Maximum magnetic entropy change modulated toward room temperature in perovskite manganites La0.7−xNdx(Ca,Sr)0.3MnO3

With Nd³⁺ doping and Ca²⁺, Sr²⁺ modulating in the sol–gel technique, a series of polycrystalline perovskite samples La_0.7?xNd_x(Ca,Sr)_0.3MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25) was prepared, their maximum magnetic entropy changes were tuned to room temperature (ΔS_H = ?1.47 J/kg K at 298 k for La_0.45Nd_0.25(Ca,Sr)_0.3MnO₃), an enhancement of the maximum magnetic entropy change (ΔS_H = ?1.89 J/kg K at 315 k) and its refrigerant capacity (about 45.3 J/kg) had also been obtained under 9 kOe magnetic field variation for La_0.55Nd_0.15(Ca,Sr)_0.3MnO₃ contrast to La_0.7(Ca,Sr)_0.3MnO₃.     

Electron magnetic resonance (EMR) in sonochemically prepared doped nano-manganites

X-band electron magnetic resonance method was explored for study of sonochemically prepared nano-powders of La-based doped manganites: La_0.7Sr_0.3MnO₃ and La_0.9Ca_0.1MnO₃ at 115 ⩽ T ⩽ 600 K temperature interval, including main characteristic points of its magnetic transitions. The data obtained were compared with those for crushed bulk single crystals of the same compositions. It is shown that nano-powder of La_0.7Sr_0.3MnO₃ has the same ferromagnetic ordering as its bulk counterpart, being more homogeneous and less anisotropic. While, nano-powder of La_0.9Ca_0.1MnO₃ shows two distinct ferromagnetic phases in a marked contrast to the bulk sample, where complex magnetic structure comprising canted antiferromagnetic matrix and ferromagnetic clusters is observed. To explain the observed phenomena, the crucial role of nano-scale grain size and its interplay with spin and charge degrees of freedom in considered systems are discussed.     

Regularities in temperature,magnetic field and pressure effect on the resistive properties of magnetic semiconductors

The influence of hydrostatic pressure, magnetic field and temperature on resistivity behaviour of bulk and film samples La_0.9Mn_1.1O₃ and La_0.56Ca_0.24Mn_1.2O₃ at action of magnetic field and temperature has been analysed. It is established that the maximum of magnetoresistive and the revealed baroresistive, magnetobaroresistive effects coincide at the same temperature T_pp. This temperature is equal to the “metal-semiconductor” phase transition temperature T_ms. “Cooling” and “heating” effects of pressure and magnetic field have been revealed. A mutual correspondence of T–P–H (6.2 K, 1 kbar, 2.7 kOe) influence on polycrystalline sample La_0.9Mn_1.1O₃ resistivity has been determined. The linear change of T_ms(P) and T_ms(H) in La_0.9Mn_1.1O₃, La_0.56Ca_0.24Mn_1.2O₃ resistivity have been found. An importance of the regularities of elastic-deforming correspondence of T–H–P influence on magnetic, resistivity properties, phase transitions and effects was elucidated and explained. An alternating influence of T–H–P and its role in resistivity has been pointed. A correlation between structural, elastic and resistive properties is specified.     

Performance of perovskite-related oxide cathodes in contact with lanthanum silicate electrolyte

The cathodic performance of selected mixed-conducting electrodes, including perovskite-type SrMn_0.6Nb_0.4O_3 ? δ, Sr_0.7Ce_0.3Mn_0.9Cr_0.1O_3 ? δ and Gd_0.6Ca_0.4Mn_0.9Ni_0.1O_3 ? δ, and Ruddlesden–Popper La₂Ni_0.5Cu_0.5O_4 + δ, LaSr₂Mn_1.6Ni_0.4O_7 ? δ, La₄Ni_3 ? xCu_xO_10 ? δ (x = 0–0.1) and La_3.95Sr_0.05Ni₂CoO_10 ? δ, was evaluated in contact with apatite-type La₁₀Si₅AlO_26.5 solid electrolyte at 873–1073 K and atmospheric oxygen pressure. The electrochemical activity of porous nickelate-based layers was found to correlate with the concentration of mobile ionic charge carriers and bulk oxygen transport, thus lowering in the series La₄Ni_2.9Cu_0.1O_10 ? δ > La₄Ni₃O_10 ? δ > La_3.95Sr_0.05Ni₂CoO_10 ? δ and decreasing on copper doping in K₂NiF₄-type La₂Ni_1 ? xCu_xO_4 ? δ. The relatively high overpotentials of nickelate-based cathodes, varying in the range ? 240 to ? 370 mV at 1073 K and current density of ? 200 mA/cm², are primarily associated with surface diffusion of silica from La₁₀Si₅AlO_26.5, which partially blocks the electrochemical reaction zone. As compared to the intergrowth nickelate materials, the manganite-based electrodes exhibit substantially worse electrochemical properties, in correlation with the level of oxygen-ionic and electronic conduction in Mn-containing phases. The effects of cation interdiffusion between the cell components as a performance-deteriorating factor are briefly discussed.     

