Roberto Luiz Moreira

Unusual Angular Dependence of the Raman Response in Black Phosphorus

Anisotropic materials are characterized by a unique optical response, which is highly polarization-dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals that are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals and a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.

Infrared phonon dynamics of a multiferroic BiFeO3 single crystal

R. P. S. M. Lobo,1,* R. L. Moreira,2 D. Lebeugle,3 and D. Colson3 1Laboratoire Photons et Matiere (CNRS UPR 5), ESPCI, Universite Pierre et Marie Curie, 10 rue Vauquelin, 75231 Paris Cedex 5, France 2Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Caixa Postale 702, Belo Horizonte, Minas Gerais, 31270-901, Brazil 3Service de Physique de l’Etat Condense (CNRS URA 2464), DSM/DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Received 17 October 2007; published 28 November 2007

Raman-spectroscopic evaluation of the long-range order in Ba(B1/3′B2/3″)O3 ceramics

Ordered Ba(B1/3′B2/3″)O3 ceramics are currently used as dielectric resonators for microwave and millimeter wave technologies. The degree of ordering is generally determined by x-ray techniques. In this work, we demonstrate that Raman spectroscopy can be used to evaluate the degree of long-range order of these materials. We also show how varying the degree of order of samples allows partial assignment of the optical vibrational modes.

Raman-spectroscopic study of lanthanide trifluorides with the β-YF3 structure

We have performed polarized Raman scattering measurements on several Czochralski-grown lanthanide trifluoride (LnF3) crystals presenting the β-YF3 structure (space group: Pnma). The phonon mode assignments for the DyF3 and LuF3 spectra are given for the first time. Besides this, the Raman spectra of TbF3, ErF3 and YbF3 complete and extend previous studies on these materials. The vibrational modes of LnF3 compounds are then correlated with those of the archetype β-YF3 crystal. The totally symmetrical fundamentals present a strong dependence on the scattering geometry, similar to what is shown by molecular crystals with weakly interacting molecules. Most of the Raman features are well behaved with the lanthanide ion substitution. Although the observed modes always involve all ions, it was possible to discern modes that are dominated by the motion of either the lanthanide or the fluorine ions. The intensity of some low frequency modes has proved to be quite dependent on the orthorhombic distortion of the quasi-hexagonal structure.

Raman scattering and X-ray diffraction investigations on hydrothermal barium magnesium niobate ceramics

Abstract Ba(B′1/3B″2/3)O3 perovskites are important ceramic materials used as resonators in microwave technologies due to their adequate dielectric properties. These properties are mainly determined by the characteristics of the optical polar phonons of the material. A few spectroscopic investigations of lattice vibrations in Ba(B′1/3B″2/3)O3 ceramics were realized in highly ordered samples. In this work, we use Raman spectroscopy and X-ray diffraction to investigate hydrothermal Ba(Mg1/3Nb2/3)O3–BMN–ceramics with different order degree. Spectroscopic and structural data showed sensitivity to the sample microstructural evolution during sintering. Besides, they allowed independent determination of the long range order degree of the materials. The complete Raman mode assignment for the disordered and ordered BMN structure is also presented.

Microstructural dependence of the magnetic properties of sintered NiZn ferrites from hydrothermal powders

The magnetic properties after sintering of Ni0.34Zn0.60Fe2.06O4 from hydrothermal submicrometer-sized powders were evaluated. The results from hysteresis magnetic loops showed high saturation magnetization (5000 < 4πMs < 7000 G), low remanence (160 < 4πMr < 300 G) and coercitivity (22 < Hc < 35 Oe) values. The microstructural evolution was correlated to the magnetic parameters and the relations between them were discussed. The absolute values of saturation were compared to other NiZn ferrites of similar composition and obtained from different methods described in the literature.

Infrared Spectroscopic Investigation of Chain Conformations and Interactions in P(VDF-TrFE)/PMMA Blends

The blending between poly(methyl methacrylate) (PMMA) and ferroelectric (vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) copolymer chains has been inves- tigated by Fourier transform infrared (FTIR) spectroscopy over the full range of com- position, for the copolymer with 50 mol % of trifluorethylene (TrFE). The FTIR spectra revealed an absorption band at 1643 cm -1 , characteristic of the blend and absent in the individual constituents. We attributed this band to the interaction of the carbonyl group of the PMMA side chains with the disordered helical chains present in the amorphous region of the P(VDF-TrFE). We investigated the consequences of adding PMMA onto the formation of the all trans conformation of the copolymer chains and we demonstrated that the effects of thermal heating on the spectra are relevant only for the samples where the ferroelectric semicrystalline phase is present.

