Navegando por Autor "Moreira, Roberto Luiz"

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Comment on ‘‘prediction of lattice constant in cubic perovskites".
(2007) Moreira, Roberto Luiz; Dias, Anderson
In 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 average relative errors between predicted and observed lattice constants of 132 materials were below 1%. Here, corrected coefficients for that empirical expression are obtained, which would likely be useful for investigation of general perovskite materials.
Crystal structure and phonon modes of ilmenite-type NaBiO3 investigated by Raman and infrared spectroscopies.
(2010) Dias, Anderson; Moreira, Roberto Luiz
NaBiO3 is an ilmenite-type compound presenting a trigonal structure. In this work, we have performed optical spectroscopic investigations using Raman scattering and infrared reflectivity for this material. By using group-theory calculations, it was possible to determine the number of optically activemodes. Fitting procedures besides Kramers–Kronig (KK) procedures lead to a consistent collection of phononmodes. The Raman spectrumshowed nine active first-ordermodes, while the infrared one revealed eight polar phonons, in good agreement with the theoretical predictions. The results allowed us to confirm the R3 (#148) space group for this compound, and to establish a set of active phonons not yet reported in the literature.
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.
Crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 complex perovskites investigated by Raman scattering.
(2010) Dias, Anderson; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz
This work investigates the crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 perovskites by Raman spectroscopy. The samples were produced by conventional solid-state processing at 1200 ◦C. X-ray diffraction showed that single-phase homogeneous materials were produced, which are cubic or pseudo-cubic in symmetry. The existing controversies in the literature for these complex perovskiteswere investigated by comparing experimental Raman data with group-theory analysis. Ceramicswith Ca andWorMowere found to be cubic, space group Fm3m. For thesematerials, four Raman-active bands were observed and the fitting parameters showed that the Ba(Ca1/2Mo1/2)O3 ceramic presents bands at lower wavenumbers if compared with the Ba(Ca1/2W1/2)O3 sample. For the Ba(Sr1/2W1/2)O3 material, two hypotheses were investigated for monoclinic or triclinic structures. The experimental results showed 12 Raman-active modes for this ceramic, which is in perfect agreement with the theoretical predictions for a monoclinic (I2/m) structure.
Disorder-induced symmetry lowering in Ba(Y1/2Nb1/2)O3 ceramics probed by Raman spectroscopy.
(2008) Dias, Anderson; Sá, Rodrigo Gomes; Moreira, Roberto Luiz
This work investigates the evolution of the crystal structure of microwave-hydrothermal synthesized Ba(Y1/2Nb1/2)O3 powders as a function of firing temperature by Raman spectroscopy. The samples were produced at 200 °C and fired at temperatures ranging from 600 to 1600 °C. Raman spectra were obtained at room temperature for all samples and the results showed that materials fired at 1600 °C exhibited tetragonal (I4/m or C5 4h) structure, whereas those fired at lower temperatures exhibited the triclinic (P1 or C1 i ) structure. The results were compared with those observed for ceramics obtained by conventional solid-state methods. It is believed that the lowering of the symmetry verified in materials fired below 1600 °C is a consequence of the local disorder of Y+3 and Nb+5 ions in octahedral sites.
Exfoliation and characterization of a two-dimensional serpentine-based material.
(2019) Santos, Joyce Cristina da Cruz; Barboza, Ana Paula Moreira; Matos, Matheus Josué de Souza; Barcelos, Ingrid David; Fernandes, Thales Fernando Damasceno; Soares, Edmar Avellar; Moreira, Roberto Luiz; Neves, Bernardo Ruegger Almeida
We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using density functional theory, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are observed and compared with the FTIR characterization of the parent bulk material. They show that there is no degradation of serpentine's structural properties during its mechanical exfoliation down to nanometer-thin sheets. Therefore, our results introduce the serpentine mineral as an attractive low-cost candidate in 2D materials applications.
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.
Hydrothermal synthesis and polarized micro-Raman spectroscopy of copper molybdates.
