Response of La0.8Sr0.2CoO3-δ to perturbations on the CoO3 sublattice

Emission and transmission M?ssbauer studies of La_0.8Sr_0.2CoO_3-δ perovskites doped with ∼0.02 stoichiometric units of oxygen vacancy or 2.5% iron corroborate the occurrence of electronic phase separation in these systems. The effect of the small perturbation of the CoO₃ sublattice with either iron ions or oxygen vacancies on the bulk magnetization as well as on the M?ssbauer spectra is in good agreement with the double exchange based cluster model. The magnetoresistance does not show any peak near the Curie temperature, but reaches -84% in a field of 7.5 T at T = 8 K. Below T_C ≈ 180 K the M?ssbauer spectra distinctly include the contribution from paramagnetic and ferromagnetic regions, providing direct evidence for phase separation. No contribution to the spectra from Fe⁴⁺ ions can be observed, which is an unambiguous evidence that at low concentration iron (either directly doped or formed from ⁵⁷Co by nuclear decay) is accommodated in the cobaltate lattice as Fe³⁺ ion.     

Optical properties of thin epitaxial Ba0.8Sr0.2TiO3 films

The properties of nanodimensional (Ba_0.8, Sr_0.2)TiO₃ films on single-crystal magnesia substrates are studied. The films are applied by rf sputtering and grow in the layer-by-layer mode. The lattice parameters are measured by the X-ray diffraction method. The transmission of the films with different thicknesses is studied in the wavelength range 190?C1100 nm. When analyzing experimental optical parameters, additional relaxation parameters depicting a final lifetime of the oscillator are used to characterize the refractive index and absorption factor in the dispersion relation. Such an approach allows a more accurate approximation of experimental data.     

Temperature evolution of the cluster state in La0.8Ca0.2MnO3 and La0.8Ca0.2CoO3

The results of investigations of the transport and magnetic properties (ac linear and nonlinear (second- and third-order) susceptibilities) of La_0.8Ca_0.2MnO₃ and La_0.8Ca_0.2CoO₃ single crystals have been presented. It has been found that both compounds in the paramagnetic phase contain ferromagnetic clusters with close magnetic characteristics. At high temperatures, ferromagnetic clusters nucleate in preferred sites associated with chemical inhomogeneities. Cooling below a specific temperature is accompanied by homogeneous nucleation of clusters. These two stages are observed in both compounds. In the doped cobaltite, the coalescence of clusters begins to develop at the third stage, whereas in the manganite, their behavior changes due to the development of ferromagnetic ordering of the matrix. These features indicate that the cluster state in doped manganites and cobaltites has a common nature. The difference in the behavior of ferromagnetic clusters is a consequence of the magnetically active character of the matrix in the case of manganites and the neutral character of the matrix in the case of cobaltites.     

Effect of low-angle boundaries on the dielectric properties of epitaxial Ba0.8Sr0.2TiO3 films

The effect of low-angle boundaries on the dielectric properties of epitaxial Ba_0.8Sr_0.2TiO₃ films is studied by comparing films differing in crystalline-block size. It is found that the permittivity diminishes considerably when the block sizes are reduced. The maximum of the temperature-dependent permittivity is shifted towards lower temperatures, and the sensitivity of the permittivity to an electric field is reduced. Moreover, it is found that the maximum in the permittivity temperature dependence is displaced towards lower temperatures when the applied measured voltage is increased and becomes higher than the coercive voltage. The width of a hysteresis loop decreases significantly when the frequency of the controlling field is reduced. The reasons for the observed behavior are analyzed.     

