Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis

Microstructural, magnetothermal and dielectric properties of YCrO₃ powders prepared by combustion and solid state methods have been studied by a combination of XRD, specific heat, magnetization and permittivity measurements. The TEM and XRD characterization confirm that the combustion powders are amorphous plate-like agglomerates of nano-sized crystalline particles. A more uniform grain size along with an increase of the relative density is observed by SEM in the sintered samples prepared by combustion route with respect to those produced by solid state reaction. Similar to the material obtained through solid state synthesis, the material prepared by the combustion method also shows spin canted antiferromagnetic ordering of Cr⁺³ (S=3/2) at ∼140 K, which is shown by magnetization as well as λ-type anomaly in the total specific heat. Furthermore, the magnetic contribution to the total specific heat reveals spin fluctuations above T_N and a spin reorientation transition at about 60 K. Both YCrO₃ compounds show a diffuse phase transition at about 450 K, typical of a relaxor ferroelectric, which is characterized by a broad peak in the real part of the dielectric permittivity as a function of temperature, with the peak decreasing in magnitude and shifting to higher temperature as the frequency increases. The relaxor dipoles are due to the local non-centrosymmetric structure. Furthermore, the high loss tangent in a broad range of temperature as well as conductivity analysis indicates a hopping mechanism for the electronic conductivity as we believe it is a consequence of the outer d³-shell, which have detrimental effects on the polarization and the pooling process in the YCrO₃ bulk material. The more uniform particle size and higher density material synthesized through the combustion process leads to an improvement in the dielectric Properties.     

A Novel Synthesis of Alkaline Earth Silicate Phosphor Sr3MgSi2O8： Eu^2＋, Dy^3＋

A novel method for synthesizing long afterglow silicate phosphor Sr3MgSi2O8：Eu^2＋,Dy^3＋using TEOS and inorganic powders as reactants was reported. Acetic acid as a catalyzer controlled the hydrolysis of TEOS by adjusting pH value of the system. The morphologies of precursor were characterized by transmission electron microscope （TEM）. The structure and optical properties of the phosphor powders were systematically investigated by means of X-ray diffraction and spectrofluorometry. TEM images have reflected the core-shell structure and quasi-spherical morphology of the precursor particles. It was found that the single-phase Sr3MgSi2O8 crystalline structures were obtained at 1050 and 1250 ℃ for the samples prepared with the nano-coating method and the solid state reaction, respectively. The emission intensities of the phosphors prepared by the present method were higher than those by the conventional process. Also, the afterglow characteristic was better than that prepared by solid-state reaction in the comparable condition.     

Analytical Techniques Applied in Optimization of Electrophoretic Deposition of Thin Film YBCO Superconductors

 YBa₂Cu₃O _7−x (x = 0.1–0.2) compounds (YBCO) were produced by the oxalate coprecipitation and the solid state reaction methods. The powders obtained were used for the production of YBCO superconducting coatings on Pt/Si wafers, by the electrophoretic deposition technique. The optimum process conditions for the production of both powders and coatings were found by using a combination of modern analytical techniques. The thermal treatment of the samples was followed by thermogravimetry (TG) and differential scanning calorimetry (DSC). The optimization and characterization of the superconducting properties of the powders and coatings were achieved by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), optical microscopy, magnetic susceptibility and electrical resistivity measurements.     

Preparation and Some Properties of Lithium Vanadium Bronzes

Lithium vanadium bronzes with composition formula Li_xV₂O₅ (0.04 ≤ × ≤ 0.92) have been prepared by solid‐state reaction at 650 °C in argon atmosphere. The obtained products were characterized by X‐ray powder diffraction and IR spectroscopy. The results reveal that four phases are present in the range from x = 0.04 to 0.92, namely α, β, β′, and γ phase. The magnetic susceptibility for the investigated bronzes was measured using the conventional Gouy's method. The values of the effective magnetic moments, as calculated from experimental data, indicate the presence of V⁴⁺ ions in all bronze samples. The electrical conductivity as a function of temperature and lithium content was measured in the temperature range from room temperature to 483 K. The electrical conductivity of the bronzes is found to be affected by lithium content. The values of the electrical conductivity increase with temperature for the prepared samples and both electronic and ionic conduction are discussed.     

