A transição de fase estrutural de ferroelétrico para paraelétrico foi estudada por espalhamento Raman em cerâmicas de titanato de chumbo (PbTiO3) dopadas com lantânio (La). O modo vibracional A1 contém alguns picos que tem sido associados na literatura com a concentração de PbTiO3 (Ca, Ba), e os efeitos das altas temperaturas ao se aproximar da temperatura de Curie (Tc). Um novo estudo Raman mostra que o comportamento anômalo do modo A1 não é apenas causado pelos efeitos da concentração ou da temperatura próxima de Tc, mas por anarmonicidade e defeitos na rede do PbTiO3 são termodinamicamente inevitáveis. Este efeito foi estudado em uma ampla faixa de temperatura de 10 K a Tc em cada amostra (La = 0, 5, 10, 15, 20%), mostrando que o efeito depende não somente das altas temperaturas no PT puro.

The structural phase transition from ferroelectric to paraelectric was studied by Raman scattering in the ceramic lead titanate (PbTiO3) doped with lanthanum (La). A1 vibrational mode contains some sub-peaks that have been associated in literature with PbTiO3 concentration (Ca, Ba), and effects of high temperatures as it approaches the Curie temperature (Tc). Raman new study shows that the anomalous behavior of the A1 mode is not only caused by the effects of concentration or temperature near Tc, but by anharmonicity and defects in the network in the PbTiO3 thermodynamically are inevitable. This effect was studied by a wide temperature range from 10 K to Tc in each sample (La = 0, 5, 10, 15, 20%), showing that the effect depends not only on the high temperatures in the pure PT.

Single crystal samples of L-leucine, C6H13NO2, a fundamental aliphatic amino acid of the human body, have been studied by Raman spectroscopy at temperatures from 300 to 430 K over the spectral range from 50 to 3100 cm-1. A tentative assignment of all bands is given. For high temperatures, several modifications on the Raman spectra were observed at about 353 K, giving evidence that the L-leucine crystal undergoes a structural phase transition. An interpretation for this phase transition in terms of group theory analysis is given.

Continuous chips removed by single point diamond turning of single crystal silicon have been investigated by means of Scanning Electron Microscopy/Transmission Electron Microscopy and micro-Raman Spectroscopy. Three different chip structures were probed with the use of electron diffraction pattern: (i) totally amorphous lamellar structure, (ii) amorphous structure with remnant crystalline material and, (iii) partially amorphous together with amorphous with remnant crystalline material. Furthermore, micro-Raman spectroscopy from the chips left in the cutting tool rake face showed different silicon phases. We have found, from a detailed analysis of the debris, five different structural phases of silicon in the same debris. It is proposed that material removal mechanisms may change along the cutting edge from shearing (yielding lamellar structures) to extrusion. Shearing results from structural changes related to phase transformation induced by pressure and shear deformation. Extrusion, yielding crystalline structures in the chips, may be attributed to a pressure drop (due to an increase in the contact area) from the tool tip towards the region of the cutting edge where brittle-to-ductile transition occurs. From this region upwards, pressure(stress) would be insufficient to trigger phase transformation and therefore amorphous phase would not form integrally along the chip width.

A grande maioria dos estudos fotoluminescentes foram realizados com materiais cristalinos, cuja emissão ocorre em temperaturas criogênicas, limitando a aplicação destes materiais. Por sua vez, determinados sólidos com alta densidade de defeitos (amorfos), como titanatos, tungstatos, zirconatos, entre outros, apresentam fotoluminescência à temperatura ambiente, semelhante à do silício amorfo. Essa descoberta levou à necessidade de se desenvolver novos modelos para explicar o comportamento desses materiais, mais adequados ao estudo desses materiais. Esta revisão tem como objetivo explicar os resultados de fotoluminescência, utilizando uma abordagem teórico-experimental, com base nas recentes pesquisas desta propriedade em óxidos amorfos.

The majority of the studies about photoluminescence had been carried out with crystalline materials, whose emission occurs at cryogenic temperatures, limiting the application of these materials. In turn, some amorphous solids, as titanates, tungstates, zirconates, among others, present photoluminescence at room temperature, seemed with silicon. This discovery led to the necessity of developing new models to explain the behavior of these materials. This revision intends to elucidate the results on photoluminescence, using a theoretical-experimental treatment, on the basis of the recent researches of this property in amorphous oxides.

Molecular dynamics simulation was used to study structural and dynamical properties of InSb. The effective potential takes into account two and three-body interactions, considering atomic-size effects and charge-charge, charge-dipole, and dipole-dipole interactions between 1000 particles, 500 In and 500 Sb, initially within a cubic box of side L=32.397 Å . The effect of hydrostatic pressure and temperature on the structural properties like pair distribution function, coordination number, volume change and bond angle distribution and on dynamical properties like vibrational density of states, phonon anharmonicity, dynamic Debye-Waller factor, thermal expansion coefficient and structural phase transformations are correctly described, in excellent agreement with the experimental results.

Discutiu-se a fotoluminescência para o tungstato de cálcio amorfo, observada à temperatura ambiente. Verificou-se que há concordância entre os resultados experimentais e teóricos. Neste trabalho foram simuladas as estruturas cristalinas e amorfas do tungstato de cálcio (CaWO4), comparando-se as respectivas estruturas eletrônicas. Os resultados dos cálculos teóricos indicam a formação de novos níveis de energia na banda de valência e de condução do amorfo. Estes níveis eletrônicos extras são os responsáveis pela formação da cauda na curva do espectro de absorção. Correspondentemente, medidas experimentais de absorção óptica mostraram a presença da cauda, com relação ao espectro do cristalino. Desta forma, pode-se interpretar a formação da cauda, como sendo associada aos defeitos promovidos na estrutura desordenada do material amorfo.

We discuss the nature of visible photoluminescence at room temperature in amorphous calcium tungstate taking into account the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the calcium tungstate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous calcium tungstate. The results of our theoretical calculations of amorphous calcium tungstate indicate that the formation of threefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). These delocalized electronics levels are related to the formation of a tail in the absorbency spectrum curve. The results indicate that amorphous calcium tungstate has the conduction band near the band gap dominated by Ca states contribution. Experimental optical absorption measurements showed the presence of a tail. These 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 delocalized states in the band gap.