J. Mendes Filho

Optical properties of Sm3+ doped lead fluoroborate glasses

We present a study of emission, absorption and vibrational properties of Sm3+ doped lead fluoroborate glasses (PbO–PbF2–B2O3–Sm2O3) as a function of the Sm3+ content. Changes in the position and the intensity parameters of the transitions are closely related to structural changes in the glass network. A Judd–Ofelt scheme was used to estimate the intensity parameters Ωλ and radiative properties for Sm3+ in the glass. The behaviour of Ω2 and Ω6 parameters suggested a decrease in the asymmetry degree of the local ligand field at Sm3+ sites and an increase in the covalency of Sm–O bond, respectively. This hypothesis was verified by analysing the structural role of Sm2O3 through Raman and infrared spectroscopy measurements.

Optical properties of L-alanine Organic Crystals

This work reports on the optical properties of L-alanine crystals grown from aqueous solution using the well-known slow evaporation method. Linear properties including absorption spectra and refractive indices are determined, as well as nonlinear properties of second harmonic generation (SHG), such as the phase-matching loci and efficiency. Although the SHG efficiency is smaller than those of other known materials, the study of L-alanine crystals is very important since it is part of more complex aminoacids including L-arginine and asparagine, with much higher SHG efficiency. Some mechanical properties are also presented, namely hardness and thermal expansion coefficients.

Structural properties of CaCu3Ti4O12 obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of CaCu3Ti4O12 (CCTO), for the first time, using two different experimental procedures. The milled CCTO were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. For two different milling procedures, CCTO was obtained after a couple of hours of milling (in average 30 h of milling, depending on the reaction procedure). The X-ray diffraction (XRD) patterns indicate that the crystallite size is within the range of 20 � /35 nm. After 100 h of milling the formation of CCTO was confirmed by X-ray powder diffraction in both procedures, with good stability. We also prepare the CCTO ceramic using the traditional procedure described in the literature and compared the physical properties of these samples with those ones obtained by milling process and good agreement was observed. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time leads to the formation of nanocrystalline CCTO, as seen by XRD analysis. These materials are attractive for capacitor applications and certainly for microelectronics, microwav ed evices (cell mobile phones for example), where the decrease of the size of the devices are crucial. This milling process presents the advantage that melting is not necessary and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material can be compacted and transformed in solid ceramic samples or used in others procedures of film preparation. The high efficiency of the process opens a way to produce commercial amount of nanocrystalline powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry. # 2002 Elsevier Science B.V. All rights reserved.

Raman study of morphotropic phase boundary in PbZr 1 − x Ti x O 3 at low temperatures

The Raman spectra of

Pressure-induced phase transitions in monohydrated l-asparagine aminoacid crystals

{\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}

Polarized Raman Spectra and Infrared Analysis of Vibrational Modes in L-Threonine Crystals

ceramics with titanium concentration varying between

Monoclinic phase ofPbZr0.52Ti0.48O3ceramics: Raman and phenomenological thermodynamic studies

x=0.40

High temperature Raman spectra of L-leucine crystals

and 0.60 were measured at 7 K. By observing the concentration-frequency dependence of the vibrational modes, we identified the boundaries among the rhombohedral, monoclinic, and tetragonal ferroelectric phases. The analysis of the spectra was made in the view of group theory analysis making possible the assignment of some modes for the monoclinic phase.

Coexistence of triclinic and monoclinic phases in WO3 ceramics

Abstract Single-crystal samples of monohydrated l-asparagine, C4N2O3H8. H2O, a fundamental aminoacid of the human body and other complex beings, have been studied by Raman spectroscopy in a diamond-anvil cell up to pressures of 2.0 GPa. From the analysis of results we observed that the crystal undergoes a series of three phase transitions. All transitions are accompanied by drastic changes in the external modes and in some internal modes of the crystal, which shows in a clear way the transitions induced by the hydrostatic pressure. Pressure coefficients for external modes are also given.

Structural and dielectric properties of Nd3+-doped Ba0.77Ca0.23TiO3 ceramics

Polarized Raman and infrared espectra of l-threonine crystals have been obtained at room temperature and an assignment of normal modes is proposed based on group theory analysis. The usefulness of these assignments comes from the fact that some modes are related to important vibrations that can show changes under different pressure and temperature conditions.