Search results for "RAMAN"
showing 10 items of 1328 documents
Stability of the fergusonite phase in GdNbO 4 by high pressure XRD and Raman experiments
2017
Abstract We describe the results of high pressure x-ray diffraction and Raman measurements on gadolinium orthoniobate. The ambient pressure monoclinic fergusonite phase remains stable in a remarkable large pressure range. There is no significative evolution of the monoclinic distortion up to 25 GPa , the maximum pressure achieved. Instead, the anisotropic compressibility is associated to the stiffness of NbO 4 tetrahedra in respect to the GdO 8 polyhedra. The high pressure evolution of external modes parallels the wavenumber dependence on ionic radius along the lanthanide series. The chemical pressure analogy is attributed to the compression of GdO 8 polyhedra. There is no evidence of any p…
Crystal structure and phase transition of TlReO4: a combined experimental and theoretical study
2020
Abstract The present work describes a density-functional theory (DFT) study of TlReO4 in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P21/c) is observed at room temperature while two isostructural tetragonal structures (space group I41/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic str…
Raman studies of isotope effects in Si and GaAs
1999
Abstract We have measured by Raman scattering changes of the optic phonon energy and line width in Si and GaAs with isotopic composition. The phonon energies of isotopically pure samples show the expected dependence on the average atomic mass in Si and the reduced mass in GaAs, respectively, as well as small anharmonic contributions. In isotopically disordered samples we find frequency shifts of 1.15(20) cm−1 for 28Si0.530Si0.5 and 0.31(20) cm−1 for the TO phonon of natGaAs, induced by mass disorder which also contributes to the line broadening. We give theoretical estimates of these effects.
FIRST-PRINCIPLES LCAO CALCULATIONS ON 5D TRANSITION METAL OXIDES: ELECTRONIC AND PHONON PROPERTIES
2009
ABSTRACT First-principles quantum chemistry calculations within the periodic linear combination of atomic orbitals (LCAO) formalism have been used to probe electronic and phonon properties of crystalline 5d transition metal oxides ReO3, ZnWO4 and CaWO4. The obtained equilibrium crystal structure is in good agreement with known crystallographic data. Rhenium trioxide is correctly predicted to be a metal, whereas both tungstates—a wide gap insulating compounds. The phonon frequencies, calculated by the frozen phonon method, agree rather well with those obtained by infrared and Raman spectroscopies.
High-pressure synthesis of boron-rich chalcogenides B12S and B12Se
2022
The authors thank Drs. I. Dovgaliuk and T. Chauveau for assistance with Rietveld analysis; and Drs. V. Bushlya and A. Jamali for help with EDX/SEM measurements. This work was financially supported by the European Union's Horizon 2020 Research and Innovation Program under Flintstone2020 project (grant agreement No 689279).
Optical Contrast and Raman Spectroscopy Techniques Applied to Few-Layer 2D Hexagonal Boron Nitride
2019
The successful integration of few-layer thick hexagonal boron nitride (hBN) into devices based on two-dimensional materials requires fast and non-destructive techniques to quantify their thickness. Optical contrast methods and Raman spectroscopy have been widely used to estimate the thickness of two-dimensional semiconductors and semi-metals. However, they have so far not been applied to two-dimensional insulators. In this work, we demonstrate the ability of optical contrast techniques to estimate the thickness of few-layer hBN on SiO2/Si substrates, which was also measured by atomic force microscopy. Optical contrast of hBN on SiO2/Si substrates exhibits a linear trend with the number of h…
The structural, phonon and optical properties of [CH3NH3]M0.5CrxAl0.5−x(HCOO)3 (M = Na, K; x = 0, 0.025, 0.5) metal–organic framework perovskites for…
2019
We report the structural, phonon and luminescence studies of six heterometallic perovskite-type metal–organic frameworks (MOFs) templated by methylammonium cations (CH3NH3+ and MeA+) with the following formulae: [MeA]Na0.5Cr0.5(HCOO)3 (MeANaCr), [MeA]K0.5Cr0.5(HCOO)3 (MeAKCr), [MeA]Na0.5Al0.5(HCOO)3 (MeANaAl), [MeA]K0.5Al0.5(HCOO)3 (MeAKAl), [MeA]Na0.5Cr0.025Al0.475(HCOO)3 (MeANaAlCr, 5 mol% of Cr3+ ions) and [MeA]K0.5Cr0.025Al0.475(HCOO)3 (MeAKAlCr, 5 mol% of Cr3+ ions). All of them crystallise in a monoclinic system (P21/n space group) with one MeA+ cation in an asymmetric unit forming four medium-strength hydrogen bonds (HBs) with a metal-formate framework. The DSC measurements and XRD s…
Splitting of the surface phonon modes in wurtzite nanowires
2013
We analyze the surface optical modes of GaN nanowires (NW) and perform a comparative study with the characteristics expected for other polar NWs. The theoretical analysis of the modes is performed within the context of the effective medium theory that takes into account the dipolar interaction between neighboring NWs (Maxwell-Garnett approximation). It is shown that deviations of the exciting light from the NWs axis, which coincides with the wurtzite c-axis, result in the anticrossing of two distinct surface phonon branches, leading to their splitting in axial and planar components and the appearance of two peaks in the Raman spectra. Additional calculations are performed that determine th…
Anticrossing of axial and planar surface-related phonon modes in Raman spectra of self-assembled GaN nanowires
2012
cited By 17; International audience; GaN columnar nanostructures usually called nanowires have been investigated by micro-Raman spectroscopy. In addition to conventional Raman scattering by confined optical phonons of a wurtzite structure (i.e., E 2h and QLO modes), an unusual two peaks band centered near 700 cm -1 is observed and analyzed as a function of several experimental parameters (polarization, filling factor, incidence angle). The surface character of these two modes is experimentally confirmed by their high sensitivity to the dielectric constant of the as-grown nanowires surrounding medium. Calculations describing the nanowires' environment by means of an effective dielectric func…
Raman scattering inβ-ZnS
2004
The first- and second-order Raman spectra of cubic ZnS $(\ensuremath{\beta}$-ZnS, zinc-blende) are revisited. We consider spectra measured with two laser lines for samples with different isotopic compositions, aiming at a definitive assignment of the observed Raman features and the mechanisms which determine the linewidth of the first order TO and LO Raman phonons. For this purpose, the dependence of the observed spectra on temperature and pressure is investigated. The linewidth of the TO phonons is found to vary strongly with pressure and isotopic masses. Pressure runs, up to 15 GPa, were performed at 16 K and 300 K. Whereas well-defined TO Raman phonons were observed at low temperature in…