Navegando por Autor "Borges, Raquel Moreira"

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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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    Structural and thermal evolution studies of LaSbO4 ceramics prepared by solid-state reaction method.
    (2013) Siqueira, Kisla Prislen Félix; Borges, Raquel Moreira; Soares, Júlia Cristina; Dias, Anderson
    This work investigates the thermal evolution of LaSbO4 ceramics produced by solid-state method in the temperature range 700e1500 _C, for 6 h. Besides the expected phase transitions, a thermal decomposition at high temperatures to La3SbO7 was observed and discussed. The results showed that the phase LaSbO4 can be obtained only below 1450 _C, while temperatures lower than 1100 _C are not able to produce crystalline structures. The samples were submitted to X-ray diffraction and Raman scattering measurements aiming to determine their crystal structures. LaSbO4 materials exhibited monoclinic structures, space group P21/m ¼ C2 2h(#11), with Z ¼ 4. It was verified that all the 36 Raman-active modes predicted by group-theory calculations were observed. Also, the symmetries could be discerned by using polarized Raman scattering, which allow us to assign the gerade modes. Luminescence properties of LaSbO4 were investigated and the results showed that this compound exhibits sensitivity of the host lattice to UV excitation. PL spectra excited at 360 nm have a blue emission band maximum at 428 nm, corresponding to the self-activated luminescence center of LaSbO4.