Experimental and theoretical study on the structure, optical properties, and growth of metallic silver nanostructures in Ag3PO4

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

A relationship between structural and electronic order-disorder effects and optical properties in crystalline TiO2 nanomaterials

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural characterization of Li-MN doped powders prepared by Pechini's method

Pechini's method has been successfully used to prepare Li-doped MgNb2O6(MN) at short time and low temperature. It consists in the preparation of metal citrate solution, which is polymerized at 250°C to form a high viscous resin. This resin was burned in a box type furnace at 400°C/2h and ground in a mortar. Successive steps of calcination up to 900°C were used to form a crystalline precursor. SEM, DTA and XRD were used to characterize the powders. MN precursor powders containing from 0.1 to 5.0 mol% of LiNbO3 additive was prepared aiming better dielectric properties and microstructural characteristics of the PMN prepared from columbite route. SEM analysis showed that particles increased by sintering, forming large agglomerates. The surface area is also substantially reduced with the increase in additive amount above 1.0 mol%. In XRD pattern of the precursor material with 5.0 mol% of additive was observed the LiNbO3 phase of trigonal structure. XRD data were used for Rietveld refinement and a decrease in microstrain and pronounced increase in crystallite size with the increase of LiNbO3 were observed. It is in agreement with the particle morphologies observed by SEM analysis.

Modeling the crystallographic structure of Ho(Ni,Co,Mn)O3±δ perovskite-type manganite

Crystallographic and microstructural properties of Ho(Ni,Co,Mn)O3± perovskite-type multiferroic material are reported. Samples were synthesized with a modified polymeric precursor method. The synchrotron X-ray powder diffraction (SXRPD) technique associated to Rietveld refinement method was used to perform structural characterization. The crystallographic structures, as well as microstructural properties, were studied to determine unit cell parameters and volume, angles and atomic positions, crystallite size and strain. X-ray energies below the absorption edges of the transition metals helped to determine the mean preferred atomic occupancy for the substituent atoms. Furthermore, analyzing the degree of distortion of the polyhedra centered at the transitions metal atoms led to understanding the structural model of the synthesized phase. X-ray photoelectron spectroscopy (XPS) was performed to evaluate the valence states of the elements, and the tolerance factor and oxygen content. The obtained results indicated a small decrease distortion in structure, close to the HoMnO3 basis compound. In addition, the substituent atoms showed the same distribution and, on average, preferentially occupied the center of the unit cell.

Estudo da relação de ordem-desordem estrutural em nanopartículas de TiO2

Pós-graduação em Química - IQ

Caracterização estrutural da cerâmica Ti x(Sm0,2Ce0,8)1-xO 2-delta pelo método de Rietveld

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Quantitative structural analysis of the transition from LT-LixCoO2 to HT-LixCoO2 using the rietveld method: correlation between structure and electrochemical performance

A quantitative phase analysis was made of LiXCoO2 powders obtained by two distinct chemical methodologies at different temperatures (from 400 to 700degreesC). A phase analysis was made using Rietveld refinements based on X-ray diffraction data, considering the LiXCoO2 powders as a multiphase system that simultaneously contained two main phases with distinct, layered and spinel-type structures. The sults showed the coexistence of both structures in LiXCoO2 obtained at low temperature (400 and 500degreesC), although only the layered structure was detected at higher temperatures (600 and 700degreesC, regardless of the chemical powder process employed. The electrochemical performance, evaluated mainly by the cycling reversibility of LiXCoO2 in the form of cathode insertion electrodes, revealed that there is a close correlation between structural features and the electrochemical response, with one of the redox processes (3.3 v/3.9 v) associated only with the presence of the spinel-type structure. (C) 2003 Elsevier B.V. All rights reserved.

Rietveld refinements of SnO(2) ceramic powders doped with ZnO, WO(3), CoO, Nb(2)O(5) and MoO(3)

Polycrystalline materials of SnO(2) doped with ZnO, WO(3), CoO, Nb(2)O(5), and MoO(3). were synthesized by solid state reaction. X-ray powder diffraction data were collected with Cu K(alpha) radiation from a Rigaku-Rint 2000 rotating anode source. The structural and profile parameters were refined by the Rietveld method using GSAS [2]. The obtained residual parameters are R(wp) = 11,93% and R(Bragg) = 4,19%. The refined profile parameters indicate no anisotropic crystallite microstrain. The refinement results and Fourier differences calculations indicate that the dopants do not occupy interstitial sites in the crystal structure of SnO(2).

