ 0.10, which corresponds to the P2(1)/n ? I2/m phase transformation.     

Structure,chemical stability and electrical properties of BaCe0.9Y0.1O3−δ modified with V2O5

Wet vacuum impregnation method was applied in order to evaluate the possibility of the formation of the material in BaCe_0.9Y_0.1O_3?δ–V₂O₅ system. Single-phase BaCe_0.9Y_0.1O_3?δ samples, synthesised by solid-state reaction method, were impregnated with the solution of vanadium(V) oxide precursor. Multi-step, multi-cycle impregnation procedure was applied to enhance the impregnation efficiency. Partial decomposition of Y-doped BaCeO₃ in contact with the solution of the precursor, resulting in the formation of vanadium containing phases (CeVO₄ and BaV₂O₆) on the materials surface, was observed. However, the presence of vanadium was also confirmed for the inner parts of the materials. The synthesised materials were submitted for exposition test to evaluate their chemical stability towards CO₂/H₂O. All BaCe_0.9Y_0.1O₃-based materials modified by impregnation revealed higher chemical stability in comparison with single-phase un-modified BaCe_0.9Y_0.1O_3?δ, since the amount of barium carbonate formed during the exposition was significantly lower. The total electrical conductivity of the received multi-phase materials was generally slightly lower than for the reference BaCe_0.9Y_0.1O_3?δ sample, since the presence of the additional phases had a blocking effect on materials conductivity. The values of BaCeO₃ lattice parameters and the Seebeck coefficient did not show the modification of the defects structure of Y-doped BaCeO₃ during applied synthesis procedure.

     

Synthesis and structural mechanisms of the 2201-type ferrites and polytypes: Fe2(Sr2 − xAx)FeO6.5 − δ/2 (A = Ba,La, Tl,Pb and Bi)

The Fe₂(Sr_2 ? xA_x)FeO_6.5 ? δ/2 systems have been investigated, by doping the iron rich 2201-type parent structure with Ba²⁺, La³⁺ and 5d¹⁰ post-transition cations. The syntheses have been carried out up to the limit of the 2201-type solid solutions, in order to test the role of the double iron layer Fe₂O_2.5 ? δ/2. The localisation of the charge carriers in these compounds is consistent with their strong antiferro-magnetism. The investigation was then carried out in the transition part of the diagram up to the formation of stable phases. The study of structural mechanisms was carried using high resolution electron microscopy (transmission and scanning transmission), electron diffraction and energy dispersive spectroscopy. Different non-stoichiometry mechanisms are observed, depending on the electronic structure and chemical properties of the doping elements. The specific behavior of the modulated double iron layer is discussed.     

Structure,nonstoichiometry and magnetic properties of the perovskites Sr1−xCaxMnO3−δ

The structural, thermal and magnetic properties of the perovskite-type alkaline-earth manganites of the series Sr_1−xCa_xMnO_3−δ (0⩽x⩽1) were investigated. SrMnO_3−δ forms a hexagonal perovskite lattice and shows a first-order transformation to a highly defective cubic high-temperature modification. By substituting Ca for Sr (x>0.25) the hexagonal perovskite is suppressed and a cubic (or orthorhombic) lattice becomes stabilized for all temperatures. For x=0.5 and 0.75 cubic perovskites with a large nonstoichiometry (e.g., δ=0.25 for x=0.5) are obtained at 1400 °C. The defective perovskites are prepared by either quenching from high temperature or by cooling in an inert atmosphere. The oxygen vacancies are easily filled by subsequent reoxidation at low temperature (400–600 °C) and stoichiometric samples are obtained. Orthorhombic perovskites are formed at T⩽1200 °C with the nonstoichiometry δ increasing with increasing temperature (e.g., δ=0.06 at 1000 °C and δ=0.14 at 1200 °C for x=0.5). Slow cooling in air results in almost complete reoxidation (δ=0). CaMnO_3−δ is an orthorhombic perovskite with a large range of nonstoichiometry (0⩽δ⩽0.30). The cubic to hexagonal phase transformation of the Sr-rich samples is accompanied by a large expansion of the lattice that is reduced by Ca substitution. The Ca/Sr-manganites are antiferromagnets with T_N of 170 K for x=0.5 and δ=0.02 and 120 K for x=1 and δ=0.05.     