镧掺杂对大洋锰结核制备铁氧体磁性的影响

大洋锰结核又称铁锰结核、多金属结核,常见于4 000～6 000 m的海底沉积物表层,在世界各大洋底均有分布,预计总储量超过3×1012 t[1].天然锰结核含有Mn和Fe,以及Cu、Co、Ni、铂族和稀土等60多种金属元素,其中锰占20%～30%,铁约占3～20%[2].从化学成分而言,大洋锰结核与Mn-铁氧体(铁酸锰)十分接近.以锰结核为原料合成Mn-铁氧体可以为这种天然资源的开发利用提供新思路.     

Preparation of nanocrystalline CuAlO<Subscript>2</Subscript> through sol–gel route

Nanocrystalline powders of CuAlO₂ were synthesized through sol–gel method using nitrate-citrate route and also through solid state reaction method. We used a new set of precursor materials for the synthesis of CuAlO₂ through sol–gel route which were not reported in the past. A little lowering of the synthesis temperature (1,000 °C) was observed in case of sol–gel process compared to the solid state reaction method (1,100 °C) and also at shorter time duration. The particle size of the synthesized powders was determined through small angle X-ray scattering. It has been observed that the particle size prepared by nitrate-citrate technique is less than the particle size prepared by the solid-state reaction method. Chemical states of the atomic species were determined by X-ray photoelectron spectroscopy. The formation of phase pure CuAlO₂ were also confirmed by Fourier transformed infrared spectroscopy. A number of solvents were also used for finding the best possible combinations for obtaining phase pure CuAlO₂ at 1,000 °C and it was observed that only the combination of nitrate salts, citric acid and ethanol resulted phase pure CuAlO₂.     

Ferromagnetic semiconductor ZnGeAs<Subscript>2</Subscript> {Mn} with a curie point of 367 K

Germanium zinc diarsenide crystals with various manganese fractions, (ZnGeMn)As₂, were prepared by reacting high-purity powders of zinc diarsenide, germanium, arsenic, and manganese. The manganese solubility at temperatures near the ZnGeAs₂ melting temperature was 3.5 wt % as determined by X-ray fluorescence analysis. The unit cell volume decreased with rising manganese concentration. Magnetization in fields up to 50 kOe, magnetic susceptibility, electrical resistance, and Hall constants were measured over wide temperature ranges. Manganese-doped samples had spontaneous magnetization and high Curie temperatures reaching 367 K. Their magnetic properties are characteristic of spin glasses in low magnetic fields. In higher fields, the spin state changed to spontaneous magnetization.     

Complexes of copper(II) with nicotinic acid and some related ligands

Summary Coordination compounds of copper(II) chloride, bromide, thiocyanate and sulphate with nicotinic acid, nicotinamide and isonicotinamide have been prepared and characterized by molecular conductance, magnetic susceptibility, electronic and i.r. spectral measurements down to 200 cm^–1 in the solid state. Possible stereochemistries for these complexes in the solid state are discussed.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

Visible-light-driven photocatalyst of Bi2WO6 nanoparticles prepared via amorphous complex precursor and photocatalytic properties

The visible-light-driven photocatalyst Bi₂WO₆ nanoparticles have been prepared by calcining amorphous complex precursor at a relatively low temperature of above 450 ^oC. The effects of calcination temperature and time on the structures and properties of Bi₂WO₆ nanoparticles have been investigated in detail. The photocatalytic activity of the Bi₂WO₆ powders were evaluated by degradation of RhB molecules in water under visible light irradiation (λ>400 nm). The results showed that the particle size and grain size of Bi₂WO₆ increased with the calcination temperature and time. The photocatalytic activity of the best sample was about 8.8 times higher than that of the sample prepared by traditional solid state reaction and the photo-degradations was a zero-order reaction. The best route to enhance the photocatalytic activity of Bi₂WO₆ was to prepare the sample at a lower temperature for a longer time, due to the samples with better crystallization and smaller particle size.     