(2018) Martins, Guilherme Mendes; Coelho, Pâmela de Oliveira; Moreira, Roberto Luiz; Dias, Anderson
Conventional and microwave-assisted hydrothermal methods were used to synthesize Cu3(MoO4)2(OH)2 ceramics. Different experimental conditions of temperature, time and precipitation rate were employed to explore the growth mechanisms, structural and morphological properties of the obtained copper molybdates. The use of microwave-assisted reactors at 150 °C/10–30 min produced a mixture of copper molybdate hydrates, while only Cu3(MoO4)2(OH)2 was obtained by using microwave processing at longer times (above 120 min) or conventional hydrothermal reactors (110–250 °C/24 h). Experiments conducted after fast and slow coprecipitation rates resulted in different crystallographic phases and morphologies. Flower-like and rod-like micrometer-sized ceramics were produced with high anisotropy and single-crystalline nature. A better understanding on the growth mechanisms, as well as on the structural and morphological characteristics of copper molybdates was attained. Based upon the relative enhancement of Raman bands in parallel or in perpendicular configuration, a symmetry assignment of 34 (17Ag + 17Bg) external modes for Cu3(MoO4)2(OH)2 ceramics (42 bands are expected) was possible. Besides, five (of six possible) internal OH-stretching modes were identified in the spectral region above 3200 cm−1.
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.
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.
Investigation of polymorphism and vibrational properties of MnMoO4 microcrystals prepared by a hydrothermal process.
(2018) Martins, Guilherme Mendes; Coelho, Pâmela de Oliveira; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz; Dias, Anderson
MnMoO4 microcrystals were synthesized by hydrothermal methods, and their structural, morphological, and vibrational properties were investigated. Conventional reactors and microwave-heated hydrothermal vessels were used to synthesize micrometer-sized MnMoO4 polymorphs at different temperature and time conditions. MnMoO4·H2O crystals were obtained in both reactors at temperatures below 200 °C, for times up to 24 h. These microcrystals belong to the triclinic P1̅ space group, and their 26 characteristic Raman bands were identified. A monoclinic MnMoO4 polymorph belonging to the P2/c structure was synthesized at temperatures higher than 200 °C only into conventional hydrothermal reactors. Polarized micro-Raman analyses have evidenced and allowed the assignment of 18 phonon modes predicted by group-theory calculations for this polymorph. Another MnMoO4 polymorph, within the monoclinic C2/m space group, was obtained by two processing routes: (i) heating the P2/c microcrystals at 600 °C; or (ii) heating the MnMoO4·H2O phase at 250 °C. For this monoclinic phase, 33 Raman-active bands were identified and assigned, in very good agreement with group-theoretical calculations, which predict 36 modes for the C2/m polymorph. Well-faceted, highly crystalline microcrystals were clearly observed by scanning and transmission electron microscopies, in perfect agreement with X-ray diffraction and Raman spectroscopic analysis. Finally, the appearance of characteristic phonon modes related to molybdenum in octahedral coordination suggests an incipient crystallization of a new, unreported α-MnMoO4 polymorph, at least in a short-range degree.
Micro far-infrared dielectric response of lanthanide orthotantalates for applications in microwave circuitry.
(2017) Dias, Anderson; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz
Lanthanide 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 depicted from the unpolarized infrared reflectivity spectrum, while group theory tools foreseen 33 infrared-active modes e the absent mode are likely hidden by accidental degeneracy. The smaller lanthanides (Nd, Dy and Lu) exhibited all the 15 predicted infrared-active bands, in perfect agreement with group-theory calculations, despite the mixing of polarization symmetry due to the polycrystalline nature of the samples. The intrinsic (infrared) dielectric properties were determined for all samples indicating that these orthotantalate ceramics could be candidates for microwave (MW) circuitry applications. LaTaO4 ceramics exhibited the best MWdielectric response among the investigated LnTaO4 ( 3 r ¼ 21.2 and estimated Qu f z 77 THz), followed by DyTaO4 ( 3 r ¼ 19.9 and Qu f z 75 THz), NdTaO4 ( 3 r ¼ 18.7 and Qu f z 55 THz), and LuTaO4 ( 3 r ¼ 16.2 and Qu f z 60 THz).
Microwave-hydrothermal preparation of alkaline-earth-metal tungstates.
(2010) Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz; Valadares, Marcelo; Dias, Anderson
Microwave-hydrothermal processing was employed to synthesize nanostructured alkaline-earth-metal tungstate compounds in environmentally friendly conditions not yet found in the literature: 110 C for times ranging from 5 to 20 min. X-ray diffraction showed that crystalline, single-phase materials were produced, while electron microscopy evidenced nanostructured particles with different morphologies. Raman spectroscopy was used to probe short-range ordering and also to obtain a reliable set of spectra containing all the Raman-active bands predicted by group-theory calculations. The Raman spectra showed no extra feature, indicating that our method gave wellordered materials without important tetrahedral distortions. Sintered ceramics were submitted to polarized Raman scattering in parallel- and cross-polarized configurations. The Eg modes were totally evidenced in a cross-polarized configuration, while Ag and Bg modes appeared in the parallel arrangement, in complete agreement with the theoretical predictions. Photoluminescence measurements showed broad, complex emission bands, which varied from the blue to the red visible light regions. These results are related to shallow and deep defects into the band gap, through the presence of oxygen vacancies in the complex clusters with metal cations and tungsten atoms.
