Showing papers on "Lanthanum published in 1998"

The Structural Role of Lanthanum and Yttrium in Aluminosilicate Glasses: A 27Al and 17O MAS NMR Study

Abstract: Aluminosilicate glasses containing rare-earth and rare-earth analogue cations such as Y3+ and La3+ are interesting for a variety of technological applications, as well as for elucidating general principles of glass formation and structure. We present high-resolution 17O and 27Al NMR data on a series of lanthanum and yttrium aluminosilicates, which show the significant increase in concentrations of AlO5 and AlO6 groups caused by the smaller radii and higher field strengths of the modifier cation in the latter. Triple-quantum magic-angle spinning (3QMAS) spectra for both nuclides show much better resolution than standard MAS and demonstrate that the latter can give misleading results for compositional effects on speciation. Oxygen sites that resemble those in crystalline Y2O3 and La2O3 were not detected, suggesting that such extreme clustering is not significant in the compositions studied. The role of highly coordinated Al, and the configurational complexity generated by the formation of such species, is d...

Low‐Temperature Preparation of Nanocrystalline Lead Zirconate Titanate and Lead Lanthanum Zirconate Titanate Powders Using Triethanolamine

Abstract: Nanocrystalline lead zirconate titanate (PZT) powders, with a Zr:Ti ratio of 60:40, have been prepared from a solution of triethanolamine (TEA) and Ti4+, Zr4+, and Pb2+ ions. The metal ions were in solution through complex formation with TEA. The soluble metal-ion–TEA complex formed the precursor material when it was completely dehydrated. Heat treatment of the precursor at 450°C resulted in single-phase PZT powders. The precursor and the heat-treated powders have been characterized by using thermal analysis and X-ray diffractometry (XRD) studies. The average particle size, as measured from X-ray line broadening and transmission electron microscopy studies, was ∼20 nm. PZT powders modified with 3 mol% of lanthanum (PLZT) also were prepared through this route and were investigated via XRD studies. The dielectric constants of the PZT and PLZT powders were 12475 and 11262, and their corresponding Curie temperatures were 362° and 315°C, respectively.

Self-Assembly of {2}-Metallacryptands and {2}-Metallacryptates

Abstract: Reaction of H2L (1) with potassium or strontium hydride, or lanthanum(III) chloride, followed by iron(III) chloride, yielded the {2}-ironcryptates 2a–c. The mono-, di-, and trivalent guest cations are endohedrally encapsulated. In contrast, the dinuclear trispyridinium ironcryptand 2d was generated from the reaction of H2L (1) with only iron(III) chloride. The potassium metallacryptate 2a′ was formed from the triple-helicate 2d by addition of potassium carbonate. The new compounds 2b, 2c, and 2d were unequivocally characterised by X-ray diffraction analyses.

Phase formation and ferroelectricity of sol-gel derived (Pb, La)TiO3 thin films

Abstract: Ferroelectric lead lanthanum titanate (PLT) thin films with various lanthanum concentration were fabricated by sol-gel spin-on process onto Pt/Ti/SiO2/Si substrates. We investigated the crystal structure, microstructure, and dielectric and ferroelectric properties of PLT films according to lanthanum doping concentration. The films annealed at 600 °C for 30 min have the single perovskite phase having only tetragonal or pseudocubic structure. The film microstructures were changed according to the doping concentration of La. The PLT films showed the typical polarization-electric field hysteresis loops and capacitance-voltage characteristics representing the ferroelectric switching property. The coercive field and remanent polarization decreased due to the phase transformation from ferroelectric to paraelectric phase with increasing La concentration. The effective dielectric constant and the leakage current density increased according to the increase of La content.

Mechanical properties of calcium- and strontium-substituted lanthanum chromite

Abstract: Mechanical properties of acceptor (calcium and strontium)-substituted lanthanum chromites are reported as a function of composition, temperature and environment. The strength dependence on temperature for these perovskite conductors was found to depend on the acceptor type, with the calcium-substituted chromites showing a significant reduction in strength with increasing temperature, while the strength of strontium-substituted chromites was essentially invariant with temperature. The decrease in strength observed upon annealing in highly reducing environments was correlated to changes in lattice structure, stoichiometry and fracture morphology. A significant observation was the decrease in the cohesive strength of the grains relative to grain boundaries, beyond a critical oxygen vacancy concentration in the chromites. The structural changes in the chromite lattice upon reduction also resulted in decreased fracture toughness. © 1998 Chapman & Hall

Probing the malate hypothesis of differential aluminum tolerance in wheat by using other rhizotoxic ions as proxies for Al

