Xeractinol - A new flavanonol C-glucoside from Paepalanthus argenteus var. argenteus (Bongard) Hensold (Eriocaulaceae)

New compound isolated from methanolic extract from the leaves of Paepalanthus argenteus var. argenteus (Bongard) Hensold was characterized as xeractinol, a new dihydroflavonol C-glucoside. The structure was elucidated on the basis of extensive spectroscopic analysis (1D and 2D NMR, MS, HREIMS, IR and UV). Ab initio electronic structure calculations support our proposal to the molecular structure. The dihydroflavonol herein isolated may serve as taxonomic marker of Paepalanthus subgenus Xeractis, because this flavonoid have not been reported in any other taxon of Eriocaulaceae.

Conditions giving rise to intense visible room temperature photoluminescence in SrWO4 thin films: the role of disorder

The nature of intense visible photoluminescence at room temperature of SrWO4 (SWO) non-crystalline thin films is discussed in the light of experimental results and theoretical calculations. The SWO thin films were synthesized by the polymeric precursors method. Their structural properties have been obtained by X-ray diffraction data and the corresponding photoluminescence (PL) spectra have been measured. The UV-vis optical spectra measurements suggest the creation of localized states in the disordered structure. The photoluminescence measurements reveal that the PL changes with the degree of disorder in the SWO thin film. To understand the origin of visible PL at room temperature in disordered SWO, we performed quantum-mechanical calculations on crystalline and disordered SWO periodic models. Their electronic structures are analyzed in terms of DOS, hand dispersion and charge densities. We used DFT method with the hybrid non-local B3LYP approximation. The polarization induced by the symmetry break and the existence of localized levels favors the creation of trapped holes and electrons, giving origin to the room temperature photoluminescence phenomenon in the SWO thin films. (c) 2004 Elsevier B.V. All rights reserved.

On the stabilizing behavior of zirconia: A combined experimental and theoretical study

Reactive zirconia powder was synthesized by the complexation of zirconium metal from zirconium hydroxide using a solution of 8-hydroxiquinoline. The kinetics of zirconia crystallization was followed by X-ray diffraction, scanning electron microscopy and surface area measured by the nitrogen adsorption/desorption technique. The results indicated that zirconia with a surface area as high as 100 m(2)/g can be obtained by this method after calcination at 500degreesC. Zirconia presents three polymorphic phases (monoclinic, tetragonal and cubic), which are reversibly interconversible. The cluster model Zr4O8 and Z(r)4O(7)(+2) was used for a theoretical study of the stabilization process. The ab initio RHF method was employed with the Gaussian94 program and the total energies and the energy gap of the different phases were calculated and compared with the experimental energy gap. The theoretical results show good reproducibility of the energy gap for zirconia. (C) 2004 Kluwer Academic Publishers.

Towards an insight on the photoluminescence of disordered CaWO4 from a joint experimental and theoretical analysis

A joint experimental and theoretical study has been carried out to rationalize for the first time the photoluminescence (PL) properties of disordered CaWO4 (CWO) thin films. From the experimental side, thin films of CWO have been synthesized following a soft chemical processing, their structure has been confirmed by X-ray diffraction data and corresponding PL properties have been measured using the 488 nm line of an argon ion laser. Although we observe PL at room temperature for the crystalline thin films, the structurally disordered samples present much more intense emission. From the theoretical side, first principles quantum mechanical calculations, based on density functional theory at B3LYP level, have been employed to study the electronic structure of a crystalline (CWO-c) and asymmetric (CWO-a) periodic model. Electronic properties are analyzed in the light of the experimental results and their relevance in relation to the PL behavior of CWO is discussed. The symmetry breaking process on going from CWO-c to CWO-a creates localized electronic levels above the valence band and a negative charge transfer process takes place from threefold, WO3, to fourfold, WO4,. tungsten coordinations. The correlation of both effects seems to be responsible for the PL of amorphous CWO. (c) 2005 Elsevier B.V. All rights reserved.

An investigation of metal oxides which are photoluminiscent at room temperature

Amorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trends. (C) 2004 Elsevier B.V. All rights reserved.

Jahn-Teller effect on the structure of the Sm-doped PbTiO3: A theoretical approach

Samarium doped PbTiO3 (PT:Sm) and pure PbTiO3 (PT) powders were obtained by polymeric precursor method. These powders were characterized by X-ray diffraction (XRD) and theoretical calculations using the CRYSTAL98 program. The effect of the samarium atom is taken into account only indirectly. The experimental models were compared with the cubic (ideal) and tetragonal theoretical models. The structure deformations existent in the experimental compounds were analyzed from the tiny structural differences that lead to perturbations in the crystal orbital splittings. This paper proposes an efficient alternative methodology for defining structural distortions. (c) 2007 Elsevier B.V. All rights reserved.

