Showing papers on "Lanthanum published in 2001"

Formation of a stratified lanthanum silicate dielectric by reaction with Si(001)

Abstract: We have characterized the structure and electrical properties of lanthanum silicate layers formed on Si(001) by reaction of lanthanum oxide with the substrate. Postoxidation of the deposited films results in the formation of a stacked dielectric with a lanthanum silicate layer atop an interfacial layer of SiO2. This structure combines the interfacial properties of SiO2 with the large permittivity of lanthanum silicate. Although the resulting film has leakage properties far superior to an equivalent thickness of SiO2, there is evidence of significant quantities of ionic charge that must be eliminated before use in electronic applications.

Structural and transport characteristics ofthe LAMOX family of fast oxide-ion conductors, based on lanthanum molybdenumoxide La2Mo2O9

Abstract: A new family of fast O2− conductors, which exhibit anionic conductivity comparable to that of stabilised zirconia, is presented. The parent compound of this new family, hereafter called LAMOX, is lanthanum molybdate La2Mo2O9. Various substitutions have been attempted: on the lanthanum site (La2 − xAx)Mo2O9 with A = Sr, Ba, K, or Bi; on the molybdenum site La2(Mo2 − xBx)O9 with B = Re, S, W, Cr and V; and on the oxygen site with fluorine. Most of these substitutions suppress the phase transition which occurs in La2Mo2O9 around 580 °C from a low temperature α form to a high temperature (more conducting) β form, and stabilise the β form at room temperature. Several members of the LAMOX series are studied through X-ray and neutron diffraction, and conductivity measurements. Large O2− thermal factors and local static disorder agree well with the anionic nature of the conductivity. Partly vacant sites with short inter-site distances suggest a most probable conduction path with tridimensional character.

Magnetic susceptibility tensor anisotropies for a lanthanide ion series in a fixed protein matrix.

Abstract: The full series of lanthanide ions (except the radioactive promethium and the S-state gadolinium) has been incorporated into the C-terminal calcium binding site of the dicalcium protein calbindin D9k. A fairly constant coordination environment is maintained throughout the series. At variance with several lanthanide complexes with small chelating ligands investigated in the past, the large protein moiety provides a large number of NMR signals whose hyperfine shifts can be exclusively ascribed to pseudocontact shifts (PCS). The chemical shifts of 1H and 15N backbone and side chain amide NH groups were accurately measured through HSQC experiments. 1097 PCS were estimated from these by subtracting the diamagnetic contributions measured on HSQC spectra of either the 4f0 lanthanum(III) or the 4f14 lutetium(III) derivatives and used to define a quality factor for the structure. The differences in diamagnetic chemical shifts between the two diamagnetic blanks were relatively small, although some were not negligib...

Mono-guanidinate complexes of lanthanum: synthesis, structure and their use in lactide polymerization

Abstract: La[N(SiMe3)2]3 reacted with 1,3-dicyclohexylcarbodiimide in refluxing toluene to yield the mono-guanidinate complex La[CyNC(N(SiMe3)2)NCy](N(SiMe3)2)21. Compound 1 is monomeric in solution; X-ray structural analysis reveals an unassociated complex in the solid state, with a four-coordinate lanthanum center. Complex 1 reacts with 2,6-di-tert-butylphenol (2 equivalents) in cold pentane to yield the bis(phenoxide) complex La[CyNC(N(SiMe3)2)NCy](OC6H3tBu2-2,6)2. X-Ray analysis indicates a similar structure to that of 1, with a four-coordinate lanthanum center coordinated by a single guanidinate ligand and two phenoxide groups. This compound catalyses the ring-opening polymerization of D,L-lactide to polylactide. Although high molecular weight polymer is obtained, polydispersities are broad and no control over the stereochemistry of the polymer is observed.

Interplay of structure and transport properties of sodium-doped lanthanum manganite

Abstract: The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La1-xNaxMnO3 (0.07≤x≤0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07≤x≤0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La1-xNaxMnO3 compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO6 octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT2 behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

High temperature thermal stabilization of alumina modified by lanthanum species

Abstract: The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET), X-ray powder diffraction (XRD), N2 adsorption–desorption isotherms, thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600–1150°C. Although powder La2O3, which is mechanically mixed with γ-Al2O3 or pseudo boehmite, can effectively retard the α phase transformation by solid phase interaction with Al2O3, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of γ-Al2O3, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600°C and for atomic ratio of La/Al up to 0.1 or at 1150°C and La/Al≤0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La2O3, LaAlO3 and crystalline La2O3. At T≤1000°C the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000°C results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO3 mainly retards the surface area loss resulting from the α phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from α phase transformation at high temperature ≥1000°C has been proposed in this paper.

