Synthesis and optical properties of La1−XCeXMnO3 studied by infrared reflectivity measurements

The bulk polycrystalline La_1−xCe_xMnO₃ (0.0 ≤ x ≤ 0.8) manganites were synthesized through citrate sol-gel auto-combustion method for a wide range of composition 0.0 ≤ x ≤ 0.8. The x-ray diffraction (XRD) analysis suggests the single phase rhombohedral (R-3c) symmetry for parent LaMnO₃ compound, however a secondary cubic (Fm-3 m) phase of cerium oxide (CeO₂) is appeared when Ce is incorporated into the LaMnO₃. A detailed XRD study of these materials were discussed, and specifically the infrared reflectivity measurements on La_1−xCe_xMnO₃ materials are discussed in this study for the first time. The phonon softening and hardening was observed for different modes but softening was found to be more pronounced which implied strong electron–phonon coupling. The most affected phonon mode was appeared for the doping range 0.2 ≤ x ≤ 0.4, in which the largest softening was observed. The Lyddane–Sachs–Teller relation is verified by experimental FTIR data.     

Preparation and properties of Ba1−xCaxTiO3 nanopowders obtained by mechanochemical synthesis

ABSTRACT

The calcium-substituted barium titanate nanopowders Ba_1?xCa_xTiO₃ (0.2 ≤ x ≤ 0.3) have been obtained at room temperature by mechanochemical synthesis. The formation of the perovskite phase was controlled by X-ray diffraction studies at various milling duration. The powders possess the perovskite crystallographic structure directly after milling longer than 10 h. The dielectric properties of the ceramics obtained by sintering of the nanopowders were investigated in the temperature range between 300 and 500 K. The temperature dependence of permittivity exhibited a single anomaly, which corresponds to the ferroelectric–paraelectric phase transition.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La_1−xSr_xCoO_3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.     

Valence state of manganese ions in the La1−α BiLaβMnLa1−δ OLa3±γ ceramics

The valence state of the Mn ions in ceramic samples of the La_1?α BiLa_βMnLa_1?δ OLa_3±γ composition (LBMO) has been studied using the X-ray photoelectron spectroscopy. The presence of Mn³⁺ and Mn⁴⁺ ions in these compounds has been shown. The relative content Mn³⁺/Mn⁴⁺ has been determined by means of fitting the experimental Mn 2p spectra by the superposition of theoretical spectra of Mn³⁺ and Mn⁴⁺ ions. The elemental composition of the samples has been determined by the X-ray photoelectron spectroscopy and electron probe microanalysis. It has been established that the relative content of Mn⁴⁺ ions correlates with parameter δ, which characterizes the deviation of the actual elemental composition of the La_1?α BiLa_βMnLa_1?δ OLa_3±γ ceramics from stoichiometry La_{1 ? x} Bi _x MnO₃.     

Synthesis and superconductivity properties of Ba1−xKxBiO3

Ba_1?xK_xBiO₃ with x from 0.315 to 0.6 were successfully synthesized by molten salts method and characterized by XRD and magnetic susceptibility measurements. It is found that Ba_1?xK_xBiO₃ powders could directly be precipitated from KOH melts. Superconductivity has been observed in all samples and the highest superconducting transition temperature was found to be T_c = 30.6 K with x = 0.4. The lattice constant linearly depended on the potassium content in accord with the equation of a = 4.3548–0.1743x, and the decrease of the mole ratio of Bi³⁺/Bi⁵⁺ resulted in the increase of the potassium content, which suggested the disproportionation of Bi valence.     

Synthesis,structure and magnetic entropy change of polycrystalline La1−xKxMnO3+δ

The use of a sol–gel synthetic method allows the preparation, at temperatures as low as 873 K, of single-phase perovskites in the potassium-doped lanthanide manganites La_1−xK_xMnO_3+δ. Samples prepared at 1273 K remain single phase and are constituted by submicrometer homogeneous particles, without deviation from nominal stoichiometries upon heating. These materials exhibit paramagnetic-to-ferromagnetic phase transitions at 230, 283, 338, and 344 K for x=0.075, 0.10, 0.165, and 0.20, respectively. By measuring the sample magnetization as a function of field and temperature we have determined the magnetic entropy change associated with these transitions. It is suggested by the results that these materials can be utilized as suitable magnetic refrigerants with very large temperature span.     