Abstract: An Al-stimulated efflux of malate from the root apex has been proposed as the primary mechanism whereby some wheat (Triticum aestivum L.) genotypes demonstrate marked resistance to the rhixotoxic metal Al. Appealing in its simplicity, the model has not been unequivocally validated, and suffers from some significant discrepancies between estimated, steady-state concentrations of malate at the root surface and concentrations that are necessary to explain the resistance of the superior genotypes. Using two other rhizotoxic ions that are also chelated by malate, Cu(II) and La(III), we specifically probed whether the quantities of malate released by tolerant genotypes could effectively detoxify Al. Experiments with exogenous additions of malate to solution showed that ≥200 μM malate is required to account for the difference between Scout 66 (Al-sensitive) and Atlas 66 (Al-tolerant) wheats, and that this level of malate can also partially alleviate the toxicities of Cu and La. When simultaneously exposed to a mildly rhizotoxic level of Al (25 μM) to induce malate efflux, Atlas exhibited a pronounced reduction in sensitivity to Cu. When, La was used as the proxy ion, however, no such Al-induced tolerance to La was observed, a result that refutes the significance of malate efflux to Al tolerance. Additional experiments using Al, Cu, and La in combination suggested that a trivalent ion can alleviate Cu toxicity directly (i.e. via competition for apoplastic binding), providing an alternative explanation for the ability of Al to detoxify Cu in Atlas. Using a weight-of-evidence approach, we argue that malate efflux plays at most a minor role in the differential Al tolerance of wheat, and that a more integrative, multifaceted model of tolerance is needed.

preparation and characterization of lanthanum zirconate

Abstract: Lanthanum zirconate has been prepared by citrate synthesis, by coprecipitation and by solid state decomposition of metal nitrate–urea mixtures. The relative advantages of these methods with respect to surface area and reproducibility are investigated. The lanthanum zirconate phase is formed by the urea decomposition method without any additional heat treatment. In the case of the citrate and coprecipitation methods the pyrochlore phase is formed only after heating the precursor above 900 °C. The citrate method yields the highest surface area, the coprecipitation method gives more reproducible results. However, after washing with ethanol the coprecipitated sample has much higher surface area compared with the citrate method. © 1998 Kluwer Academic Publishers

Formation of nonlinear dielectric films for electrically tunable microwave devices

Abstract: A thin film structure including a lanthanum aluminum oxide substrate, a thin layer of homoepitaxial lanthanum aluminum oxide thereon, and a layer of a nonlinear dielectric material thereon the thin layer of homoepitaxial lanthanum aluminum oxide is provided together with microwave and electro-optical devices including such a thin film structure.

Bioaccumulation of lanthanum, uranium and thorium, and use of a model system to develop a method for the biologically‐mediated removal of plutonium from solution

Abstract: Removal of La 3+ , UO 2 2+ and Th 4+ from aqueous solution by a Citrobacter sp. was dependent on phosphatase-mediated phosphate release and the residence time in a plug-flow reactor (PFR) containing polyacrylamide gel-immobilized cells. In a stirred tank reactor (STR) lanthanum phosphate accumulated on the biomass rapidly, in preference to uranium or thorium phosphates. Thorium removal was not affected by the presence of uranium but was promoted in the presence of lanthanum. Analysis of the accumulated polycrystalline material by X-ray powder diffraction (XRD) analysis and proton induced X-ray emission (PIXE) suggested the formation of a mixed crystal of lanthanum and thorium phosphate. La 3+ , UO 2 2+ and Th 4+ are analogues of the corresponding species of Pu 3+ , PUO 2 2+ and Pu 4+ . The La/U/Th model system was used to identify some potential problems in the bioremediation of wastes containing plutonium and to develop a method for the biologically-mediated removal of plutonium from solution, in a test solution of 2 39 Pu 'spiked' with a 241 Pu tracer.

Study of Transition Metal Oxide Doped LaGaO 3 as Electrode Materials for LSGM- Based Solid Oxide Fuel Cells

Abstract: Transition metal oxide doped lanthanum gallates, La0.9Sr0.1Ga0.8M0.2O3 (where M=Co, Mn, Cr, Fe, or V), are studied as mixed ionic-electronic conductors (MIECs) for electrode applications. The electrochemical properties of these materials in air and in H2 are characterized using impedance spectroscopy, open cell voltage measurement, and gas permeation measurement. Three single cells based on La0.9Sr0.1Ga0.8 Mg0.2O3 (LSGM) electrolyte (1.13 to 1.65 mm thick) but with different electrode materials are studied under identical conditions to characterize the effectiveness of the lanthanum gallate-based MIECs for electrode applications. At 800 °C, a single cell using La0.9Sr0.1- Ga0.8Co0.2O3 as the cathode and La0.9Sr0.1Ga0.8Mn0.2O3 as the anode shows a maximum power density of 88 mW/cm2, which is better than that of a cell using Pt as both electrodes (20 mW/cm2) and that of a cell using La0.6Sr0.4CoO3 (LSC) as the cathode and CeO2-Ni as the anode (61 mW/cm2) under identical conditions. The performance of LSGM-based fuel cells with MIEC electrodes may be further improved by reducing the electrolyte thickness and by optimizing the microstructures of the electrodes through processing.