Experimental and theoretical aspects of the stabilization of zirconia.

Using the Rietveld method, phases of ceria-doped zirconia, calcined at temperatures of 600 and 900 degrees C, were quantitatively analysed for different concentrations of ceria. The results show that the stabilization of zirconia depends on the dopant concentration and calcination temperature. Moreover, the theoretical calculation using the ab initio Hartree-Fock-Roothaan method indicates that the most stable phases for ceria-stabilized zirconia are cubic or tetragonal, in accordance with experimental results. (C) 1999 Kluwer Academic Publishers.

Theoretical studies on the stability of N-methylformamide in both liquid and gas phases

Ab initio (restricted Hartree-Fock and DFT) and molecular mechanics calculations at MM2 level were performed for N-methylformamide (NMF) molecule and for three dimers in order to investigate the relative stability of the cis and trans conformers. The ab initio calculations show that no intramolecular interaction is relevant for the stability of the conformers explored. The trans conformer is the most stable. The MM calculations revealed that a double H-bonded cyclic cis-cis dimer is the most stable among the studied dimers, followed by a 'linear' H-bonded trans-trans dimer. This 'linear' dimer, however, is prevalent in the liquid phase. (c) 2006 Elsevier B.V. All rights reserved.

Phase separation suppression in InGaN epitaxial layers due to biaxial strain

Phase separation suppression due to external biaxial strain is observed in InxGa1-xN alloy layers by Raman scattering spectroscopy. The effect is taking place in thin epitaxial layers pseudomorphically grown by molecular-beam epitaxy on unstrained GaN(001) buffers. Ab initio calculations carried out for the alloy free energy predict and Raman measurements confirm that biaxial strain suppress the formation of phase-separated In-rich quantum dots in the InxGa1-xN layers. Since quantum dots are effective radiative recombination centers in InGaN, we conclude that strain quenches an important channel of light emission in optoelectronic devices based on pseudobinary group-III nitride semiconductors. (C) 2002 American Institute of Physics.

Structural deformation monitored by vibrational properties and orbital modeling in (Pb,Sm)TiO3 systems

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Shape controlled synthesis of CaMoO4 thin films and their photoluminescence property

CaMoO4 (CMO) disordered and ordered thin films were prepared by the complex polymerization method (CPM). The films were annealed at different temperatures and time in a conventional resistive furnace (RF) and in a microwave (MW) oven. The microstructure and surface morphology of the structure were monitored by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (HRSEM). Order and disorder were characterized by X-ray diffraction (XRD) and optical reflectance. A strong photoluminescence (PL) emission was observed in the disordered thin films and was attributed to complex cluster vacancies. The experimental results were compared with density functional and Hartree-Fock calculations. (C) 2008 Elsevier B.V. All rights reserved.

Intense and broad photoluminescence at room temperature in structurally disordered Ba[Zr0.25Ti0.75]O-3 powders: An experimental/theoretical correlation

Intense and broad photoluminescence (PL) emission at room temperature was observed on structurally disordered Ba[Zr0.25Ti0.75]O-3 (BZT) powders synthesized by the polymeric precursor method. BZT powders were annealed at 573 K for different times and at 973 K for 2 h in oxygen atmosphere. The single-phase cubic perovskite structure of the powder annealed at 973 K for 2 It was identified by X-ray diffraction and Fourier transform Raman techniques. PL emission increased with the increase of annealing time, which reached its maximum value in the powder annealed at 573 K for 192 h. First principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and disordered models. The theoretical calculations and experimental measurements of Ultraviolet-visible absorption spectroscopy indicate that the presence of intermediary energy levels in the band gap is favorable for the intense and broad PL emission at room temperature in disordered BZT powders. The PL behavior is probably due the existence of a charge gradient on the disordered structure, denoted by means of a charge transfer process from [TiO5]-[ZrO6] or [TiO6]-[ZrO5] clusters to [TiO6]-[ZrO6] clusters. (C) 2008 Elsevier Ltd. All rights reserved.

Single walled MgF2 nanotubes

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

First principles calculations on the origin of violet-blue and green light photoluminescence emission in SrZrO3 and SrTiO3 perovskites

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

Investigation on the structural properties in Er-doped PbTiO3 compounds: A correlation between experimental and theoretical results

The aim of this work is to investigate the structural properties of the (Pb1-xErx)TiO3 (PET) powders, with x varying from 0.01 to 0.08, prepared by the polymeric precursor method. Combined X-ray diffraction, Raman spectroscopy and ab initio calculation reveal a diffuse phase-transition of a tetragonal to a cubic phase. The crystalline models built allowed to calculate electronic properties and to analyze the behavior of the doping element in the structure of the material, which are consistent with the experimental results that indicate the beginning of phase-transition from tetragonal to cubic. (C) 2007 Elsevier B.V. All rights reserved.