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρ _xx and transverse ρ _xy resistivities and magnetic susceptibility χ _ac of La_0.8Sr_0.2MnO₃ single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρ _xy ∼ρ _xx , which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χ _ac. As a result, the curve of ρ _xy versus ρ _xx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρ _xx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽T _C . The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials. Zh. éksp. Teor. Fiz. 116, 671–683 (August 1999)     

Electrical transport properties and laser-induced voltage effect in La0.8Ca0.2MnO3 epitaxial thin films

La_0.8Ca_0.2MnO₃ (LCMO) thin films about 200 nm thickness were grown on untilted and tilted (5°, 10° and 15°) LaAlO₃ (100) single crystal substrates by pulsed laser deposition technique. Electrical properties of the epitaxial thin films were studied by conventional four-probe technique and the anisotropic thermoelectric properties of the films grown on the tilted substrates have been investigated by laser-induced voltage (LIV) measurements. X-ray diffraction analysis and atomic force microscopy results show that the prepared LCMO thin films have a single phase and high crystalline quality. The remarkably large temperature coefficient of resistance (TCR) values (above 11 %/K) are observed in the all films. TCR value reaches 18 %/K on the film grown on 10° tilted substrate. The intensity of LIV signals monotonously increases with the tilting angles, and the largest signal is 148 mV with the fast time response 229 ns for the film grown on 15° tilted substrate.     

Oxygen reduction at sol–gel derived La0.8Sr0.2Co0.8Fe0.2O3 cathodes

The perovskite material, La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF), substituted by Sr and Fe at the A and B sites, was prepared using the sol–gel (SG) method, followed by heating at 900 °C for 4 h. The X-ray powder diffraction pattern for the SG derived LSCF material showed good agreement with the literature data. Scanning electron microscopy showed that the LSCF structure is highly porous, facilitating gas transfer and maximizing the number of active sites for the oxygen reduction reaction (ORR) at the cathode of a solid oxide fuel cell. Transmission electron microscopy (TEM) was employed to determine the SG–LSCF particle size and distribution. The kinetics of the ORR were investigated at SG–LSCF, deposited by screen-printing on a samarium-doped ceria (SDC) electrolyte, using electrochemical impedance spectroscopy and cyclic voltammetry at temperatures ranging from 400 to 700 °C. The results showed that the SG–LSCF cathode is stable and exhibits a high exchange current density (and low charge transfer resistance), yielding an apparent activation energy for the ORR of ca. 120 kJ/mol. It was also found that the SG–LSCF on SDC cathode was approximately one order of magnitude more active than standard manganite-based composite cathodes, deposited on yttria stabilized zirconia, studied under otherwise identical operating conditions.     

Electrochemical oxidation and reduction of the La0.2Sr0.8CoO3−δ phases: Control of itinerant ferromagnetism and magnetoresistance

In order to understand how the magnetotransport properties are related to the oxygen content, the La_0.2Sr_0.8CoO_3−δ perovskite phases have been studied by using electrochemical cycling. The samples, prepared by a sol–gel technique, have been characterized from a structural and chemical point of view concomitantly to their physical properties. Upon reduction, a brownmillerite-type phase La_0.2Sr_0.8CoO_2.65, which presents a weak ferromagnet behavior, is formed and, upon oxidation, a complete stoichiometric phase with a cubic unit cell (a = 3.83Å) could be obtained with a ferromagnetic and metallic behavior. Finally, it is shown that using appropriate cut-off potential upon reduction or oxidation the oxygen content and the cobalt valence can be monitored in such transition metal oxides to reach the fully oxidized or reduced end-members.     

Co掺杂对钙钛矿锰氧化物La0.8Sr0.2MnO3磁性的影响

采用传统的高温固相反应法制备La0.8Sr0.2Mn1-xCoxO3（x=0,0.1）系列多晶样品,采用X射线衍射（XRD）测量样品的结构,并用物性测量系统（PPMS）探究样品的磁化强度随温度的变化曲线（M-T）,以及在不同的温度下磁化强度随外加磁场的变化曲线（M-H）。结果表明：当温度高于TG时,样品呈现出纯顺磁态,当TG相似文献   

稀土Pr掺杂纳米发光材料Sr_(0.2)Ca_(0.8)TiO_3的结构及光学性质

采用溶胶-凝胶法制备了Pr掺杂Sr0.2Ca0.8TiO3纳米粉体,用紫外可见光吸收光谱、X射线衍射、透射电镜进行结构表征,分析了不同温度下Sr0.2Ca0.8TiO3∶Pr的粒径大小以及结构变化,分析了其发光特性,并总结出最佳的合成温度为600℃.     

