Navegando por Autor "Granado, Eduardo"

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    Crystal structure of fluorite-related Ln3SbO7 (Ln=La–Dy) ceramics studied by synchrotron X-ray diffraction and Raman scattering.
    (2013) Siqueira, Kisla Prislen Félix; Borges, Raquel Moreira; Granado, Eduardo; Moreira, Leandro Malard; Paula, Ana Maria de; Moreira, Roberto Luiz; Bittar, Eduardo Matzenbacher; Dias, Anderson
    Ln3SbO7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) ceramics were synthesized by solid-state reaction in optimized conditions of temperature and time to yield single-phase ceramics. The crystal structures of the obtained ceramics were investigated by synchrotron X-ray diffraction, second harmonic generation (SHG) and Raman scattering. All samples exhibited fluorite-type orthorhombic structures with different oxygen arrangements as a function of the ionic radius of the lanthanide metal. For ceramics with the largest ionic radii (La–Nd), the ceramics crystallized into the Cmcm space group, while the ceramics with intermediate and smallest ionic radii (Sm–Dy) exhibited a different crystal structure belonging to the same space group, described under the Ccmm setting. The results from SHG and Raman scattering confirmed these settings and ruled out any possibility for the non-centrosymmetric C2221 space group describing the structure of the small ionic radii ceramics, solving a recent controversy in the literature. Besides, the Raman modes for all samples are reported for the first time, showing characteristic features for each group of samples.
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    Synchrotron X-ray diffraction and Raman spectroscopy of Ln3NbO7 (Ln=La, Pr, Nd, Sm-Lu) ceramics obtained by molten-salt synthesis.
    (2014) Siqueira, Kisla Prislen Félix; Soares, J. C.; Granado, Eduardo; Bittar, Eduardo Matzenbacher; Paula, Ana Maria de; Moreira, Roberto Luiz; Dias, Anderson
    Ln3NbO7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) ceramics were obtained by molten- salt synthesis and their structures were systematically investigated by synchrotron X-ray diffraction (SXRD), second harmonic generation (SHG) and Raman spectroscopy. It was observed that ceramics with the largest ionic radii (La, Pr, Nd) crystallized in to the Pmcn space group, while the ceramics with intermediate ionic radii (Sm-Gd) exhibited different crystal structure belonging to the Ccmm space group. For this last group of ceramics, this result was corroborated by SHG and Raman scattering and ruled out any possibility for the non-centrosymmetric C 2221 space group, solving a recent controversy in the literature. Finally, according to SXRD, Tb-Lu containing samples exhibited an average defect fluorite structure (Fm3m space group). Nonetheless, broad scatter in gat forbidden Bragg reflections indicates the presence of short-range domains with lower symmetry. Vibrational spectroscopy showed the presence of six Raman-active modes, inconsistent with the average cubic fluorite structure, and in line with the existence of lower-symmetry nano-domains immersed in the average fluorite structure of these ceramics.