Browsing by Author "Matinaga, Franklin Massami"

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    High-temperature antiferroelectric and ferroelectric phase transitions in phase pure LaTaO4.
    (2017) Abreu, Yuslín Gonzále; Siqueira, Kisla Prislen Félix; Matinaga, Franklin Massami; Moreira, Roberto Luiz; Dias, Anderson
    Phase-pure LaTaO4 ceramics was prepared by solid-state reaction. Dielectric spectroscopic data as well as differential scanning calorimetric experiments showed the existence of a sequence two high-temperature firstorder structural phase transitions. The first phase transition occurs above 160 °C (on heating), from the monoclinic P21/c space group at room temperature to the polar orthorhombic Cmc21 group, exhibiting a very large thermal hysteresis probably linked to the reconstructive nature of the structural transition. The second transition occurs around 225 °C to the orthorhombic Cmcm space group, with a small thermal hysteresis. Vibrational Raman spectroscopic analyses confirmed these two sequential phase transitions, as well as the thermal hysteresis observed for both first-order transitions in repeated heating and cooling cycles. The existence of a strong monoclinic distortion at room temperature could be related to the presence of defects (oxygen vacancies) in LaTaO4 ceramics, after sintering. Dielectric spectroscopy showed a strong influence of the electric conductivity on the dielectric response with activation energy of dc component of conductivity (0.62 eV) compatible with the presence of oxygen vacancies. Far-infrared data confirmed that the extra modes observed in the Raman spectra are forbidden bands, which were activated by defects into the structure.
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    Influence of the matrix on the red emission in europium self-activated orthoceramics.
    (2015) Siqueira, Kisla Prislen Félix; Lima, Patrícia Pereira de; André, Maria Rute de Amorim e Sá Ferreira; Carlos, Luís António Ferreira Martins Dias; Bittar, Eduardo Matzenbacher; Matinaga, Franklin Massami; Paniago, Roberto Magalhães; Krambrock, Klaus Wilhelm Heinrich; Moreira, Roberto Luiz; Dias, Anderson
    Different oxide host matrices of ABO4 with A3+ (Eu) and B5+ (Nb, Ta, and Sb) were prepared to investigate the solid state luminescence behavior of Eu3+ as a self-activated emitter in orthoceramics. Crystal structures, phonon modes, metal valence states, optical excitation, and emission luminescence properties including emission decay curves, colorimetry, and nonstoichiometry defects were studied using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Classical intraconfigurational (f−f transitions), interconfigurational (4fn−15d → 4fn), and charge transfer bands (X5+−O2−) were observed, besides split Stark levels indicating low local symmetry. The influence of host matrices on the optical behavior of orthocompounds was noticed by changes in the energy of the typical Eu3+ transitions. In this case, the larger blue-shifted peaks were observed for EuSbO4 and the larger red-shifted positions for EuNbO4, with respect to emission spectra of orthocompounds studied here. These results were correlated to the respective fifth ionization energies and covalent fraction as well as to the crystal structures exhibited by the different host matrices (I2/a: Nb, Ta; P21/c: Sb). The presence of more than one component for the 5D0 → 7F0 transition in each compound was observed at low temperature, and it could be justified by nonstoichiometric defects like the oxygen vacancy and Eu2+. These typical defects are observed in nonstoichiometric oxide materials, and they were analyzed in the orthoceramics by EPR and XPS spectroscopy. Finally, the lifetime of the 5D0 state and chromaticity diagrams confirmed our orthoceramics as good optical emitters in the red-end region.
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    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.