Synthesis and investigations on the stability of La0.8Sr0.2CuO2.4+δ at high temperatures

For application in solid oxide fuel cells La_0.8Sr_0.2CuO_2.4+δ was synthesized and the phase evolution was characterized after quenching from different temperatures and after slow cooling. A single phase perovskite was found after quenching from 950 °C. The electrical conductivity of the La_0.8Sr_0.2CuO_2.4+δ perovskite exhibited metallic behavior reaching values of about 270 S/cm at 800 °C in air. The thermal expansion between 30 and 800 °C gave a thermal expansion coefficient of 11.1 × 10^− 6 K^− 1.At higher temperatures, the perovskite was transformed to the K₂NiF₄-type structure via an intermediate stage that can be best described as a LaSrCuO₄ phase with preferential growing of {020} lattice planes. After sintering at 1100 °C and slow cooling in the furnace a phase mixture of (La,Sr)CuO_4+δ and (La,Sr)CuO_2.4+δ perovskite was obtained. This phase mixture showed higher electrical conductivity (400 S/cm at 800 °C) and smaller thermal expansion coefficient (9.6 × 10^− 6 K^− 1) than the single phase La_0.8Sr_0.2CuO_2.4+δ perovskite.     

Infrared optical properties of PbZr0.4Ti0.6O3/La0.5Sr0.5CoO3 heterostructures on platinized silicon substrate

PbZr_0.4Ti_0.6O₃/La_0.5Sr_0.5CoO₃ heterostructures have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution routes. Optical properties of the PbZr_0.4Ti_0.6O₃/La_0.5Sr_0.5CoO₃ heterostructures were studied by infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5–12.5 m. The optical constants of PbZr_0.4Ti_0.6O₃ and La_0.5Sr_0.5CoO₃ thin films were determined by fitting the IRSE data using a classical dielectric model and a Drude dielectric model, respectively. For PbZr_0.4Ti_0.6O₃ thin films, the refractive index decreases and the extinction coefficient increases as the wavelength increases. For La_0.5Sr_0.5CoO₃ thin films, the refractive index and the extinction coefficient increase as the wavelength increases. The absorption coefficient of La_0.5Sr_0.5CoO₃ thin films is greater than 10⁴ cm^-1 for the wavelength range of 2.5–12.5 m. The absorption coefficient of PbZr_0.4Ti_0.6O₃ thin films on a La_0.5Sr_0.5CoO₃/Pt/Ti/SiO₂/Si substrate is smaller than that on a Pt/Ti/SiO₂/Si substrate by a factor of two. PACS 77.84.Dy; 78.20.Ci; 81.20.Fw     

In-situ Mössbauer spectroscopy and thermogravimetry of La0.2Sr0.8FeO3−Δ and La0.4Sr0.6FeO3−Δ

Single phase perovskite-type oxides La_0.2Sr_0.8FeO_3−Δ (LSF28) and La_0.4Sr_0.6FeO_3−Δ (LSF46) were prepared by the glycine nitrate process. The oxygen nonstoichiometry of LSF46 was studied by precision thermogravimetry at 700 and 900 °C as a function of oxygen partial pressure. Mössbauer spectra were measured at room temperature, as well as at 500 and 700 °C (LSF28) and at 700 and 900 °C (LSF46) under varying oxygen partial pressures. Good agreement is observed for the pO₂-dependent nonstoichiometry of LSF46 at 700 and 900 °C as derived from thermogravimetry and in-situ Mössbauer spectroscopy. At low pO₂, the Mössbauer isomer shifts and quadrupole splittings indicate a phase transition in LSF46 at 700 °C and in LSF28 at the two temperatures studied from a disordered perovskite phase to a brownmillerite-like structure with larger quadrupole splitting.     

