Raman Spectra Of Single Crystals Research Articles

Comparison of the layered semiconductors GaSe, GaS, and GaSe1−xSx by Raman and photoluminescence spectroscopy

The room-temperature Raman spectra of single crystals of GaSe, GaS, and mixed compounds GaSe1−xSx with 0.02⩽x⩽0.8 were measured with a HeNe laser in confocal configuration. The changes in the spectra indicate changes of the crystal structure. The spectra of pure GaSe and of the mixed compound with x=0.02 show pronounced photoluminescence signals blueshifted from the laser line, whereas these signals do not appear for higher sulfur content. Their origin is interpreted as second-harmonic generation in the laser focus causing the formation and radiative decay of Wannier excitons. Two-photon absorption is ruled out, since the effect is absent in the centrosymmetric crystals with x>0.02. With a green laser whose photon energy is larger than the band gap, strong photoluminescence is also observed in crystals with higher sulfur content.

Single crystal Raman spectrum of uvarovite, Ca3Cr2Si3O12

Single-crystal Raman spectra of synthetic end-member uvarovite (Ca3Cr2Si3O12) and of a binary solution (59% uvarovite, 41% andradite) have been measured using single crystal techniques. For each of these garnets, 22 and 21 of the 25 Raman modes were located, respectively. The spectra for uvarovite garnets closely resemble those of the other calcic garnets, grossular, and andradite. The modes for uvarovites do not fit into the same trends as established by the other five anhydrous end-member garnets: the high energy “internal” Si–O modes do not depend on lattice constant in uvarovite. They exceed frequencies for both andradite and grossular. This is likely due to the large crystal field stabilization energy of trivalent chromium. The low energy and midrange modes are at similar frequencies to the other calcic garnets.

Spectral investigations of unsymmetrical π-donor ethylenedithio-dithiadiselenafulvalene iodomercurate, (EDT-DTDSF) 4Hg 3I 8

Spectral investigations of the new organic metal β-(EDT-DTDSF) 4Hg 3I 8 were performed. Polarized reflectance (from 500 to 17000 cm −1) and Raman spectra of single crystals were studied at room temperature. An electronic dispersion in IR spectra was analyzed versus temperature (from 4 to 300 K) in terms of the Drude model and optical transport parameters were determined. Additionally, the optical absorption spectra of the powdered salt dispersed in KBr pellets were measured in the 400–45 000 cm −1 range. The proposed assignment of vibrational features of β-(EDT-DTDSF) 4Hg 3I 8 salt was based on ab initio calculations of the normal modes of EDT-DTDSF 0 and EDT-DTDSF +.

Crystal structure, vibrational spectra and thermal decomposition study of a new, dicationic, acidic pyrophosphate: KHMgP2O7⋅2H2O

Colourless crystals of a new, dicationic, hydropyrophosphate dihydrate salt, KHMgP2O7⋅ 2H2O, are formed, when a mixture of aqueous equimolar solutions of MgCl2⋅4H2O and K4P2O7, at pH 2.12, are left to stand at ambient temperature for 4 days. The new pyrophosphate salt has been characterized by single-crystal X-ray diffraction, IR and Raman spectra, and DSC-TG thermal analysis. The structure consists of alternating layers of [HP2O7]3− groups and MgO6 octahedra, joined by K+ ions and bridging hydrogen bonds. The [P2O7]4− anions exhibit a bent eclipsed conformation. The absence of coincidences for the majority of the IR and Raman bands is in accord with the centrosymmetric structure of the material. The 31P chemical shift tensor components have been determined from the solid-state MAS NMR spectra. The structural changes occurring during thermal decomposition have been examined by TG-DSC, X-ray powder diffraction (XRD) and IR spectroscopy. When the new pyrophosphate salt is heated gradually, it first becomes amorphous and then condenses to chain metaphosphates.

The vibrational spectrum of synthetic hydrogrossular (katoite) Ca3Al2(O4H4)3: A low-temperature IR and Raman spectroscopic study