Enhanced dielectric response of GeO2-doped CaCu3Ti4O12 ceramics

CaCu3Ti4O12 ceramic samples were prepared by solid state conventional route using stoichiometric amounts of CuO, TiO2, and CaCO3. Afterward the material was doped with GeO2 with concentrations up to 6% by weight and sintered at 1050 °C for 12 h. The influence of doping on the microstructure, vibrational modes, and dielectric properties of the material was investigated by x-ray diffraction, scanning electron microscopy coupled with an energy dispersive spectrometer, and infrared and dielectric measurements between 100 Hz and 30 MHz. The materials presented huge dielectric response, which increases with doping level relative to undoped CaCu3Ti4O12. The main effect of doping on the microstructure is the segregation of Cu-rich phase in the ceramic grain boundaries. Cole–Cole modeling correlates well the effects of this segregation with the relaxation parameters obtained. The intrinsic phonon contributions for the dielectric response were obtained and discussed together with the structural evolution of the system.

Far-infrared spectroscopy in ordered and disordered BaMg1/3Nb2/3O3 microwave ceramics

Ba(Mg1/3Nb2/3)O3 ceramics with suitable microwave dielectric properties for application in wireless communications and information access technologies were studied by far-infrared spectroscopy. Samples with different B-site ordering degrees, obtained by hydrothermal synthesis followed by sintering at various temperatures, were employed in this investigation. The sixteen infrared modes predicted by factor-group analysis were observed and adjusted according to a four-parameter semiquantum model. The dispersion parameters were determined in order to calculate the real part of the dielectric permittivity and the quality factors associated with the dielectric losses in the microwave region. The materials exhibited increasing e0 and Q values up to 1100 °C, increasing more substantially when the temperature attained 1300 °C. The B-site ordering played an important role on this behavior along with the microstructural evolution above 1100 °C (grain growth), which also increased the phonon lifetime and contributed to the Q improvement. Kramers–Kronig analyses were carried out in all experimental data and the contributions of the main optical polar modes to the dielectric and microwave properties were carefully analyzed in order to identify and attribute the Ba–BO3 external mode, the inner modes related to the O–Mg–O and O–Nb–O bending vibrations, and the stretching modes of each MgO6 and NbO6 octahedron.Ba(Mg1/3Nb2/3)O3 ceramics with suitable microwave dielectric properties for application in wireless communications and information access technologies were studied by far-infrared spectroscopy. Samples with different B-site ordering degrees, obtained by hydrothermal synthesis followed by sintering at various temperatures, were employed in this investigation. The sixteen infrared modes predicted by factor-group analysis were observed and adjusted according to a four-parameter semiquantum model. The dispersion parameters were determined in order to calculate the real part of the dielectric permittivity and the quality factors associated with the dielectric losses in the microwave region. The materials exhibited increasing e0 and Q values up to 1100 °C, increasing more substantially when the temperature attained 1300 °C. The B-site ordering played an important role on this behavior along with the microstructural evolution above 1100 °C (grain growth), which also increased the phonon lifetime and contributed ...

Raman scattering study of RETiTaO6 dielectric ceramics

In this work we have investigated microwave-dielectric ceramics RETiTaO6 (RE=Al, Y, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er or Yb) by Raman scattering measurements at room temperature. The observed Raman-active phonons were analysed in conformity with two orthorhombic structures proposed for these ceramics, namely aeschynite (for lanthanide ions with atomic number between 58 and 66) or euxenite (for Y, Ho, Er and Yb). The results indicate that the phonon spectra remained relatively unchanged with respect to the RE ion substitution within each structure. For the three remaining materials with RE=La, Al and In, which are known to present multiphases, the Raman phonon spectra showed significant differences in comparison with those of ceramics with aeschynite and euxenite structure. # 2003 Elsevier Ltd. All rights reserved.