Navegando por Autor "Lobo, Ricardo P. S. M."

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Infrared dispersion analysis and Raman scattering spectra of taurine single crystals.
(2018) Moreira, Roberto Luiz; Lobo, Ricardo P. S. M.; Dias, Anderson
A comprehensive set of optical vibrational modes of monoclinic taurine crystals was determined by Raman scattering, and infrared reflectivity and transmission spectroscopies. By using appropriate scattering/reflection geometries, the vibrational modes were resolved by polarization and the most relevant modes of the crystal could be assigned. In particular, we were able to review the symmetry of the gerade modes and to resolve ambiguities in the literature. Owing to the non-orthogonal character of Bu modes in monoclinic crystals (lying on the optic axial plane), we carried out a generalized Lorentz dispersion analysis consisting of simultaneous adjust of infrared-reflectivity spectra at various light polarization angles. The Au modes (parallel to the C2-axis) were treated within the classical Lorentz model. The behavior of off-diagonal and diagonal terms of the complex dielectric tensors and the presence of anomalous dispersion were discussed as consequences of the low symmetry of the crystal.
Optical phonon features of triclinic montebrasite : dispersion analysis and non-polar Raman modes.
(2015) Almeida, Rafael Mendonça; Höfer, Sonja; Mayerhöfer, Thomas G.; Popp, Jürgen; Krambrock, Klaus Wilhelm Heinrich; Lobo, Ricardo P. S. M.; Dias, Anderson; Moreira, Roberto Luiz
Polarized infrared and Raman spectra of triclinic LiAl(PO4)(OH) [montebrasite] single crystal were recorded for appropriate optical configurations. Dispersion analysis was applied on the infrared reflectivity spectra taken at low incidence angle (11 ) to determine the oscillator parameters and the dipole directions of the polar phonons. In particular, all the 27 polar phonons, predicted by group theory for triclinic P1 structure,were determined. The obtained dielectric tensor parameters have been checked by comparison between predicted and measured infrared spectra at higher incidence angle (34 ). The azimuth and co-elevation angles obtained from the dispersion analysis showed that the response of several polar phonons is close to that of an orthorhombic system. Polarized Raman spectra obtained in several scattering geometries allowed us to obtain well-defined 24 non-polar modes, also in perfect agreement with group theory. The selection rule between Raman and infrared phonons was respected, confirming the centrosymmetric structure and ruling out any relevant influence of defects. The relatively narrow phonon bands are compatible with a highly ordered structure with fully occupied atomic sites.
Raman and infrared spectroscopic investigations of a ferroelastic phase transition in Ba2ZnTeO6 double perovskite.
(2018) Moreira, Roberto Luiz; Lobo, Ricardo P. S. M.; Ramos, Sérgio L. L. M.; Sebastian, Mailadil T.; Matinaga, Franklin Massami; Righi, Ariete; Dias, Anderson
The low-temperature vibrational properties of B a 2 ZnTe O 6 double-perovskite ceramics obtained by the solid-state route were investigated by Raman scattering and Fourier-transform infrared reflectivity. We found that this material undergoes a reversible ferroelastic phase transition at around 140 K, well compatible with a recently proposed rhombohedral-to-monoclinic structural change that would occur below 165 K. Complementary calorimetric measurements showed that the phase transition has a first-order character, with an entropy jump compatible with a displacive mechanism. The vibrational spectra show clearly the splitting of the doubly degenerate E modes into nondegenerate representations of the low-symmetry phase. In particular, the lowest-frequency Raman mode presents soft-mode behavior and splits below the critical temperature, confirming the in-plane ferroelastic deformation in the low-temperature phase.