The powder IR spectra of synthetic hydrogrossular, Ca3Al2(O4H4)(3), were recorded at temperatures between 298 and 10 K and the polarized single-crystal Raman spectra at 298 and 4 K. The results were interpreted using factor group analysis which predicts the number and symmetry of the different IR- and Raman-active modes. A first attempt was made to assign the observed bands in the IR and Raman spectra to various atomic or polyhedral motions and to make a first-order lattice dynamic analysis. The mode assignments for grossular, Ca3Al2(SiO4)(3), and hydrogrossular were compared. The measured spectra are considerably different in the high-wavenumber region, where O-H stretching modes occur, between 298 and 10/4 K. In the Raman spectra two different symmetry O-H bands are observed at 3648 cm(-1) (A(1g) + E-g) and 3653 cm(-1) (F-2g) at room temperature, while at 4 K several O-H bands are present. At room temperature an IR-active O-H band located around 3662 cm(-1) narrows and shifts to higher wavenumbers and also develops structure below about 80 K. Concomitantly, several weak intensity O-H bands located around 3600 cm(-1) begin to appear and they become sharper and increase in intensity with further decreases in temperature down to 10 K. The spectra indicate that the vibrational behavior of individual OH groups and their collective interactions measurably affect the lattice dynamic (i.e., thermodynamic) behavior of hydrogrossular. The (O4H4) group in garnet is best described as a chemical component that can substitute for SiO4 tetrahedra and not as a structural or polyhedral unit. The line widths of the IR and Raman O-H bands at low and room temperature were interpreted qualitatively based on the time scales associated with the Raman and IR experiment, which are different in magnitude. The results of this study demonstrate the need for making spectroscopic measurements at low temperatures when studying the vibrational behavior of O-H modes in minerals.

Electronic Raman spectroscopy of the vanadium(III) hexaaqua cation in guanidinium vanadium sulphate: Quintessential manifestation of the dynamical Jahn–Teller effect

Single-crystal Raman spectra are presented for the salt [C(NH2)3][V(OH2)6](SO4)2, displaying electronic transitions between the trigonal components of the vanadium(III) 3T1g(Oh) ground term. The 3A-->3E(C3) electronic Raman band is centered at approximately 2720 cm-1, and exhibits extensive structure, revealing the energies of the spinor components of the 3E(C3) term for the two crystallographically distinct [V(OH2)6]3+ cations. The data are interpreted in conjunction with parameters previously reported from an electron paramagnetic resonance study of the salt. A satisfactory reproduction of the electronic Raman profile and ground-state spin-Hamiltonian parameters is achieved by employing a (3A plus sign in circle3E)multiply sign in circle e vibronic coupling model, in which the spin-orbit splitting of the 3E(C3) is quenched significantly by the Ham effect, and the intensity of harmonics of the Jahn-Teller active vibration enhanced by their proximity to the electronic Raman bands. The model gives an excellent account of the intensities of the electronic Raman bands, which are shown to depend profoundly on both temperature and the selected component of the polarizability tensor. The electronic Raman profile changes notably upon deuteriation, a result that exposes deficiencies in the single-mode coupling model.

Polarised IR and Raman spectra of the γ-glycine single crystal

Complete (full) set of the polarised IR and Raman spectra for the γ-glycine single crystal at room temperature are presented. The polarised IR spectra were measured by the specular reflection method and the spectra of the imaginary parts of the refractive indices were computed by Kramers–Kronig transformation. The polarised properties of the bands are discussed with respect to the normal coordinate analysis (literature data) and diffraction crystal data (oriented gas model approximation). A very good agreement between the polarised properties of the bands and simple models of vibrations are observed for the stretching vibrations of the CH 2 and COO − group. It is not the case for most of the deformation vibrations of the carboxylic group and of the skeleton. The polarization properties of the stretching vibrations of the NH 3 + group are determined by their hydrogen bondings.

Raman Spectra of Cadmium Titanate

Polarized Raman spectra of CdTiO3 single crystals are recorded for the first time over the frequency range 5 < ν < 1000 cm−1 at temperatures of 10 to 1200 K. The emphasis was on the low-frequency range, where an anomalous temperature dependence of a few phonon modes was observed. At high temperatures, four phonon modes exhibiting a behavior typical of soft modes were found to exist. These phonon modes are assumed to restore the cubic symmetry of the lattice. Their extrapolated temperature dependences suggest that there exists a sequence of three hypothetical high-temperature phase transitions analogous to those observed in the genuine perovskite CaTiO3. At temperatures below 78 K, the Raman spectrum exhibits new lines associated with polar distortions of the unit cell. At low frequencies, three lines are observed whose parameters exhibit an anomalous behavior typical of soft modes in a ferroelectric phase. Several different polar states are assumed to exist at low temperatures.

