12 documents found, page 1 of 2
Micro far-infrared dielectric response of lanthanide orthotantalates for applic...
Dias, Anderson; Siqueira, Kisla Prislen Félix; Moreira, Roberto LuizLanthanide orthotantalates LnTaO4 (Ln ¼ La, Nd, Dy and Lu) were synthesized by solid-state reactions at 1300 C leading to well crystallized single-phase materials. XRD and infrared spectroscopic investigations showed that the samples exhibited three different monoclinic crystal structures depending on the lanthanide ion: P21/c (for La), I2/a (for Nd) and P2/a (for Dy and Lu). For LaTaO4, 21 polar modes could be...
High-temperature antiferroelectric and ferroelectric phase transitions in phase...
Abreu, Yuslín Gonzále; Siqueira, Kisla Prislen Félix; Matinaga, Franklin Massami; Moreira, Roberto Luiz; Dias, AndersonPhase-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, exhibi...
Structural and vibrational properties of phase - pure monoclinic NdLuO3 interla...
Soares, Júlia Cristina; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz; Dias, AndersonPhase-pure monoclinic NdLuO3 interlanthanides were successfully synthesized from hydrothermalderived precursors. LuO(OH) and Nd(OH)3 materials were firstly obtained by hydrothermal synthesis at 250 _C, for 24 h. X-ray diffraction, micro-Raman spectroscopy, scanning and transmission electron microscopies were employed to investigate the thermal behavior of these precursors and NdLuO3 interlanthanide up to 1600 _...
Optical phonon features of triclinic montebrasite : dispersion analysis and non...
Almeida, Rafael Mendonça; Höfer, Sonja; Mayerhöfer, Thomas G.; Popp, Jürgen; Krambrock, Klaus Wilhelm Heinrich; Lobo, Ricardo P. S. M.; Dias, AndersonPolarized 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 tric...
Crystal structure of fluorite-related Ln3SbO7 (Ln=La–Dy) ceramics studied by sy...
Siqueira, Kisla Prislen Félix; Borges, Raquel Moreira; Granado, Eduardo; Moreira, Leandro Malard; Paula, Ana Maria de; Moreira, Roberto LuizLn3SbO7 (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 ...
Comment on ‘‘prediction of lattice constant in cubic perovskites".
Moreira, Roberto Luiz; Dias, AndersonIn a recent work by Jiang et al. [Prediction of lattice constant in cubic perovskites, J. Phys. Chem. Solids 67 (2006) 1531–1536], the interrelationship between lattice constant, ionic radii and tolerance factor of cubic perovskites has been established and an empirical equation was obtained. However, the assumption of incorrect ionic coordination led to an incorrect mathematical expression even though the aver...
Synthesis and characterisation of La0.4Ba0.6Ti0.6RE0.4O3 (where RE =Y, Yb) cera...
Feteira, Antônio; Elsebrock, R.; Dias, Anderson; Moreira, Roberto Luiz; Lanagan, Michael T.; Sinclair, Derek C.La0.4Ba0.6Ti0.6RE0.4O3 (where RE =Y, Yb) powders were synthesised by the mixed oxide route. According to laboratory X-ray diffractometry both compositions are single-phase and can be indexed on the Pm3m space group; however, electron diffraction suggests the occurrence of anti-phase octahedral tilting or 1:1 ordering. Ceramics fired at 1600 ◦C for 4 h exhibit an average grain size of ∼20_m. Bright field TEM ima...
Synchrotron X-ray diffraction and Raman spectroscopy of Ln3NbO7 (Ln=La, Pr, Nd,...
Siqueira, Kisla Prislen Félix; Soares, J. C.; Granado, Eduardo; Bittar, Eduardo Matzenbacher; Paula, Ana Maria de; Moreira, Roberto Luiz; Dias, AndersonLn3NbO7 (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 intermed...
Raman scattering study of the high temperature phase transitions of NaTaO3.
Teixeira, Nayara Gomes; Dias, Anderson; Moreira, Roberto LuizThe phase transition sequence of sodium tantalate (NaTaO3) ceramic samples has been investigated by Raman spectroscopy, from room temperature up to 640 ◦C. At room temperature, NaTaO3 shows an orthorhombically distorted perovskite structure belonging to the Pbnm space group. Raman and infrared vibrational spectra at 20 ◦C are in relative good agreement with group theory predictions for this group. At high tempe...
Raman-spectroscopic investigations on the crystal structure and phonon modes of...
Moreira, Roberto Luiz; Khalam, L. Abdul; Sebastian, Mailadil Thomas; Dias, AndersonComplex Ba(RE1/2Ta1/2)O3 [RE = rare earth] ceramics have been prepared by solid state ceramic route and their microwave dielectric responses were measured in the GHz range. In general, the resonators showed high dielectric constants, high quality factors and good thermal stability of the resonance frequency. However, these properties are found to depend nonlinearly on the tolerance factor, indicating structural...