A New Lanthanum and Calcium Borate La2CaB10O19

Abstract: A new lanthanum calcium borate with the composition La2CaB10O19 has been synthesized. Single crystals have been grown from a melt close to the stoichiometric composition. The compound crystallizes in the monoclinic system, space group C2, with a = 11.043(3) A, b = 6.563(2) A, c = 9.129(2) A, α = γ = 90°, β = 91.47°, and two formula units per cell. The crystal structure contains B5O12 double-ring pentaborate groups, which are linked together to form an infinite two-dimensional double layer. The layer runs almost perpendicular to the c axis of the crystal. The La atoms are located in layers, while the Ca atoms are located between two layers. La2CaB10O19 exhibits an optical second-harmonic generation effect about twice as large as that of KDP (KH2PO4).

The red–ox treatments influence on the structure and properties of M2O3–CeO2–ZrO2 (M=Y, La) solid solutions

Abstract: The introduction of trivalent cation — Y3+ or La3+ — into the lattice of CeO2–ZrO2 solid solutions allows to stabilise a cubic structure at low ceria content (30 mol%). The reducibility of the samples has been compared in the experiments of temperature-programmed reduction (TPR). The introduction of lanthanum cations decreases the amount of hydrogen consumed during TPR, while the introduction of yttrium ones increases this value. At the same time, the value of temperature of the maximum speed of reduction (Tmax) is independent on the trivalent dopant. The reducibility of these solid solutions did not change during repeated red–ox treatments at temperature below 1220 K. It is connected with the high thermostability of all systems in this temperature interval. TPR up to 1470 K causes a significant shift of Tmax value to higher temperature and a slight decrease of hydrogen consumption in two to three cycles. It is suggested that this alterations are connected with the sharp decrease of the specific surface area of all samples and partially phase decomposition of CeO2–ZrO2 and Y2O3–CeO2–ZrO2 solid solutions. Raman characterisation of the oxygen sublattice of the fresh samples and of the samples after TPR has been carried out.

Heterocyclopentadienyl Complexes of Group‐3 Metals

Abstract: Heterocyclopentadienyl complexes of group-3 metals (scandium, yttrium, lanthanum and the lanthanides, and uranium) are compounds in which one or more −CH units of a cyclopentadienyl-like ligand have been replaced by a heteroelement (nitrogen, phosphorus, arsenic, or antimony). These ligands can have very diverse substitution patterns, notably with bridged and cavitand-like structures. In addition to the classical η5-coordination behaviour, the heterocyclopentadienyl ligand can adopt a very large array of coordination patterns. Some complexes have a very promising chemistry since they have been found to activate small molecules such as nitrogen and ethylene.

Studies on the structural, microstructural and optical properties of sol-gel derived lead lanthanum titanate thin films

Abstract: The La modified lead titanate Pb 1− x La x Ti 1− x /4 O 3 (PLT) ( x =0.0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) thin films were deposited on sapphire (0001) substrates by the sol–gel process. The X-ray diffraction data and micro-Raman analysis show that with the increase in La content the crystal quality of the PLT films undergoes a tetragonal-to-cubic transformation. X-ray photoelectron spectroscopy analysis shows an excellent surface stoichiometry for all compositions under study. The optical properties of these films were investigated using both transmission and reflection spectra in the wavelength range of 200–900 nm. The appearance of interference fringes is an indication of the thickness uniformity of the film. The low value of extinction coefficient (in the order of 10 −2 ) as observed in our films is a qualitative indication of excellent surface smoothness of the films. Absorption coefficient ( α ) and the band-gap energy ( E g ) are obtained for undoped and La doped films with varying La concentrations. It has been found that the refractive index and packing fraction values decrease with La doping. Lanthanum doping was found to decrease the grain size of the films and improve the densification of individual grains. Increased La content lead to clusterification of smaller grains. The observed variation of band-gap energy with La doping has been correlated with the observed microstructure of these films.

Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

Abstract: The thermal expansion of La0.9Sr0.1Cr1-xMxO3(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤x≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La0.9Sr0.1CrO3was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La0.9Sr0.1Cr1-xMxO3 perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La0.9Sr0.1Cr0.96Co0.02Al0.02O3 were 10.5 × 10−6/°C in air, 10.7 × 10−6/°C under He–H2 atmosphere (oxygen partial pressure of 4 × 10−15 atm at 1000°C), and 11.8 × 10−6/°C in H2 atmosphere.