Synthesis and electric properties of perovskite Pr0.6Ca0.4Fe0.8Co0.2O3 for SOFC applications

In this study, polycrystalline powder Pr_0.6Ca_0.4Fe_0.8Co_0.2O₃ (PCFC) was synthesized by a sol–gel process. This oxide was analyzed by X-ray powder diffraction. Synthesized Pr_0.6Ca_0.4Fe_0.8Co_0.2O₃ showed up to be single phase and belongs to the orthorhombic crystalline system with a Pbnm space group. The microstructural features of the synthesized products display particles having an irregular morphology and a size in the range of 50–100 nm. X-ray diffraction (XRD) analysis shows the chemical compatibility between the PCFC cathode and the electrolyte Sm-doped ceria since no reaction products were honored when the material was mixed and co-fired at 1,000 °C for 168 h. The thermal expansion coefficient of PCFC 16.9?×?10^?6 °C^?1 is slightly higher than that of Ce_0.8Sm_0.2O_1.9 (SDC) over the studied temperature range. The greater contribution to the total resistance of the electrode is the electrochemical resistance associated with oxygen exchange in the cathode surface (0.96 Ωcm²). The dc four-probe measurement indicated that PCFC exhibits fairly high electrical conductivity, over 100 S cm^?1 at T?≥?500 °C, making this material promising as a cathode material for intermediate temperature solid oxide fuel cells.     

Relaxor properties of Nb-modified (Ba0.8Sr0.2)TiO3 ceramics

Nb-doped (Ba_0.8Sr_0.2)TiO₃ ceramics were prepared using conventional mixed-oxide processing technique. Permittivity and loss factor were investigated as a function of temperature for various frequencies of the measuring field. The obtained results confirmed the relaxor ferroelectric behaviour of the studied ceramics, i.e. a strong frequency dispersion of the permittivity maximum and a visible shift of its temperature with frequency. Analysis of real part of permittivity allowed us to determine the value of the freezing temperature characterising the relaxor ferroelectrics. The physical processes, responsible for the relaxor behaviour of the studied ceramics are discussed.     

Catalytic behavior of La0.6Sr0.4Co0.8Fe0.2O3 perovskite-type oxide thin films deposited on YSZ during the reaction of ethanol oxidation

The perovskite-type oxide La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was studied as an ethanol oxidation catalyst-anode as a probable candidate for future application in Solid Oxide Fuel Cells (SOFCs). The investigation was carried out at atmospheric total pressure in a fully Yttria-Stabilized Zirconia (YSZ) continuous stirred tank reactor (C.S.T.R.) at a temperature range between 300–700 °C. Preliminary results show that La_0.6Sr_0.4Co_0.8Fe_0.2O₃ thin films deposited on the inner bottom of the one end closed YSZ tube present: a) mediocre ethanol oxidation catalytic activity at relatively low temperature values and b) thermal stability under the experimental conditions. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Preparation and characterization of La0.5Sr0.5CoO3 /Al-doped ZnO thin film heterojunctions

In this study, p–n heterojunctions with La_0.5Sr_0.5CoO₃ (LSCO) and Al-doped ZnO (AZO) thin films were fabricated by the radio frequency (r.f.) magnetron sputtering technique. The LSCO/AZO heterojunction was obtained by stacking the p-type LSCO thin film on the annealed n-type AZO thin film under different Ar: O₂ sputter gas ratio atmosphere. The thickness of LSCO and AZO thin films are about 400 nm and 500 nm, respectively. Good crystalline match between LSCO and AZO films was observed from the SEM and XRD analysis. The heterojunction diode clearly demonstrated rectifying behavior in the range of ?8 to +8 V in reverse shape. The turn-on voltage of the diodes is obtained around 1.5 V and is in agreement with the value obtained from the difference in the work functions of LSCO and AZO. The band structure of the heterojunction was proposed based on the results of analysis.     

Electrical properties of La0.67Sr0.16Ca0.17MnO3 perovskite

La_0.67Sr_0.16Ca_0.17MnO₃ perovskite was prepared by the solid-state reaction technique. Preliminary room-temperature structural studies confirm the formation of the sample in the rhombohedral structure. The electrical response was studied using the impedance spectroscopy technique over a broad frequency range (250 Hz–3 MHz). The results indicate that the electrical properties of the material are strongly dependent on temperature and frequency. Impedance spectrum analysis shows that the material can be described as a grain and grain boundary medium and permits to estimate the grain boundary contribution. Electronic conduction is found to be dominated by a thermally activated hopping of small polaron at high temperatures.