Preparation and properties of Y1−xHoxBa2Cu3O7−δ thin films by TFA-MOD method

Y_1?xHo_xBa₂Cu₃O_7?δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) thin films were prepared on LaAlO₃ (0 0 1) substrates by trifluoroacetate metal organic deposition (TFA-MOD) without change of the processing parameters. The highest J_c was attributed to the sample of Y_0.6Ho_0.4Ba₂Cu₃O_7?δ thin film, whose critical current density is about 1.6 times as compared to that of YBa₂Cu₃O_7?δ thin film at 77 K and self field. The flux pinning type was not varied with Ho substitution and can be attributed to δl pinning model, which is attributed to the close ionic radius between the Y³⁺ and Ho³⁺ ions. The improvement of J_c by Ho substitution without change of the processing parameters will provide an effective route to enhance the J_c of YBCO-based thin films using TFA-MOD method.     

Structural,magnetic, and physical properties of La(1–x)MnO3±δ nano-manganite

A detailed structural, magnetic and physics properties of La_1?xMnO_3±δ (LMO) nanomanganites were investigated to find out the role of cationic vacancies (La vacancy with Mn³⁺/Mn⁴⁺) in grain size modulation. Crystal structure and phase analysis of all samples were carried out by Rietveld refinement of high-resolution XRD and neutron diffraction data. We report here, the oxygen content in studied LMO compound decreases with increase in La vacancies in parent site and a parasitic Mn₃O₄ phase has been evolved in the range of 0.9 ≥ La/Mn ≥ 0.7. Para to ferro magnetic transition temperature (T_C) of all nanometric samples (La/Mn < 0.9) was found at high temperature side (≥260 K) whereas, the same for bulk one (La/Mn ≥ 0.9) was around 160 K. The enhancement of T_C (~70 K) with size reduction is attributed to broadening of bandwidth due to compaction of MnO₆ octahedra in system unit cell. In bulk sample, a secondary cluster/spin glassy phase is found below 50 K, whereas the glassy phase has been suppressed in nanoscale. Field-dependent magneto-resistance measurements are also carried out for all samples at different temperatures to get a profound insight of magneto-transport dynamics of the present system.     

Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La_1−xSr_xCoO_3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10⁻⁵ and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L_0.4Sr_0.6CoO_3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering. Paper presented at the 6th Euroconference on Solid state Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Phase transitions in La0.5Sr0.5FeO3−δ investigated by mechanical spectrum

The mechanical spectrum measurement was performed in ceramic La_0.5Sr_0.5FeO_3?δ from liquid nitrogen temperature to room temperature at kilohertz frequencies. From temperature dependent reduced modulus, a kink (corresponding temperature labeled as T_M) was observed which evidenced a phase transition by the mechanical spectrum at two flexural resonance frequencies. This elastic manifested phase transition is a charge disproportionation transition. Around 170 K, an internal friction peak (labeled as P1) was observed accompanied with a large modulus hardening with the decrease in temperature. Two mechanisms are proposed for P1 peak, one is elastic manifestation of magnetic freezing, and the other is the ordering or freezing of oxygen vacancies.     

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x Na x MnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na⁺ ion substitution for Ca²⁺ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La_0.7Ca_0.3−x Na _x MnO₃ manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ ². The temperature dependence of magnetization of all samples obeys the Bloch T ^3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na⁺ ions causes a decrease in total exchange integral Aof the samples.      

Synthesis and ionic conduction of C3A-type nasicon Na3+3x−yY1−xSi3−yPyO9

In the solid solutions of Na_3+3x−yY_1−xSi_3−yP_yO₉, new fast Na⁺ -conductors were obtained. The ionic conductivity, σ, and the activation energy, E, of the most conductive material, Na_4.425Y_0.375Si_2.55P_0.45O₉ were 5 × 10⁻³ S/cm at 300°C and 10 kcal/mol, respectively. Those values were strongly dependent upon the composition. With an increase of Na concentration, [Na], σ increased and E decreased. Accommodation of higher [Na] in the structure was madefeasible by decreasing [Y] and increasing [P]. Based on the crystal structure assumed for Na_3+3x−yY_1−xSi_3−yP_yO₉ and on the above results, a conduction model was presented. The structural consideration revealed that a conduction path is formed along the 〈1 1 1 〉 direction, where Y³⁺ ions locate in the way of Na⁺ ions.     