Distribution, speciation, and bioavailability of lanthanides in the Rhine-Meuse estuary, The Netherlands

Abstract: Changing environmental conditions may influence the fate and bioavailability of lanthanides (part of the rare earth elements [Ln]) in estuaries. The aim of this study was to quantify the variation in estuarine lanthanide solid/water distribution, speciation, and bioaccumulation. The latter was studied in the amphipod Corophium volutator under field and laboratory conditions. Calculations with the chemical equilibrium model MINEQL+ indicate that dissolved lanthanides are complexed mainly to carbonates and dissolved organic matter. In the water phase, the relative abundance of the free ion, LnCO3, and humic complexes decreases from lanthanum to lutetium, whereas the relative abundance of Ln(CO3)2 increases. Cerium and europium anomalies were found in the water. Europium anomalies were also found in some biota. The biota sediment accumulation factors (BSAFs) decreased across the series from lanthanum to lutetium. Regression analysis revealed that alkalinity correlated negatively with lanthanide uptake. This suggests that increasing complexation reduced bioavailability under the prevailing conditions. The BSAFs did not depend on salinity or pH, which may simplify sediment-quality criteria for fresh versus saline waters. Field BSAFs were significantly lower than laboratory values for the same sediments, which is explained by adaptation of the organisms to lanthanides.

Elaboration and characterization of tin oxide–lanthanum oxide mixed layers prepared by the electrostatic spray pyrolysis technique

Abstract: Lanthanum oxide–tin oxide mixed layer was found to show promising sensing properties to CO 2 in the air. In this paper, we describe the characterization of such layers fabricated by the electrostatic spray pyrolysis technique. Analysis by scanning electron microscopy (SEM) and atomic force microscopy (AFM) show a structure of grain aggregates, while energy dispersion analysis reveal that the original and final compositions are very close with an uniformly distribution in the layer. X-ray diffraction gives the nature of the deposited oxide, which is LaOCl and SnO 2 . Electrical characterization versus temperature and concentration reveal a sensitivity peak for the 2% lanthanum oxide mixed layer at about 425°C. A variable frequency (13 MHz–500 Hz) ac voltage was applied to all the sensor structures to perform the complex impedance spectrum measurements, at a temperature range 350–550°C, and a CO 2 concentration range 400–2000 ppm in synthetic air. For all these measurements, we obtain the same equivalent circuit, which fits very well the experimental data, and allows us to propose a conduction model.

Physicochemical Properties of Lanthanide and Yttrium Solutions in Fused Salts and Alloy Formation with Nickel

Abstract: The thermodynamic and kinetic properties of solutions of lanthanide (lanthanum, cerium, praseodymium, gadolinium) and yttrium chlorides in fused alkali chlorides were obtained from transient electrochemical techniques. Digital simulation was used to obtain a precise analysis of transient responses including perturbing factors such as ohmic drop, electrocrystallization, and reduction of alkali ions. A fitting procedure, included in the numerical calculation, allows an accurate determination of reaction parameters, diffusion coefficients, standard potentials, and covering factor. The technique was extended to the formation of lanthanum and gadolinium alloys with nickel.

Synthesis of New Diaryl-Substituted Triple-Decker and Tetraaryl-substituted Double-Decker Lanthanum(III) Porphyrins and Their Porphyrin Ring Rotational Speed as Compared with that of Double-Decker Cerium(IV) Porphyrins

Abstract: Tetraaryl-substituted cerium(IV) double-decker porphyrin (2D), tetraaryl-substituted lanthanum(III) double-decker porphyrin (3D), and diaryl-substituted lanthanum(III) triple-decker porphyrins (4T•Me and 4T•MeO) were newly synthesized and their porphyrin ring rotation rates were systematically estimated by means of a VT NMR spectroscopic method. In 2D the coalescence temperature (Tc) for the porphyrin ring rotation was higher than 110 °C, whereas in 3D it appeared at ca. 0 °C. In diaryl-substituted cerium(IV) double-decker porphyrin (5D) it appeared at 13 °C whereas in 4T•Me and 4T•MeO they were lower than −80 °C. These results consistently support the view that the porphyrin ring rotation rates in lanthanum(III)-based porphyrins are much faster than those in cerium(IV)-based porphyrins. The difference is reasonably explained by the difference in the ion size between these two metal ions. Since these sandwich-type porphyrins can act as novel scaffolds for designing positive allosteric recognition systems,...