Synthesis,structure, and properties of randomly mixed and layer-ordered SrMn1−xGaxO3−δ perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques, we have extended the solubility limit of Ga³⁺ in the cubic perovskite phase to x≈0.33. Higher Ga concentrations lead to mixed phases until a single-phase ordered double-perovskite structure is obtained at x=0.5, i.e., Sr₂MnGaO_6−δ. In the cubic perovskite phase the maximum oxygen content is 3−x/2, which corresponds to 100% Mn⁴⁺. All maximally oxygenated solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases. Reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.7Ga_0.3O_2.5 below 30 K. The brownmillerite phase at low oxygen content, Sr₂MnGaO₅, is found to have Icmm crystallographic symmetry. At 12 K its magnetic structure is found to order in the Icm′m′ magnetic symmetry corresponding to a G-type antiferromagnetic structure of Mn³⁺ ions. At higher oxygen content, Sr₂MnGaO_5.5 is found to have Cmmm crystallographic symmetry with disordered oxygen vacancies. At 12 K two competing long-range magnetic structures are found for the Mn⁴⁺ sublattice having C_Im′m′m symmetry (G-type), and C_Pm′m′m symmetry (C-type), together with a G-type short-range magnetic correlations.     

High-temperature electrical conductivity and electrochemical activity in oxygen redox reaction of La-doped Sr2FeCo0.5Mo0.5O6-δ

Journal of Solid State Electrochemistry - High-temperature electrical conductivity and electrochemical activity in the oxygen redox reaction of Sr2FeCo0.5Mo0.5O6-δ (SFCM) and...     

A cobalt-free electrode material La0.5Sr0.5Fe0.8Cu0.2O3 − δ for symmetrical solid oxide fuel cells

Cobalt-free perovskite oxide La_0.5Sr_0.5Fe_0.8Cu_0.2O_3 − δ (LSFC) was applied as both anode and cathode for symmetrical solid oxide fuel cells (SSOFCs). The LSFC shows a reversible transition between a cubic perovskite phase in air and a mixture of SrFeLaO₄, a K₂NiF₄-type layered perovskite oxide, metallic Cu and LaFeO₃ in reducing atmosphere at elevated temperature. The average thermal expansion coefficient of LSFC in air is 17.7 × 10^− 6 K^− 1 at 25 °C to 900 °C. By adopting LSFC as initial electrodes to fabricate electrolyte supported SSOFCs, the cells generate maximum power output of 1054, 795 and 577 mW cm^− 2 with humidified H₂ fuel (~ 3% H₂O) and 895, 721 and 482 mW cm^− 2 with humidified syngas fuel (H₂:CO = 1:1) at 900, 850 and 800 °C, respectively. Moreover, the cell with humidified H₂ fuel demonstrates a reasonable stability at 800 °C under 0.7 V for 100 h.     

The oxygen deficient cubic perovskite SrFe1−xScxO3−δ (x ≤ 0.5; 0.1 ≤ δ ≤ 0.5): Structural features and physical properties

Solid state synthesis method has been used to stabilize oxygen deficient perovskite phases SrFe_1?xSc_xO_3?δ (0 ≤ x ≤ 0.5). The good homogeneity of samples is confirmed by energy dispersive spectroscopy (EDS) analysis performed with a transmission electronic microscope (TEM). By combining X-ray and electronic diffraction (ED), it is demonstrated that the cationic substitution on the B site of the perovskite induces a decrease of the oxygen content but without inducing long range ordering phenomenon. On this basis, X-ray patterns of compounds were indexed in the cubic Pm3m space group. The oxidation states of iron evidenced by Mössbauer spectroscopy, are in good agreement with the oxygen stoichiometries determined by cerimetric titration. In the SrFe_1?xSc_xO_3?δ series, the Fe³⁺/Fe⁴⁺ origin of the electronic conductivity is clearly evidenced. The limit compound SrFe_0.5Sc_0.5O_2.5 is highly resistive and characterized by a cluster glass-like behaviour. Finally, negative magnetoresistivity properties are revealed for the x = 0.1 and x = 0.2 samples, reaching ?10% around the magnetic transition temperature in a 7T magnetic field.     