Crystal structure refinement of Co-doped lanthanum chromites

Results of crystal structure refinements and phase quantification for samples of Co-doped lanthanum chromites with nominal composition LaCr(1-x)Co(x)O(3), for x=0.00, 0.10, 0.20, and 0.30, prepared by combustion synthesis are presented. The resulting powders were characterized by scanning electron microscopy and X-ray diffraction (XRD). The XRD patterns were obtained with Cu K alpha radiation for non-doped lanthanum chromite sample and additionally with Cr K alpha radiation for Co-doped lanthanum chromites samples, in order to enhance the signal from scattering. Rietveld analysis of XRD data showed that the studied samples presented the lanthanum chromite with an orthorhombic structure (Pnma), except for the composition with x=0.30, in which the space group was found to be R (3) over barc. (C) 2008 International Centre for Diffraction Data.

Structural refinement, growth process, photoluminescence and photocatalytic properties of (Ba1-xPr2x/3)WO4 crystals synthesized by the coprecipitation method

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Quantitative structural analysis of the transition from LT-Li xCoO2 to HT-LixCoO2 using the rietveld method: Correlation between structure and electrochemical performance

A quantitative phase analysis was made of LixCoO2 powders obtained by two distinct chemical methodologies at different temperatures (from 400 to 700°C). A phase analysis was made using Rietveld refinements based on X-ray diffraction data, considering the Li xCoO2 powders as a multiphase system that simultaneously contained two main phases with distinct, layered and spinel-type structures. The results showed the coexistence of both structures in LixCoO 2 obtained at low temperature (400 and 500°C), although only the layered structure was detected at higher temperatures (600 and 700°C), regardless of the chemical powder process employed. The electrochemical performance, evaluated mainly by the cycling reversibility of Li xCoO2 in the form of cathode insertion electrodes, revealed that there is a close correlation between structural features and the electrochemical response, with one of the redox processes (3.3 v/3.9 v) associated only with the presence of the spinel-type structure. © 2003 Elsevier B.V. All rights reserved.

Aplicação da difração de raios X por policristais e do método de Rietveld de refinamento de estruturas cristalinas no estudo de polimorfos cristalinos de fármacos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Structural characterization and Curie temperature determination of a sodium strontium niobate ferroelectric nanostructured powder

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Síntese e caracterização estrutural do niobato de potássio e estrôncio com estrutura tipo Tetragonal Tungstênio Bronze TTB

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural and electrochemical properties of LiCoO2 prepared by combustion synthesis

LiCoO2 powders were prepared by combustion synthesis, using metallic nitrates as the oxidant and metal sources and urea as fuel. A small amount of the LiCoO2 phase was obtained directly from the combustion reaction, however, a heat treatment was necessary for the phase crystallization. The heat treatment was performed at the temperature range from 400 up to 700 degreesC for 12 h. The powders were characterized by X-ray diffraction (XRD), X ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and specific surface area values were obtained by BET isotherms. Composite electrodes were prepared using a mixture of LiCoO2, carbon black and poly(vinylidene fluoride) (PVDF) in the 85:10:5% w/w ratio. The electrochemical behavior of these composites was evaluated in ethylene carbonate/dimethylcarbonate solution, using lithium perchlorate as supporting electrolyte. Cyclic voltammograms showed one reversible redox process at 4.0/3.85 V and one irreversible redox process at 3.3 V for the LiCoO2 obtained after a post-heat treatment at 400 and 500 degreesC.Raman spectroscopy showed the possible presence of LiCoO2 with cubic structure for the material obtained at 400 and 500 degreesC. This result is in agreement with X-ray data with structural refinement for the LiCoO2 powders obtained at different temperatures using the Rietveld method. Data from this method showed the coexistence of cubic LiCoO2 (spinel) and rhombohedral (layered) structures when LiCoO2 was obtained at lower temperatures (400 and 500 degreesC). The single rhombohedral structure for LiCoO2 was obtained after post-heat treatment at 600 degreesC. The maximum energy capacity in the first discharge was 136 mA g(-1) for the composite electrode based on LiCoO2 obtained after heat treatment at 700 degreesC. (C) 2002 Elsevier B.V. B.V. All rights reserved.