ChemInform Abstract: Crystal Structure and Physical Properties of the Quaternary Manganese‐Bearing Pavonite Homologue Mn1.34Sn6.66Bi8Se20.

The title compound is prepared by solid state reaction of the elements (773 K, 72 h) and characterized by single crystal XRD, magnetic susceptibility, thermal and electrical conductivity, thermopower, and IR measurements.     

Synthesis and characterization of La2Mo2O9 obtained from freeze-dried precursors

La₂Mo₂O₉ ceramics have been prepared from freeze-dried precursors and their properties compared to those of lantanum molybdate obtained by conventional solid state (SS) reaction. All materials have been characterized by X-ray diffraction, scanning electron microscopy and thermal analysis (TGA/DTA/DSC and dilatometry) to characterize the phase formation and phase transition. When the freeze-dried method was applied, the synthesis temperature required to obtain dense samples was much lower than that for powders obtained by SS reaction. The morphology and structure of the oxide particle are significantly dependent on the synthesis method. The grain size is smaller, whereas the density of sintered pellets is higher for the freeze-dried precursor powder when compared with the SS reaction method. Impedance spectroscopy was used to measure the electrical conductivity of La₂Mo₂O₉ from 548 to 1123 K, in air, and to characterize the blocking effects of grain boundaries.     

Development of porous cathode powders for SOFC and influence of cathode structure on the oxygen electroreduction kinetics

Meso/macroporous La_1−xSr_xCoO_3−δ powder with the specific surface area higher than 140 m² g⁻¹ has been synthesized from the corresponding nitrates, using solution thermal decomposition method. These nanopowders have been used for preparation of SOFC cathodes, demonstrating lower oxygen electroreduction activation energy than that for less porous cathodes prepared from the powders synthesized using traditional solid state reaction method. To increase macroporosity of the cathodes the special pore forming agent has been added into the raw cathode paste. The very low total polarization resistance and activation energy values have been obtained for oxygen electroreduction, depending on the cathode porosity and potential applied.     

Comparison of the Efficiency of the One and Two-Step Process for the Production of BSCCO (2212) Powders by Means of Analytical Techniques

 The efficiency of the production of the high temperature superconducting powders Bi₂Sr₂CaCu₂O_8+x (BSCCO 2212) using the solid state reaction in one or two step processes under different thermal treatment was compared by means of different modern analytical techniques. Through the same techniques the optimization of the production of the production of Bi-2212 powder produced by the two step process, was achieved. X-ray diffraction analysis (XRD) and Raman spectroscopy were used to characterize the products for their stoichiometry and phase-purity. The grain size of the powders was observed by scanning electron microscopy (SEM), while their superconducting properties were tested by electrical DC-resistivity and magnetic susceptibility measurements using a superconducting quantum interference device (SQUID). It resulted that the two step process gives a high quality BSCCO 2212 superconducting powder with T _c = 85 K, in a shorter time and with a greater recovery rate than the one step process. Received May 3, 1999. Revision April 27, 2000.     

ChemInform Abstract: Cobalt—Zinc Molybdates as New Blue Pigments Involving Co2+ in Distorted Trigonal Bipyramids and Octahedra.

Zn_1-xCo_xMoO₄ (x < 0.3) powders are prepared by solid state reaction of stoichiometric mixtures of MoO₃, Co₃O₄, and ZnO (alumina crucible, 700 °C, 20 h and 800 °C, 10 h).     