Raman spectra of lead chalcogenide single crystals

Raman spectra of single crystals of lead chalcogenides (PbTe, PbSe, PbS) were studied at room temperature and ambient pressure. The structure of spectra for all compounds is rather similar showing the bands in one- and two-phonon range. Possible spectra identification is discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman scattering of light in doped cubic boron nitride

The Raman spectra of single crystals of cubic boron nitride (cBN) doped with beryllium and also synthesized in the absence of dopants are investigated. It is shown that the wide bands at 215 and 535 cm−1 can be registered in the Raman spectra of the ( $$\bar 1\bar 1\bar 1$$ ) faces of undoped cBN crystals and that the Be impurity penetrates into the cBN crystals predominantly in the direction of these faces. In the spectra of the investigated cBN samples doped with Be, the contour of the TO and LO phonon lines does not take the Fano shape, in contrast to diamond, which can point to an insufficient concentration of the dopant for attaining the Fano interference or to the absence of it in cBN. The wide bands have been investigated on the assumption that they are “boson”-like and result from small-size disorders in both undoped and doped cBN samples with the sp 3 bond deformed by Be.

Polarised IR and Raman spectra of monoglycine dihydrogenphosphate single crystal

Polarised Raman, IR and FIR spectra of the monoglycine dihydrogenphosphate (glycinium dihydrogenphosphate) single crystal samples are presented and discussed with respect to the crystal structure on the basis of oriented gas model approximation. The IR and FIR spectra were measured by specular reflection method and spectra of an imaginary part of the refractive indices were computed by the Kramers-Kronig transformation. The polarisation properties of the internal vibrations of the glycinium cation, H 2PO 4 1− anion and hydrogen bonds are predicted and compared to the experimentally determined.

A temperature-dependent single-crystal Raman spectroscopic study of fayalite: evidence for phonon-magnetic excitation coupling

The polarized single-crystal Raman spectrum of synthetic fayalite, Fe(2)SiO(4), was recorded between 5 and 773 K in order to investigate its lattice dynamic behavior. A broad absorption envelope is observed at wavenumbers between 800 and 960 cm(-1) and it contains two intense bands at 816 and 840 cm(-1) at 293 K in the (cc) spectrum. The integral area of the envelope decreases upon cooling from 293 K and reaches a minimum around 55 K. It then increases again with a further decrease in temperature down to 5 K. It is proposed that the envelope in the (cc) spectra consists of seven different modes, some of which are symmetry-forbidden, that arise from combination scattering of nonsymmetric internal SiO(4)-stretching modes of B(ig) symmetry (i = 1, 2, 3) and low-energy excitations. The individual modes can be observed under different polarizations and agree in number and wavenumber with those obtained by fitting the broad envelope with Lorentzians. An analysis of the Raman spectrum as a function of temperature, using the known magnetic properties of fayalite, allows the assignment of the low-energy excitations to short-range magnetic interactions. Modulation of the Fe(2+)(1)-Fe(2+)(2) exchange energy leads to phonon-magnetic excitation coupling and the main role in the Fe(2+)(1)-Fe(2+)(2) magnetic interaction occurs via superexchange through the oxygens. The magnetic excitations are not magnons in the usual sense, that is as quasiparticles having a long wavelength in an ordered system. The degree of observed broadening of the SiO(4)-stretching modes is consonant with a Fe(2+)(1)-Fe(2+)(2) exchange energy of 4.7 cm(-1) presented by Schmidt et al. (1992). At temperatures above 300 K the line width of the mode at 840 cm(-1) decreases slightly, whereas those of low energy lattice modes increase. This suggests that a decrease in mode broadening due to weakened magnetic interactions compensates any thermally related broadening. Complete Fe(2+) spin disorder may not be reached until at least 530 K. Results from this study show that estimates of third-law entropies for silicates using simple crystal-chemical considerations that do not account for magnetic properties cannot give accurate values for many transition-metal-containing phases.

Phase transition in an occupationally disordered sulfate system

We have investigated the behavior of the Li 1.43Cs 0.57SO 4 single-crystal Raman spectra and dielectric permittivity as a function of temperature. It was observed that the crystal, which is an occupationally disordered system, with orthorhombic C 2 v 2 structure at room temperature, undergoes a phase transition at 230 K. In the new phase the SO 4 tetrahedra remain in two non-equivalent sites and there is no evidence of the doubling of the unit cell.

High pressure raman study of Bi-2212

The hydrostatic pressure dependence of the Raman spectra of Bi2Sr2CaCu2O8 single crystals has been investigated. The energy of the A g and B 1g modes was found to increase with pressure in agreement with previously reported measurements, except the strong mode at ∼465 cm−1, which softens with pressure, while another peak at ∼458 cm−1 appears more pronounced at low temperatures and high pressures. The energy of both modes does not seem to change with increasing pressure, up to ∼5 GPa, although the average energy of the wide band has been found to soften, which is in disagreement with previously published results. Based on the modifications observed in some phonons at ∼1.8 GPa, which correlate with the reduction of T c and the deformations of the CuO5 pyramids, we attribute the mode at ∼465 cm−1 to the vibrations of the apex oxygen atoms. All modes due to the oxygen atoms were found to be strongly anharmonic.

