Navegando por Autor "Sá, Rodrigo Gomes"

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    Disorder-induced symmetry lowering in Ba(Y1/2Nb1/2)O3 ceramics probed by Raman spectroscopy.
    (2008) Dias, Anderson; Sá, Rodrigo Gomes; Moreira, Roberto Luiz
    This work investigates the evolution of the crystal structure of microwave-hydrothermal synthesized Ba(Y1/2Nb1/2)O3 powders as a function of firing temperature by Raman spectroscopy. The samples were produced at 200 °C and fired at temperatures ranging from 600 to 1600 °C. Raman spectra were obtained at room temperature for all samples and the results showed that materials fired at 1600 °C exhibited tetragonal (I4/m or C5 4h) structure, whereas those fired at lower temperatures exhibited the triclinic (P1 or C1 i ) structure. The results were compared with those observed for ceramics obtained by conventional solid-state methods. It is believed that the lowering of the symmetry verified in materials fired below 1600 °C is a consequence of the local disorder of Y+3 and Nb+5 ions in octahedral sites.
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    Microwave-hydrothermal synthesis of nanostructured Na-birnessites and phase transformation by arsenic(III) oxidation.
    (2008) Dias, Anderson; Sá, Rodrigo Gomes; Spitale, Matheus Calonge; Athayde, Maycon; Ciminelli, Virgínia Sampaio Teixeira
    Microwave-hydrothermal synthesis was employed to produce Na-birnessites. Crystalline, single-phase materials were obtained at temperatures as low as 120 8C and times as short as 1 min. X-ray diffraction and Raman spectroscopy were used to characterize the structural features of the nanostructured powders. Birnessites possessed a monoclinic structure in space group C2/m with nine Raman-active bands, all of which were observed for the first time due to optimized acquisition of the spectroscopic data. The highly reactive materials produced were submitted to sorption experiments with As(III). An oxidative precipitation occurred with the production of Mn(II) arsenate at higher arsenic concentrations. In addition, the formation of hausmannite (Mn3O4) was confirmed by X-ray diffraction and Raman analyses of the reacted solid phase. The observed 14 Raman-active modes were adjusted according to the tetragonal I41/amd space group for hausmannite. An additional band related to the breathing mode of the arsenate was observed, leading to the conclusion that adsorption onto hausmannite takes place in addition to the oxidative precipitation of manganese arsenate.