Multiferroic and exchange bias in La0.85Sr0.15FeO3−δ perovskite nanoparticles

Multiferroic and exchange bias was observed in La_0.85Sr_0.15FeO_3?δ nanoparticles sample which was prepared by co-precipitation method. These nanoparticles have an average size of about 18.49?nm and orthorhombic phase (Pbnm) with small impurities. The anomalous peaks of the dielectric as a function of temperature indicate that the Neel temperature at T_N?=?321?K and a ferroelectric to paraelectric transition at T_C?=?555?K which confirmed by differential scanning calorimetric (DSC). The dielectric response at low frequency is dominated by Debye relaxation behaviour and the resonance behaviour is dominated above 1.7?GHz. Mössbauer spectrum revealed the presence of magnetic ordering temperature (T_N) lies above room temperature (RT). Moreover, the oxygen deficient (δ?=?0.038) was calculated from Mössbauer spectrum. The hysteresis loop (B-H) of the nanoparticle sample exhibits exchange bias as a result of the exchange coupling at the interface between the ferromagnetic surface with canted spins and the antiferromagnetic core of the particles.     

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La_0.7Ca_0.3MnO_3±δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens.The use of argon atmosphere enlarges the crystalline c-axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T_C of the samples was observed.Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.     

Stress evolution and lattice distortion induced by thickness variation and lattice misfit in La0.67Sr0.33MnO3−δ films

La_0.67Sr_0.33MnO_3?δ thin films with different thicknesses are prepared in order to investigate the structural variation induced by film thickness and lattice misfit. The X-ray diffraction results show the in-built stress evolution from a full strained thin layer (～10 nm) to a completely relaxed thick layer (～150 nm), which can be well explained by the Poisson effect. Raman spectroscopy measurements reveal the complicated correlation between the Jahn–Teller (JT) distortion and film thickness. Important octahedron modes reflecting JT distortion are completely caused by the relaxed layer. It is observed that broad JT bands are formed in the films with large thickness of the relaxed layer and the residual stress in the layer leads to an obvious blue shift. In contrast, for films with the thin relaxed layer, JT modes are present as a sharper structure and move to low frequency, indicating towards a much better oxygen stoichiometry.     

High pressure Raman study of La1-xCaxMnO3-δ manganites

We report a high-pressure Raman study on two members of the La_1-xCa_xMnO_3-δ manganite family (x = 0.20, δ = 0 and δ = 0.08). The results obtained for the δ = 0 sample show a different behavior in the low and high pressure regime which is ascribed to the onset of a new pressure-activated interaction previously invoked in other manganite compounds. The comparison of our results with literature data gives further support to the identification of the Jahn-Teller sensitive stretching mode and shows that pressure-induced octahedral symmetrization is more effective in systems exhibiting a lower metallic character. On the contrary the new interaction sets in at a pressure which decreases on increasing the metallic character of the system indicating an important role of the Mn–Mn hopping integral in its activation.     

Synthesis and characterization of hollow ��-Fe2O3 sub-micron spheres prepared by sol�Cgel

In this work we report the preparation of magnetic hematite hollow sub-micron spheres (??-Fe₂O₃) by colloidal suspensions of ferric nitrate nine-hydrate (Fe(NO₃)₃·9H₂O) particles in citric acid solution by following the sol?Cgel method. After the gel formation, the samples were annealed at different temperatures in an oxidizing atmosphere. Annealing at 180°C resulted in an amorphous phase, without iron oxide formation. Annealing at 250°C resulted in coexisting phases of hematite, maghemite and magnetite, whereas at 400°C, only hematite and maghemite were found. Pure hematite hollow sub-micron spheres with porous shells were formed after annealing at 600°C. The characterization was performed by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM).     

Synthesis and characterization of BaZrO3-doped YBa2Cu3O7−δ microtapes with improved critical current densities

We have recently demonstrated that through a sol–gel route, superconductor crystallization in the presence of simple biopolymers results in a drastic alteration of morphology, producing technologically useful nanowires and porous architectures. Morphological control is of the utmost importance to bulk high-temperature superconductors, as grain boundaries act as weak links in limiting the achievable critical current density (J_c). Here we show that, as expected, the incorporation of nanoparticulate barium zirconate (BaZrO₃) species into a biopolymer-mediated synthetic protocol for YBa₂Cu₃O_7?δ (Y123) leads to a significantly improved in-field J_c compared to that observed in a sample without BaZrO₃ additions. To ameliorate degradation of the BaZrO₃ species in this protocol, we demonstrate that by drawing the precursor sol into fibers, a microtape architecture is able to be formed, leading to lengthy, anisotropic structures having enhanced J_c through the retention of the BaZrO₃ species.