Metal compounds, mixed or sulphated, as phosphate binders

Abstract: A mixed metal compound for pharmaceutical use is free from aluminium and has a phosphate binding capacity of at least 30 %, by weight of the total weight of phosphate present, over a pH range of from 2-8. The compound is especially useful for treatment of hyperphosphataemia. The metals are preferably iron (III) and at least one of calcium, magnesium, lanthanum and cerium. A metal sulphate for pharmaceutical use is selected from at least one of calcium, lanthanum and cerium sulphate compounds and has a phosphate binding capacity of at least 30 % by weight of the total phosphate present, over a pH range from 2-8.

Chemistry-crystallization-microstructure relations of sol-gel derived lanthanum modified lead titanate thin films

Abstract: Lanthanum modified lead titanate thin films have been obtained by the deposition of sol-gel solutions onto platinized (100) silicon substrates. Crystallization of perovskite films was achieved by thermal treatments at 650°C with slow or rapid heatings. Lead oxide excesses were used in the precursor solutions to counterbalance the lead losses produced during the thermal treatment. Rapid heatings and large excesses of lead produce a preferred orientation of the films. These films have more homogeneous and denser microstructures than slow heated films without lead excess.

Process for the manufacture of 1,1,1,3,3-pentafluoropropane

Abstract: A process is disclosed for producing the hydrofluorocarbon CF3CH2CHF2. The process involves dehydrofluorinating CF3CH2CF3 at an elevated temperature in the vapor phase over a catalyst of (1) aluminum fluoride, (2) fluorided alumina, (3) metal supported on a trivalent aluminum compound containing fluoride anion, (4) lanthanum fluoride, (5) fluorided lanthanum oxide, (6) metal supported on a trivalent lanthanum compound containing fluoride anion, (7) trivalent chromium compounds and/or (8) catalysts of (a) at least one compound selected from the oxides, fluorides and oxyfluorides of magnesium, zinc and mixtures of magnesium and zinc, and optionally (b) at least one compound selected from the oxides, fluorides and oxyfluorides of aluminum, provided that the atomic ratio of aluminum to the total of magnesium and zinc in said catalyst is about 1:4, or less, to produce a product containing CF3CH=CF2 and HF; and reacting the CF3CH=CF2 in the vapor phase with hydrogen over a hydrogenation catalyst in the presence of HF to produce CF3CH2CHF2.

Characterization of zirconium, lanthanum and lead oxide deposits prepared by cathodic electrosynthesis

Abstract: Cathodic electrosynthesis of zirconium, lanthanum and lead oxides was performed from aqueous solutions of ZrOCl2 · 8H2O, La(NO3)3 · 6H2O and Pb(NO3)2, respectively. The deposits were characterized by X-ray diffraction, thermogravimetric and differential thermal analyses. Crystallite sizes of zirconia were derived at different temperatures from X-ray broadening data. The influence of hydrogen peroxide on the electrosynthesis process, crystallization and phase evolution of deposits has been studied. A possible mechanism of electrosynthesis and the role of hydrogen peroxide are discussed.

Measurement of the electron affinity of lanthanum

Abstract: The electron affinity of lanthanum has been measured using laser photoelectron energy spectroscopy. This is the first electron affinity measurement for lanthanum and one of the first measurements of an electron affinity of a rare-earth series element. The electron affinity of lanthanum was measured to be . At least one bound excited state of was also observed in the photoelectron spectra, and the binding energy relative to the ground state of lanthanum was measured as . The present experimental measurements are compared to a recent calculation.

Synthesis and Properties of Lanthanum– Pyrene Complexes—Structure of [(Cp*La)3(μ‐Cl)3(thf)(μ‐η2:η6:η6‐C16H10)], the First Complex with a Pyrene Trianion

Abstract: Different bonding modes are characteristic for the lanthanum centers of the title compound, a trinuclear lanthanum-pyrene complex in which an arene trianion is present for the first time (see picture for the structure). Thus, La1 and La3 reside in a tetrahedral environment, the La2 center in a distorted trigonal-bipyramidal one.

Synthesis of (Pb,La) (Zr,Ti)O3 films using a diol based sol–gel route

Abstract: Morphotropic phase boundary compositions of lead zirconate titanate (PZT) modified with 2, 5 and 10 mol% lanthanum (PLZT) have been prepared using a diol based sol–gel route. Thin films of these PLZT compositions were fabricated on platinized silicon substrates by a spin coating technique. The effects of firing temperature and lanthanum modifications were investigated with regard to phase development, microstructure, and ferroelectric and dielectric characteristics. A strong 〈1 1 1〉 orientation developed as the amount of lanthanum doping increased. The results indicate that the values of remanent polarization, Pr, and dielectric constant, er, decrease, relative to unmodified PZT, for films modified with 2 and 5 mol% lanthanum. The 5 mol% La films for example had a Pr of 14 μC cm−2 and an er value of 700 compared to 31 μC cm−2 and 1480 for undoped PZT films. At these La concentrations there was also an improvement in the leakage current density by two orders of magnitude compared with unmodified PZT. The 10 mol% La sample did not exhibit any switchable polarization behaviour.