Dielectric relaxation and relaxor behavior in bilayered perovskites

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

SrZrO3 powders obtained by chemical method: Synthesis, characterization and optical absorption behaviour

Strontium zirconate (SrZrO3) powders have been synthesized by the polymeric precursor method after heat treatment at different temperatures for 2 h in oxygen atmosphere. The decomposition of precursor powder was followed by thermogravimetric analysis, X-ray diffraction (XRD) and Fourier transform Raman (FT-Raman). The UV-vis absorption spectroscopy measurements suggested the presence of intermediary energy levels in the band gap of structurally disordered powders. XRD, Rietveld refinement and FT-Raman revealed that the powders are free of secondary phases and crystallizes in the orthorhombic structure. (C) 2007 Elsevier Masson SAS. All rights reserved.

Understanding the origin of photoluminescence in disordered Ca0.60Sr0.40WO4: An experimental and first-principles study

Visible photoluminescence (PL) was observed for the first time at room temperature in structurally disordered calcium strontium tungstate powder, Ca0.60Sr0.40WO4 (CSW), obtained by the polymeric precursor method. The PL behavior of CSW powders has been analyzed as a function of the disorder rate, based on experimental and theoretical studies. Quantum mechanical theory based on density functional theory at the B3LYP level has been employed to study the electronic structure of two periodic models representing both crystalline and disordered powders. Their electronic structures have been analyzed in terms of density of states, band dispersion and charge densities. The calculations indicate a break in symmetry when passing from crystalline to disordered models, creating localized electronic levels above the valence band. Moreover, a negative charge transfer process takes place from the [WO3] cluster to the [WO4] cluster. The polarization induced by the break in symmetry and the existence of localized levels favors the creation of trapped holes and electrons, originating the PL phenomenon. (c) 2007 Elsevier B.V. All rights reserved.

Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3 Nanospheres

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Determination of Nitrite and Nitrate in Brazilian Meats Using High Shear Homogenization

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Avaliação de contaminantes inorgânicos e orgânicos em álcool combustível utilizando eletroforese capilar

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

Desenvolvimento e validação de método analítico para determinação de benzoato, sorbato, metil e propilparabenos em produtos alimentícios utilizando a eletroforese capilar

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Photoluminescence at room temperature in amorphous SrTiO3 thin films obtained by chemical solution deposition

Intense photoluminescence in highly disordered strontium titanate amorphous thin films prepared by the polymeric precursor method was observed at room temperature (300 K). The luminescence spectra of SrTiO3 amorphous thin films at room temperature revealed an intense single-emission band in the visible region. X-ray absorption near edge structure was used to probe the local atomic structure of SrTiO3 amorphous and crystalline thin films. Photoluminescence intensity in the 535 nm range was found to be correlated with the presence of non-bridging oxygen defects. A discussion is presented of the nature of this photoluminescence, which may be related to the disordered structure in SrTiO3 amorphous thin films. (C) 2002 Elsevier B.V. B.V. All rights reserved.

The role of defect states in the creation of photoluminescence in SrTiO3

We discuss the nature of visible photoluminescence at room temperature in amorphous strontium titanate in the light of the results of a recent experimental and quantum mechanical theoretical study. Our calculation of the electronic structure involves the use of first-principles molecular calculations to simulate the variation of the electronic structure in the strontium titanate crystalline phase, which is known to have a direct band gap, and we also make an in-depth examination of amorphous strontium titanate. The results of our simulations of amorphous strontium titanate indicate that the formation of five-fold coordination in the amorphous system may introduce delocalized electronic levels in the highest occupied molecular orbital and the lowest unoccupied molecular orbital. These delocalized electronic levels are ascribed to the formation of a tail in the absorbance-spectrum curve. Optical absorption measurements experimentally showed the presence of a tail. The results are interpreted by the nature of these exponential optical edges and tails associated with defects promoted by the disordered structure of the amorphous material. We associate them with localized states in the band gap.

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298 - 673 K temperature range in polycrystalline Pb0.50Sr0.50TiO3 thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb0.50Sr0.50TiO3 thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO3 perovskite.

Ferroelectric and optical properties of Ba0.8Sr0.2TiO3 thin film

Barium strontium titanate (Ba0.8Sr0.2TiO3) thin films have been prepared on Pt/Ti/SiO2/Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba0.8Sr0.2TiO3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm(-1) peaks. The tetragonal-to-cubic phase transition in the Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed at about 35 degreesC, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba0.8Sr0.2TiO3 thin films showed a remnant polarization (P-r) of 6.5 muC/cm(2), a coercive field (E-c) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (E-g) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27-2.10, respectively. (C) 2002 American Institute of Physics.

Cracks developed during SrTiO3 thin-film preparation from polymeric precursors

Strontium titanate (SrTiO3) thin films were prepared by dip-coating Si(111) single-crystal substrates in citrate solutions of ethylene glycol, considering several citric acid/ethylene glycol (CA/EG) ratios. Measurements of intrinsic viscosity indicate that increasing the amount of EG increases the precursors' polymeric chains and increases the weight loss. After deposition the substrates were dried on a hotplate (approximate to 150 degrees C); this was followed by heat treatment at temperatures ranging from 500 to 700 degrees C using heating and cooling rates of 1 degrees C min(-1). SEM and optical microscopy investigations of the sintered films obtained from different CA/EG ratios indicate that there is a critical thickness above which the films present cracks. This critical thickness for SrTiO3 films deposited on the Si(111) substrate is about 150 nm, Measurements of crack spacing as a function of film thickness indicate that the origin of cracks cannot be explained by the elastic behavior of the film but rather by the viscoelastic relaxation of the film during pyrolysis and sintering. Copyright (C) 1999 John Wiley & Sons, Ltd.

Room-temperature photoluminescence in structurally disordered SrWO4

Intense and broad visible photoluminescent (PL) band in structurally disordered SrWO4 compounds was observed at room temperature. The polycrystalline scheelite strontium tungstate (SrWO4) samples prepared by the polymeric precursor method at different temperatures of annealing were structurally characterized by x-ray diffraction and Fourier transform Raman spectroscopy measurements. Quantum-mechanical calculations showed that the local disorder in the cluster of the network modifiers Sr has a very important role in the charge transfer. The experimental and theoretical results are in good agreement, indicating that the generation of the intense visible PL band can be related to short-range order-disorder degree in the scheelite structure. (c) 2006 American Institute of Physics.

Nanoparticle synthesis of La1-xSrxMnO3 (0.1, 0.2 and 0.3) perovskites

Synthesis of La1-xSrxMnO3 (x = 0.1, 0.2 and 0.3) by homogenous coprecipitation method using urea as precipitant agent Is reported. The particles are smaller than 200 nm after heating at 950 degreesC. Temperature dependence of the electrical resistivity was found to be similar to the reported value for single crystals of these manganites.