Comparison of magnetocaloric properties of the Mn2−xFexP0.5As0.5 (x = 1.0 and 0.7) compounds

The Mn_2−xFe_xP_0.5As_0.5 compounds (x = 0.7 and 1.0) studied exhibit the magnetic phase transitions, which are accompanied by a magnetic entropy change. For x = 1 the PM–FM transition is of the first order one with a weak (2–3 K) thermal hysteresis in the vicinity of T_C = 275 K. The Mn_1.3Fe_0.7P_0.5As_0.5 compound possesses two magnetic transitions: the second-order PM–FM transition at T_C = 190 K, followed by the FM–AFM transition at T_N = 90 K, leading to normal and inverse magnetocaloric effects, respectively. The maximum values of magnetic entropy change are equal to 17 J kg⁻¹ K⁻¹ in MnFeP_0.5As_0.5 and 5 J kg⁻¹ K⁻¹ in Mn_1.3Fe_0.7P_0.5As_0.5 for a field change of 5 T. The magnetic entropy changes were calculated using both the isofield magnetization curves versus temperature and the isothermal magnetization curves versus applied magnetic field. The magnetocaloric effect in MnFeAs_0.5P_0.5 is discussed in the terms of both the thermodynamic Maxwell relation and the Clausius–Clapeyron equation.     

Relationship between the crystal structure,conductive and catalytic properties of perovskites Bi4Fe2xV2−2xO11−δ

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Electronic transport and magnetic properties of the perovskites La0.8Sr0.2Co1−xFexO3; x ≤ 0.3

Transport and magnetic properties of LaCoO₃-based compounds, doped with 20% Sr and 2.5, 5, 10, 15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by ⁵⁷Fe Mössbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co → Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed.     

Crystal structure and electrical conductivity of the “one-dimensional solid electrolyteN,N-dimethylmorpholinium pentaiodotetraargentate

The new solid electrolyteN, N-dimethylmorpholinium pentaiodotetraargentate, [(CH₃)N(CH₂)₂-(CH₂)₂O]Ag₄I₅ (DMMAg₄I₅), has a crystal structure which allows the Ag⁺ ions to move effectively in only one dimension. The channels for this motion result from the sharing of opposite faces of icosahedra, having iodides at their centers, forming infinite chains of icosahedra along thea axis. The joining of the icosahedra forms three more face-sharing tetrahedra at each junction, or a total of 23 tetrahedra per eight Ag⁺ ions, which are distributed non-uniformly over the tetrahedral sites. The limited number of pathways for the Ag⁺ ion diffusion leads to a low average conductivity, 1 × 10⁻⁴ Ω⁻¹ cm⁻¹ and a high activation enthalpy, 0.35 eV. Crystals of DMMAg₄I₅ belong to space groupP2₁2₁2₁ (D⁴₂) witha = 13.167(4), b = 23.437(3), c = 12.758(3)A˚; the unit cell contains eight formula units. The structure of dimethylmorpholinium iodide (DMMI) confirms the chair model for the DMM⁺ ion. Crystals of DMMI belong to space groupP2₁m(C²_2h), witha = 8.861(4), b = 8.020(2), c = 6.685(2)A˚, β = 101.07(3)°; each unit cell contains two formula units.     

Thermodynamic study of (alkyl esters + α,ω-alkyl dihalides) III. HmEandVmE for 20 binary mixtures {xCu − 1H2u − 1CO2C4H9 + (1 − x)α,ω-ClCH2(CH2)v − 2CH2Cl}, where u = 1 to 4, α = 1 and v = ω = 2 to 6

In this article, the experimental data of excess molar enthalpies $H_{\mathrm{m}}^{\mathrm{E}}$ and excess molar volumes $V_{\mathrm{m}}^{\mathrm{E}}$ are presented for a set of 20 binary mixtures comprised of the first four butyl alkanoates (methanoate to butanoate) and five α,ω-dichloroalkanes (1,2-dichloroethane to 1,6-dichlorohexane), obtained at atmospheric pressure and at a temperature of 298.15 K. The results indicate the existence of specific interactions between both kinds of compounds resulting in exothermic processes for most mixtures, except for those containing butyl methanoate which give rise to net endo/exothermic effects. The $V_{\mathrm{m}}^{\mathrm{E}}$ are positive for mixtures of (butyl esters + 1,2-dichloroethane or 1,3-dichloropropane) and negative for the remaining ones. The change in $H_{\mathrm{m}}^{\mathrm{E}}$ with the dichloroethane chain length for a same ester is regular although the $V_{\mathrm{m}}^{\mathrm{E}}$ presents an irregular variation. It can, therefore, be deuced from this that the mixing process involves both effects, exothermic/endothermic and expansion/contraction, simultaneously. The behaviour of the mixtures is interpreted on the basis of the results observed and attributed to different effects taking place among the molecules studied.To improve application of the UNIFAC model using the version of Dang and Tassios, average values were recalculated again for parameters of the ester/chloride interaction, distinguishing, during its application, the functional group of the acid part of the ester. In spite of this, the model does not adequately reproduce the systems’ behaviour.