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.
Optical phonon modes and infrared dielectric properties of monoclinic CoWO 4 microcrystals.
(2016) Moreira, Roberto Luiz; Almeida, Rafael Mendonça; Siqueira, Kisla Prislen Félix; Abreu, Cíntia Grossi de; Dias, Anderson
The phonon characteristics of CoWO4 microcrystals with monoclinic Wolframite structure were investigated by far-infrared (IR) and Raman spectroscopies. Near-normal spectra were taken for IR light polarization along the principal b-axis (Au modes) and along several angles within the ac-plane (Bu modes). The IR spectra were analyzed with a generalized Drude– Lorentz model, and all predicted polar phonon modes were fully determined, including their symmetries, the dielectric Lorentz parameters and the non-orthogonal phonon polarizations for the Bu modes. Anomalous dispersion and negative values for the real and imaginary parts of the off-diagonal components of the dielectric tensor functions were identified and discussed under the light of the varying phonon polarization directions (spread out in the ac-plane). The obtained static and background dielectric tensors gave an average permittivity of 16.1 (at microwave region), refractive indices along the principal dielectric axes of 2.22, 2.33 and 2.44 (at 1 μm), the optical axes, and an estimated value for the biaxial angle of the crystal. Polarized Raman spectra on appropriate scattering configurations revealed the 18 non-polar gerade phonons of CoWO4 crystals, with their correct symmetries attributed.
Polarization‐resolved Raman modes of monoclinic SrAl2O4 ceramics.
(2018) Dias, Anderson; Moreira, Roberto Luiz
SrAl2O4 materials were obtained by molten salt synthesis and sintered at high temperatures to produce a ceramic body suitable for polarization‐resolved Raman spectroscopy measurements. Optimized experimental conditions were employed to study the complex vibrational spectrum of these ceramics under polarized light. A reliable set of experimental bands was obtained: 69 Raman modes were in fact observed, out of the 81 bands foreseen for the stable monoclinic polymorph of the material, from which 38 could be assigned as totally symmetric A‐type modes, and 31 attributed as asymmetric B‐type ones. Significant differences between experimental data and theoretical predictions indicated that additional efforts are necessary to understand these two sets of vibrational information. The knowledge of the optical phonon features reported here can help to understand the exceptional luminescence properties of SrAl2O4 in its monoclinic phase.
Polarized Micro-Raman Scattering of CaNb 2 O 6 single crystal fibers obtained by laser heated pedestal growth.
(2010) Moreira, Roberto Luiz; Teixeira, Nayara Gomes; Andreeta, Marcello Rubens Barsi; Hernandes, Antonio Carlos; Dias, Anderson
Polarized Raman scattering and infrared spectroscopy of a naturalmanganocolumbite single crystal.
(2010) Moreira, Roberto Luiz; Rubinger, Carla Patricia Lacerda; Krambrock, Klaus Wilhelm Heinrich; Dias, Anderson
A well-ordered natural manganocolumbite single crystal of high quality was used as a prototype for the first determination of the polarized optical phonon modes of materials with the columbite structure. Electron microprobe and X-ray diffraction characterizations determined the chemical formula asMn0.60Fe0.40(Nb0.80Ta0.20)2O6, a cationic ordering of 81%, and the crystal structure as belonging to the Pbcn group. Polarized Raman and infrared-reflectivity spectroscopies on oriented samples allowed us to discern 50 of the 54 predicted gerade (Raman) modes and 31 of the 38 predicted ungerade (infrared) modes for the columbite structure. The selection rules were verified, and polarization leaks only due to slight sample misorientation, confirming the high purity, ordering and quality of the material. From the polar phonon spectra, intrinsic dielectric merit factors <εr> = 29.2 and of 64 THz were determined, showing adequate values for designing applications in microwave circuitry.
Production of Sr-deficient bismuth tantalates from microwave–hydrothermal derived precursors : structural and dielectric properties
(2007) Dias, Anderson; Moreira, Roberto Luiz
Strontium bismuth tantalates were produced for the first time from microwave–hydrothermal precursors at 200 _C, for 2 h. Structural and dielectric properties were investigated by X-ray diffraction and complex impedance spectroscopy. A high ferroelectric–paraelectric transition temperature of 375 _C ðTcÞ was observed, together with two different dielectric regimes for the ac electrical conductivity below Tc. The activation energies were calculated as 0.155 and 0.531 eV, and are related to conduction by oxygen vacancies. It was concluded that the low activation energies showed by these materials could contribute to their fatigue-free nature.
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.