ChemInform Abstract: Effect of BaF2 on the Synthesis of the Single-Phase Cubic Y3Al5O12:Tb.

The luminescence efficiency of the YAG:Tb phosphor (yttrium aluminium garnet, Y₃Al₅O₁₂) has been improved by the application of BaF₂ flux to the firing process.     

Thermochemistry of Gd2BaCuO5 and LuBa2Cu3Ox

Solution calorimetry, using 6.0 M HCl as a solvent, is used to study the thermochemistry of Gd2BaCuO5 and the high-temperature superconductor LuBa2Cu3O6.92. For the first time, the standard formation enthalpies of these phases have been determined as follows: DeltafH(o)(Gd2BaCuO 5, s, 298.15 K) = -2618.6 +/- 7.4 kJ/mol; DeltafH(o)(LuBa2Cu3O6.92, s, 298.15 K) = -2693.1 +/- 11.9 kJ/mol. The thermodynamic stability at room temperature has been assessed. The results show that Gd211 and Lu123 are thermodynamically stable with respect to binary oxides and unstable with respect to interaction with CO 2 at ambient temperatures. Lu123 is thermodynamically stable with respect to assemblages containing combinations of Lu2O3, CuO, and BaCuO2 and thermodynamically unstable with respect to interactions with water.     

Structural and Magnetic Properties of Pr2BaCuO5 Oxide

The Pr₂BaCuO₅ oxide has been prepared by using a precursor method. X‐ray and neutron diffraction data reveal that this mixed oxide crystallizes with the so‐called Nd₂BaPtO₅‐type structure, showing tetragonal symmetry and space group P4/mbm. Magnetic susceptibility, magnetization and specific heat measurements reveal that Pr₂BaCuO₅ oxide behaves as antiferromagnet, where the Pr³⁺ and Cu²⁺ sublattices become simultaneously ordered at 15 K. Preliminary neutron diffraction studies confirm this proposed antiferromagnetic ordering and it can be described by the wave vector k = [0,0,0].     

Phase equilibria in BaF2(Y,Ln)F3 systems

Data are presented on phase equilibria at 877 ± 10°C in the systems BaF₂(Y,Ln)F₃, where Ln = SmLu. All the systems show cubic solid solutions based on BaF₂ and variable composition phases

of structure derived from the CaF₂ type (rhombohedral distortion). Syngony and unit cell dimensions have been determined on monocrystals; crystallographic parameters of ten trigonal phases have been adduced. The existence of solid solutions of BaF₂ in high-temperature α-LnF₃(LaF₃ type) at the given isothermal section indicates stabilization of the LaF₃ structure type by heterovalent isomorphous replacement. In the systems BaF₂(Y,Ln)F₃ with Ln = DyYb, monoclinic compounds BaR₂F₈ are formed. X-ray parameters, derived from single crystal and polycrystalline specimens, of six monoclinic BaR₂F₈ compounds are presented. In the BaF₂LuF₃ system we have isolated and studied for the first time a compound of the approximate composition BaLu₂F₈ which crystallizes in rhombic syngony and has a marked range of homogeneity.     

Formation of the Compound La2BaCuO5 (La4Ba2Cu2O10)

Synthesis of the compound La₂BaCuO₅ (La₄Ba₂Cu₂O_{1 0}), whose X-ray diffraction pattern is identical to that available in the literature for the solid solution La_3.6Ba_2.4Cu_1.8O_9.6, was studied.     

Mechanism of formation of YBa2Cu4O8 superconductor in the sol-gel synthesis

It was shown that a single phase YBa₂Cu₄O₈ (124-phase) could be formed from gels at 1 atm oxygen pressure, and the mechanism of its formation was elucidated. It was found that there are two key routes for the sol-gel formation of the 124-phase, one involving the tetragonal YBa₂Cu₃O _y with a low concentration of oxygen defects (tetra-I phase) and the other involving the Ba₂Cu₃O_5.9 as important intermediates of the 124-phase. The rapid formation of these intermediate compounds from the gel was attributed to the small particle size of the oxides and carbonates precipitating at the initial stage of heating. It was thought that the small particles characteristic of sol-gel processing lead to the rapid formation of the intermediate compounds and subsequent precipitation of the 124-phase.     

Study of nano-sized (ZnFe2O4)y particles/CuTl-1223 superconductor composites

Zinc ferrite (ZnFe₂O₄)_y nanoparticles/Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10−δ (CuTl-1223) superconductor composites with y = 0–2 wt.% were prepared by adding ZnFe₂O₄ nanoparticles into CuTl-1223 superconductor matrix and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transforms infrared (FTIR) spectroscopy and dc-resistivity (ρ) measurements. The bulk CuTl-1223 superconductor matrix was synthesized by solid-state reaction and Zinc ferrite (ZnFe₂O₄) nanoparticles were separately prepared by sol–gel method. XRD analysis revealed the tetragonal and spinel structure of CuTl-1223 superconductor and ZnFe₂O₄ nanoparticles, respectively. The XRD analysis showed that increased concentration of ZnFe₂O₄ nanoparticles doesn't disturb the tetragonal structure of host CuTl-1223 superconductor matrix and has no appreciable effect on its lattice parameters. The SEM images confirm the granular structure of the host superconductor matrix. The presence of ZnFe₂O₄ nanoparticles in host superconductor matrix is confirmed by using FTIR study. Variation of zero resistivity critical temperature {T_c (0)} depends upon the concentration of the nanoparticles in the host superconductor matrix. The overall suppression of T_c (0) and diamagnetism with increasing nanoparticles concentration is most probably due to trapping of mobile free carriers and reflection of spin charge due to presence of paramagnetic ZnFe₂O₄ nanoparticles. There is possibility for the incorporation of Fe and Zn in the lattice sites during the synthesis process, which may also cause the reduction of T_c (0) of the final composites.     