Spin Crossover Studies on Bis{hydro-tris(1,2,4-triazolyl)borato}iron(II)

The spin-crossover behavior of bis{hydro-tris(1,2,4-triazolyl)borato}iron(II) is investigated in bulk and as a magnetically diluted sample in the solid state and in solution as a function of temperature by magnetic susceptibility measurements and differential scanning calorimetry. In aqueous solution the low-spin to high-spin transition is shown in turn to decrease the longitudinal relaxation time T₁ of water protons with increasing temperature. The solid-state magnetically diluted samples were prepared by cocrystallization with the isostructural zinc complex.     

Effect of Cadmium Substitution on Structural and Magnetic Properties of Nano Sized Nickel Ferrite

The structure and crystal phase of the nanocrystalline powders of Ni_1-xCd_xFe₂O₄ (0≤x≤0.5) mixed ferrite, synthesized by wet chemical co-precipitation method, were characterized by X-ray diffraction. Results showed that the lattice parameter increased with increasing Cd concentration. Microstructure was studied by scanning electron microscopy. TG/DTA stud-ies were carried out on co-precipitated sulphate complexes. These studies revealed the low ferritization temperature (650 oC) of the ferrite system synthesized by presently adopted route of synthesis and occurrence of simultaneous decomposition and ferritization processes.Further studies by infrared spectroscopy were also conducted. Moreover, magnetic properties of the prepared nanoparticles were studied by magnetization and a.c. susceptibility mea-surements. The response of prepared Ni_1-xCd_xFe₂O₄ mixed ferrites to magnetic field was investigated. Results show that, magnetic susceptibility, Curie temperature, and effective magnetic moment decreased as the Cd content increases.     

The characterization of Ba3Re2O9 and Sr3Re2O9

The compounds Ba₃Re₂O₉ and Sr₃Re₂O₉ were prepared by the solid state reaction of the corresponding alkaline-earth oxide with ReO₃ at 750 to 900°C in sealed, evacuated, fused silica tubes. The two compounds are isostructural, having the nine-layer ABO₃ structure with vacant central octahedra. The unit cell parameters are given. The magnetic susceptibility for Ba₃Re₂O₉ indicates Curie-Weiss behavior with a Re⁶⁺ moment having localized electrons. The magnetic data for Sr₃Re₂O₉ suggest delocalized electron behavior from its temperature-independent susceptibility. Both compounds appear to have semiconducting properties, but the strontium analog is a better conductor. Both compounds are unstable when heated in air above 400°C. They are readily decomposed by chemical oxidizing agents.     

3- and 4-picolylamine complexes of bivalent metal halides and pseudohalides

Summary Coordination compounds formed by the interaction of some bivalent metal halides and pseudohalides with the potentially bidentate ligands, 3-picolylamine and 4-picolyl-amine, have been prepared and characterized by molecular conductance, magnetic susceptibility, electronic and i.r. spectral measurements in the solid state. The i.r. spectral studies indicate that, in addition to their monodentate bonding through pyridine ring nitrogens, these ligands also bond through their amino nitrogen and, in some cases, act as bidentate bridging or chelating ligands. Tentative stereochemistries of the complexes isolated in the solid state are discussed. The ligand field parameters 10 Dq, B, Dq/B, andv ₂/v ₁ are calculated for the cobalt(II) and nickel(II) complexes and are consistent with their proposed stereochemistries.     

Caractérisation de peroxofluorotantalates(V). Mise en évidence d'isomères géométriques par résonance magnétique nucléaire (19F et 1H)

A series of tantalum(V) peroxofluoro complexes of the type Et₄N{Ta(O-O)-F₃(diket)} (diket = β-keto-enolate) has been prepared by the reaction of β-diketones with the new compound Et₄N{Ta(O-O)F₄(H₂O)}. The complexes have been characterised in the solid state on the basis of their analytical and infrared spectral data. When the bidentate chelating ligand is unsymmetrical, proton and fluorine nuclear magnetic resonance spectra demonstrate that the products exist as geometrical isomers in acetonitrile solutions.The orientation of the peroxo group according to the nature of the metal-ligand bonds is discussed.