Effect of Nd ion on the magnetic properties of Ni–Mn ferrite nanocrystal

The nanocrystalline Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ ferrites were prepared by the emulsion method. X-ray diffractometer (XRD), transmission electron microscope (TEM), Mössbauer spectra and vibrating samples magnetometer (VSM) were used to study the structure, morphology and magnetic properties. The magnetic properties of Nd³⁺-doped Ni_0.7Mn_0.3Fe₂O₄ nanocrystal ferrites were investigated in detail. The Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ nanocrystal ferrite with particle size of 10.7 nm shows superparamagnetism.     

Improvement of refrigerant capacity of La0.7Ca0.3MnO3 material with a few percent Co doping

The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials has been investigated. Large low-field-induced magnetic entropy changes are observed in La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials. The La_0.7Ca_0.3MnO₃ material experiences a large entropy change with a first-order magnetic phase transition at the Curie temperature, T_C. On the other hand, La_0.7Ca_0.3Mn_0.95Co_0.05O₃ displays a smaller entropy change with a second order phase transition. While a first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at T_C, this is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance (MR, defined as ρ(0)/ρ(H)-1) under a field of 5 T is about 206% and 333% for La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃, respectively. The refrigeration capacity (RCP) is enhanced in La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE=3.163 for La_0.7Ca_0.3Mn_0.95Co_0.05O₃ and 4.63 J/kg K for La_0.7Ca_0.3MnO₃ at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can operate with permanent magnets rather than superconducting ones as the magnetic field source.     

Structural and magnetic ordering in La0.6Ca0.4MnO3

The structural and magnetic ordering in La_0.6Ca_0.4MnO₃ has been studied by neutron powder diffraction as a function of temperature between 15 and 300 K. The para-ferromagnetic transition at T∼250 K is accompanied by significant structural distortions in the form of octahedral Mn–O₆ rotations. At 15 K, the total refined ferromagnetic moment on the Mn site was obtained as 3.1 μ_B, in reasonable agreement with the total expected average moment of mixed Mn³⁺/Mn⁴⁺ matrix.     

Evidence of ferromagnetic domains in the perovskite La0.7Sr0.3Co0.9Mn0.1O3

The magnetic and electronic ground states of the polycrystalline perovskite compound La_0.7Sr_0.3Co_0.9Mn_0.1O₃ have been studied using AC susceptibility, resistivity and neutron depolarization techniques. Results of neutron depolarization study establish the existence of ferromagnetic domains of ∼3 μm size below 180 K. The substitution of 10 at% Mn ions at the Co site in the compound La_0.7Sr_0.3CoO₃ reduces the effective e_g electron transfer and suppresses the double exchange interaction. The competition between the reduced ferromagnetic double exchange interaction and the coexisting antiferromagnetic interaction along with the random nature of substitution leads to a randomly canted ferromagnetic ground state for the substituted compound. Resistivity study confirms that the randomly canted ferromagnetic ground state is insulating.     

The effect of average A-site cation radius on TC in perovskite manganites

Three series of samples Ln_0.7T_0.3MnO₃ (I), Ln_0.7T_0.3Mn_0.9Cr_0.1O₃ (II), and Ln_0.7T_0.3Mn_0.9Fe_0.1O₃ (III) (Ln=La, (La, Nd), (La, Y), T=Ca, CaSr, Sr) were prepared by sol–gel technique. The effect of the average A-site cation radius 〈r_A〉 on the ferromagnetic transition temperature T_C has been investigated. T_C was obtained from M–T curves measured by vibrating sample magnetometer. For a little Nd³⁺ doped at the A-site, the T_C of Ln_0.7T_0.3MnO₃ (I) drops linearly with decreasing 〈r_A〉 (1.18 Å<〈r_A〉<1.25 Å). Similar behavior has been observed in series II and III, which have lower T_C values than series I due to the weakening of the influence of double-exchange interactions upon substituting Fe and Cr for Mn.     