Soft chemistry synthesis of powders in the BaF2-ScF3 system

The BaF₂-ScF₃ system is studied using coprecipitation from aqueous solutions. Compound Ba₃Sc₂F₁₂ is found to form over a wide range (10–60 mol %) of scandium concentrations in the initial solution. At higher scandium concentrations in solution, a new phase of provisional composition BaSc₂F₈ · 2H₂O (hexagonal system, a = 9.6346 Å, c = 4.0483 Å) was obtained. Scandium fluoride hydrolysis is not observed by energy-dispersive X-ray analysis.     

Mechanism of formation of monodisperse polystyrene hollow particles prepared by membrane emulsification technique. Effect of hexadecane amount on the formation of hollow particles

In a previous study, it was found that monodisperse polystyrene (PSt) hollow particles can be prepared under special conditions by combining a Shirasu Porous Glass (SPG) emulsification technique and subsequent suspension polymerization process. The dispersed phase mainly containing St, hexadecane (HD), and initiator, was pressed through the uniform pores of a SPG membrane into the continuous phase to form uniform droplets. Then, the droplets were polymerized at 70°C. It was proposed that rapid phase separation between PSt and HD was a main reason responsible for the formation of hollow particle. Rapid phase separation confined the HD inside the droplets, it belonged to a non-equilibrium morphology. In this study, HD/St ratio was increased to a high value to confirm the above proposition by promoting rapid phase separation further between HD and PSt, to prevent monomer diffusion into aqueous phase, and to obtain hollow particle with a large hole.     

Chemical interactions in the systems BaCuO2-PbO and YBa2Cu3O7-PbO

Chemical interactions were found to occur in the systems BaCuO₂-PbO and YBa₂Cu₃O₇-PbO at 600–800C. The chemical peculiarities of these reactions are explained on the basis of redox transformations, with formation of the phase BaPbO₃.

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Zusammenfassung Bei 600–800C konnte im System BaCuO₂-PbO und YBa₂Cu₃O₇-PbO eine chemische Wechselwirkung festgestellt werden. Die chemische Besonderheit der Reaktionen wurde mit Hilfe von Redoxumformungen unter Bildung der Phase BaPbO₃ erklÄrt.

     

Determination of Y,Ba and Cu in superconductor films by X-ray fluorescence analysis

Radionuclide X-ray fluorescence analysis was used for the determination of Y, Ba and Cu in thin high-temperature superconducting films. Atomic emission ICP spectrometry was used to estimate the precision and accuracy of analytical results. Reasonable agreement between both methods was obtained when a polynomial calibration curve was applied.     

SrF2和BaF2熔化温度的分子动力学模拟

利用壳层模型分子动力学方法,考虑萤石结构分子中的预熔化现象,对SrF2和BaF2的分子动力学模拟熔化温度进行修正,获得了高压下SrF2和BaF2的熔化温度．同时给出了300K、0．1MPa-7GPa和10．1MPa-3GPa时SrF2和BaF2的状态方程,与已有研究结果的最大误差分别为0．3％和2．2％．计算所得SrF2和BaF2常压下的熔点与已有的实验结果符合较好．对于SrF2和BaF2分子体积变化和已有的熔化模拟的差别也做了比较和讨论．     

Phase equilibria in the P2O5-rich part of the system Y2O3−Na2O−P2O5

In the ternary system Y₂O₃?Na₂O?P₂O₅, the partial system Y(PO₃)₃?NaPO₃?P₂O₅ was examined by means of differential thermal analysis and X-ray powder diffraction; its phase diagram is given.     

A concerted three-body formation X+Y+C2H4 in the photodissociation of CH2XCH2Y (X,Y=Br,Cl) at 193 nm

The photodissociation of CH2XCH2Y (X,Y=Br,Cl) through absorption of 193 nm photons was investigated using product translational spectroscopy. No stable CH2BrCH2 or CH2ClCH2 was detected. The recorded time-of-flight spectra indicate that these internally excited radicals dissociated into Y+C2H4 in a concerted reaction with the first C-X bond rupture. Product anisotropy implies that the overall reaction time for three-body formation is in a fraction of rotational period. According to an asynchronous concerted reaction model, the measured spectra were simulated with product translational energy distributions coupled by asymmetric angular distributions. For the mixed halide, CH2BrCH2Cl, triple products Br+Cl+C2H4 can be originated from the cleavage of either the C-Br bond or the C-Cl bond. The results are discussed and where appropriate, comparisons with previous investigations of the related molecules are included.     

Pore formation during the pyrolysis of C-Nb2O5 and C-Ta2O5 xerogels

Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb₂O₅ or Ta₂O₅ sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I Type IV) and of the hysteresis loops (H4H2H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Thermodynamics of the adsorption-accompanied formation of new-phase particles

A thermodynamic model was proposed to describe the process of the formation of inorganic nanoparticles in a solution containing well-adsorbable organic compounds. An equation for the critical nucleus size was derived and analyzed. It was shown that intense adsorption of organic compounds leads to a strong decrease in the critical nucleation size of the forming nanoparticles.