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991.
Iván da Silva Cristina González-Silgo Juan Rodríguez-Carvajal Lourdes Mestres 《Journal of solid state chemistry》2005,178(5):1601-1608
The crystal structure of potassium, rubidium and caesium fluoroberyllates have been re-examined by neutron powder diffraction at room temperature and at 1.5 K. Previously, their structures, obtained from X-ray data, were described in the Pn21a space group. However, the results obtained from Rietveld refinements, using powder neutron diffraction, at both temperatures, indicated that all structures are orthorhombic with space group Pnma. The known phase transition at high temperature is probably related to the appearance of a hexagonal pseudo-symmetry instead of the elimination of the mirror plane between the above mentioned orthorhombic space groups. A possible phase transition, at very low temperature, was discarded considering the stereochemical criteria concerning the structural stability of A2BX4 compounds. This was confirmed by thermal analysis. On the other hand, a modulated background has been detected in all samples during the refinements. This is compatible with the presence of an amorphous phase, coexisting with the crystalline phase, or with a disordered component within the main crystalline phase. Instead of using a polynomial function, the background was modelled by Fourier filtering improving the fit for all patterns. The radial distribution function (RDF) was obtained from the analysis of the calculated background and compared with the RDF from the average crystal structure. The advantages of neutron with respect to X-ray diffraction were evidenced for this type of compound with β-K2SO4-type structure. 相似文献
992.
Anne M. Garrido Pedrosa M. J. B. Souza A. O. S. Silva Dulce M. A. Melo A. S. Araujo 《Journal of Thermal Analysis and Calorimetry》2006,84(2):503-509
The study of the incorporation
of rare earth elements as additives in Y zeolites is a very interesting field
of research, mainly by its potential application as additives in catalytic
cracking process. In this work was studied the thermal and structural properties
of cerium, holmium and samarium supported on HZSM-12 zeolite. The obtained
materials were characterized by X-ray diffraction (XRD), infrared spectroscopy
(FTIR), nitrogen adsorption, thermogravimetry (TG/DTG), differential scanning
calorimetry (DSC) and differential thermal analysis (DTA). TG/DSC/DTA analyses
showed that the dehydration temperatures of RE/HZSM-12 zeolites (RE=Ce, Ho, Sm) increase in relation to pure HZSM-12.
The acid properties were investigated by pyridine thermo desorption via TG.
The results showed two events of mass loss attributed to elimination of pyridine
adsorbed on the weak+medium acid sites and on the strong acid sites. 相似文献
993.
994.
J. L. Gautier E. Meza E. Silva C. Lamas C. Silva 《Journal of Solid State Electrochemistry》1997,1(2):126-133
The electrochemical insertion of lithium in the spinel-type manganite with the formula ZnNi
y
Mn2–
y
O4 has been studied. The galvanostatic discharge curves show that the best performance is obtained for y = 0.25, where a tetragonal to cubic structural transformation occurs. The thermodynamics and kinetics of the process of insertion
of the lithium into the tetragonal spinel Li
x
ZnNi0.25Mn1.75O4 (x = 0.05–1.3) have been studied. The molar thermodynamic quantities, such as enthalpy, entropy and free energy determined by
EMF-T measurements, varied with the lithium concentration in the oxide structure, and a major variation was observed around
x = 0.8. The chemical diffusion coefficient of lithium in these spinels was also determined. Structural analysis, degree of
oxidation and magnetic susceptibility measurements were carried out for the lithiated oxides in order to obtain the cationic
distribution as a function of x. It has been possible to demonstrate that, upon lithium insertion, Mn4+ ions on B sites are reduced to Mn3+ and then to Mn2+. A cooperative Jahn-Teller effect is present in these spinel manganese-nickel oxides.
Received: 4 February 1997 / Accepted: 11 April 1997 相似文献
995.
996.
997.
998.
José C.J.M.D.S. Menezes Ana M.V.M. Pereira Maria G.P.M.S. Neves Artur M.S. Silva Sérgio M. Santos Sabrina T. Martinez Bárbara V. Silva Ângelo C. Pinto José A.S. Cavaleiro 《Tetrahedron》2012,68(39):8330-8339
New porphyrin indolin-2-one conjugates were synthesized via palladium-catalyzed amination reactions of iodinated and dibrominated indolin-2-one derivatives with (2-amino-5,10,15,20-tetraphenylporphyrinato)nickel(II). The combination of palladium catalysts and the phosphine ligand dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos) is an effective methodology for catalyzing the coupling of 5-iodo-, 5,7-dibromo- and 4,6-dibromo-1,3,3-trimethylindolin-2-one with 2-aminoporphyrin to give the corresponding mono-(2-aminoporphyrinyl)- and di-(2-aminoporphyrinyl)-substituted indolin-2-ones in satisfactory yields under mild conditions. The mono brominated porphyrinic derivatives also underwent cross-coupling reactions under similar catalytic conditions. The studies also demonstrated that the course of the coupling process depends on factors, such as the catalytic system, number and position of the halogen substituents and the heating condition. Insights into the reactivity trends of the 5-iodo; 4,6- and 5,7-dibrominated indolin-2-one derivatives was carried out using theoretical calculations performed using density-functional theory with the B3LYP functional. 相似文献
999.
C.M. da Silva F.P. Livi E. Panarotto A.A. Gomes 《Journal of magnetism and magnetic materials》1987,70(1-3):52-54
An extension of the usual tight binding CPA to include local d-p hybridization induced by sp impurities (Al or Si) is proposed. Coupled self-consistency equations for the d and p effective medium self-energies are established. An application to Zr(Fe1−xAlx)2 pseudobinary is presented. 相似文献
1000.
K. Goeke J. Ossmann P. Schweitzer A. Silva 《The European Physical Journal A - Hadrons and Nuclei》2006,27(1):77-90
The dependence of the nucleon mass on the mass of the pion is studied in the framework of the chiral quark-soliton model.
A remarkable agreement is observed with lattice data from recent full dynamical simulations. The possibility and limitations
to use the results from the chiral quark soliton model as a guideline for the chiral extrapolation of lattice data are discussed. 相似文献