Defect chemistry and oxygen transport of (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2), Sr: Part II: Oxygen transport

This paper is the second part of a two part series, where the effects of varying the A-site dopant on the defect chemistry and transport properties of the materials (La_0.6Sr_0.4 ? xM_x)_0.99Co_0.2Fe_0.8O_3 ? δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry were reported, while the oxygen transport properties are reported here in part II. In the investigated material series, the amount of divalent dopant has been kept constant, while Sr ions have been substituted with Ca ions (smaller ionic radius) or Ba ions (larger ionic radius). The size difference induces different strains into the crystal structure in each composition. The possibility of simple relationships between various crystal strain parameters and the transport properties were analyzed. Oxygen pump controlled permeation experiments and a surface sensitive electrolyte probe were used to extract the permeability and surface resistance, r_s. The highest permeability was found for (La_0.6Sr_0.3Ca_0.1)_0.99Co_0.2Fe_0.8O_3 ? δ. The apparent activation energy of the permeability was 78 kJ/mol. The inverse surface resistance, r_s^? 1, also had an activated behavior with an activation energy close to 180 kJ/mol for most of the materials. A reversible transition to an abnormally low r_s was found in (La_0.6Sr_0.3Ca_0.1)_0.99Co_0.2Fe_0.8O_3 ? δ at T > 1223 K.     

Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ

Electronic conductivity in the potential SOFC anode material La_1−xSr_xCr_0.5Mn_0.5O_3±δ has been investigated in the range 0.2 < x < 0.3. log(σT) vs. 1/T plots indicate conduction via thermally activated polaron hopping. At 900 °C, conductivity in air increases with Sr²⁺ via an increase in [B_B] holes (B—transition metal). X-ray absorption spectroscopy (XAS) studies indicate that compensation for A-site Sr substitution and oxygen vacancy formation is via the Mn cation only; Cr maintains a 3+ oxidation state and 6-fold oxygen coordination. Electronic transport occurs by percolation between Mn cations in a disordered B-site sub-lattice. Conductivity decreases with p(O₂), which is indicative of p-type conduction behaviour, but the relationship cannot be explained by a simple redox equilibrium involving Mn³⁺, Mn⁴⁺ and oxygen, possibly due to co-existence of Mn²⁺, Mn³⁺ and Mn⁴⁺ via disproportionation as with La_1−xSr_xMnO_3±δ.     

Exchange interaction between ferromagnetic and antiferromagnetic phases in La1/3Nd1/3Ca1/3MnO3

Magnetization measurements of a La_1/3Nd_1/3Ca_1/3MnO₃ perovskite at magnetic field up to 6 T have revealed an anomalous behaviour – above 130 K the material exhibits a loop displacement about a field of 2 T. We assume that this is the result of an exchange interaction between ferromagnetic and antiferromagnetic phases in a magnetically inhomogeneous compound. At about 115 K a transition from a semiconducting to a metallic-like state has been observed.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

Tuning the magnetocaloric properties of La0.7Ca0.3MnO3 manganites through Ni-doping

The effect of Ni²⁺ doping on the magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ manganites synthesized via the auto-combustion method is reported. The aim of studying Ni²⁺-substituted La_0.7Ca_0.3Mn_1 ? xNi_xO₃ ($x=0,0.02,0.07$, and 0.1) manganites was to explore the possibility of increasing the operating temperature range for the magnetocaloric effect through tuning of the magnetic transition temperature. X-ray diffraction analysis confirmed the phase purity of the synthesized samples. The substitution of Mn³⁺ ions by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice was also corroborated through this technique. The dependence of the magnetization on the temperature reveals that all the compositions exhibit a well-defined ferromagnetic to paramagnetic transition near the Curie temperature. A systematic decrease in the values of the Curie temperature is clearly observed upon Ni²⁺ doping. Probably the replacement of Mn³⁺ by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction, which leads to a decrease in the transition temperature and the magnetic moment in the samples. By using Arrott plots, it was found that the phase transition from ferromagnetic to paramagnetic is second order. The maximum magnetic entropy changes observed for the $x=0,0.02,0.07$, and 0.1 composites was 0.85, 0.77, 0.63, and 0.59 J/kg?K, respectively, under a magnetic field of 1.5 T. In general, it was verified that the magnetic entropy change achieved for La_0.7Ca_0.3Mn_1 ? xNi_xO₃ manganites synthesized via the auto-combustion method is higher than those reported for other manganites with comparable Ni²⁺-doping levels synthesized via standard solid state reaction. The addition of Ni²⁺ increases the value of the relative cooling power as compared to that of the parent compound. The highest value of this parameter (～60 J/kg) is found for a Ni-doping level of 2% around 230 K in a field of 1.5 T.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Defect interactions in La0.3Sr0.7Fe(M′)O3−δ (M′ = Al,Ga) perovskites: Atomistic simulations and analysis of p(O2)-T-δ diagrams

Atomistic modelling showed that a key factor affecting the p(O₂) dependencies of point defect chemical potentials in perovskite-type La_0.3Sr_0.7Fe_1−xM′_xO_3−δ (M′ = Ga, Al; x = 0–0.4) under oxidizing conditions, relates to the coulombic repulsion between oxygen vacancies and/or electron holes. The configurations of A- and B-site cations with stable oxidation states have no essential influence on energetics of the mobile charge carriers, whereas the electrons formed due to iron disproportionation are expected to form defect pair clusters with oxygen vacancies. These results were used to develop thermodynamic models, adequately describing the p(O₂)-T-δ diagrams of La_0.3Sr_0.7Fe(M′)O_3−δ determined by the coulometric titration technique at 923–1223 K in the oxygen partial pressure range from 1 × 10^− 5 to 0.5 atm. The thermodynamic functions governing the oxygen intercalation process were found independent of the defect concentration. Doping with aluminum and gallium leads to increasing oxygen deficiency and induces substantial changes in the behavior of iron cations, increasing the tendencies to disproportionation and hole localization. Despite similar oxygen nonstoichiometry in the Al- and Ga-substituted ferrites at a given dopant content, the latter tendency is more pronounced in the case of aluminum-containing perovskites.     

Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

Magnetocaloric effect at room temperature in manganese perovskite La0.65Nd0.05Pb0.3MnO3 with double resistivity peaks

Series of polycrystalline manganese perovskite oxides La_0.7−xNd_xPb_0.3MnO₃ (x=0, 0.05, and 0.1) are prepared by the sol-gel technique, La_0.65Nd_0.05Pb_0.3MnO₃ were representatively investigated because the peculiar double resistivity peaks were found; the maximum magnetic entropy change ΔS_H=−2.03 J/kg K and its good refrigerant capacity 71.05 J/kg around room temperature were obtained under 9 kOe magnetic field variation. The expected double peaks of magnetocaloric effect had not occurred since magnetic entropy change originated from the differential coefficient of magnetic moment to temperature; the relatively well refrigerant capacity possibly results from the faint magnetic inhomogeneity mixed in the double exchange strong magnetic signal.     

Synthesis and characterisation of La1−xMnO3±δ nanopowders prepared by acrylamide polymerisation

La_1−xMnO_3±δ (x=−0.02 to 0.35) nanocrystalline powders were prepared by a new sol–gel method. It is used the acrylamide gelification to form an organic 3D tangled network where a solution of the respective cations is soaked. This method was adapted to cover a broad range of high impact electro–ceramic oxides, which a particular example is the CMR nanopowders reported in this work. The acrylamide sol–gel process is a fast, cheaper and easy to scale-up method for obtaining fine powders of complex oxides. This synthesis method allows performing 100 g of highly pure nanopowders in one run with simple laboratory scale. The sponge like powder obtained consists of thin sheets composed of nanocrystallites whose size varies from 66 nm to 30 nm, depending on composition. The oxygen content of the manganite powder is shown to decrease with vacancy-doping on lanthanum site. Such a evolution can be explained for La/Mn<0.9 by considering a demixtion of the powder into La_0.9MnO₃ and Mn₃O₄ phases, while for La/Mn>0.9, the high oxygen excess leads to consider vacancies on both lanthanum and manganese sites. Both hypotheses are supported by magnetic measurements, which show a constant Curie temperature of 295 K for La/Mn<0.9, while for La/Mn>0.9, the occurrence of vacancies on manganese sites progressively impedes the ferromagnetic interactions, leading to a cluster–glass behaviour in the case of the highly manganese-deficient La_0.94Mn_0.92O₃ compound.