Isomorphic substitution in vanadinite [Pb5(VO4)3Cl]—a Raman spectroscopic study

AbstractThe Raman and infrared spectra of vanadinite [Pb5(VO4)3Cl] specimens from three localities were studied. Raman spectra were found to be both orientation and temperature dependent. Measurement of the Raman spectra at liquid nitrogen temperature allowed better band separation and increased the intensities of weak bands, thus permitting band assignment. Two types of isomorphous substitution were identified: (a) substitution in the bulk of the crystal where electron beam microanalysis identifies the presence of calcium and copper and (b) surface substitution where infrared spectroscopy shows the isomorphous substitution of vanadate by phosphate and of chloride by hydroxyl groups. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Raman scattering and interaction and interference of optical vibrations in a γ-Li3PO4 superionic crystal

Polarized Raman spectra of γ-Li3PO4 single crystal are investigated at temperatures approximately up to 700 K. It is shown that vibrations of PO4 tetrahedrons and cations of the lithium sublattice can be separated in the spectra. It is found that an increase in temperature leads to interference of one-phonon optical modes of symmetry B 2g and A g , which is accompanied by antiresonance in a frequency range near 190 cm−1. Numerical analysis of the spectrum in the region of interaction of optical modes reveals a strong temperature dependence of the interaction constant, which is determined by anharmonic coupling of optical and acoustic modes.

Characterization and spectral properties of the new organic metal (BEDT-TTF) 6(Mo 8O 26)(DMF) 3

A new charge transfer salt based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the [Mo 8O 26] 4− anion was synthesized and characterized by X-ray and spectral analysis. The crystals are triclinic, space group P 1 ̄ , a=12.996(3) Å, b=13.135(3) Å, c=20.705(5) Å, α=72.01(2)°, β=85.94(2)°, γ=60.02(2)°, V=2896.9 Å 3, Z=1. The structure consists of alternating organic and inorganic layers. The inorganic layers are formed by [Mo 8O 26] 4− anions and dimethyl formamide (DMF) molecules. The organic layers are built of two different types of BEDT-TTF molecules bonded by short S … S contacts. The compound is a metal with σ≈3 S cm −1 at room temperature. IR and Raman spectra of single crystals were analyzed and an assignment of the vibrational features was proposed. The temperature dependencies of transport parameters were obtained using the Drude–Lorentz model.

Raman spectroscopic study of H2O in bikitaite: “One-dimensional ice”

The zeolite bikitaite, Li 2 [Al 2 Si 4 O 12 ]·2H 2 O, has structural channels containing infinite chains of H 2 O molecules running parallel to [010]. One hydrogen atom of an H 2 O molecule is weakly hydrogen bonded to an O atom of a neighboring molecule, while the other hydrogen atom is unbonded. The molecules are ordered and the chains they form have been called “one-dimensional ice.” Polarized Raman spectra of single crystals in the wavenumber range 40-4000 cm -1 were measured from 5 to 625 K. At low temperatures, four different O-H stretching vibrations can be observed between 3330 and 3600 cm -1 , as well as H 2 O bending vibrations at about 1640-1650 cm -1 . The two lower wavenumber hydrogen-bonded O-H stretching modes increase in wavenumber with increasing temperature, while the higher wavenumber non-hydrogen-bonded OH modes decrease in wavenumber. The temperature dependence of the linewidths of the O-H stretching modes and the degree of hydrogen bonding between neighboring H 2 O molecules show that the main cause of line broadening is modulation of the OH potential from low-energy thermal O···O vibrations in the H 2 O chains. At elevated temperatures, the different O-H stretching modes become similar in energy and only a single symmetric H 2 O stretching band is observed above 520 K. At these temperatures the H 2 O molecules lose their hydrogen bonding and are only bonded to Li cations at the walls of the channels.

Raman spectra of shocked diamond single crystals

Results from time-resolved spectroscopy, used to examine the triply degenerate Raman line of diamond shocked to peak longitudinal stresses of 45 GPa in plate impact experiments, are analyzed here. The complete lifting of the degeneracy along [110] and partial lifting of the degeneracy along [100] are of particular interest. The line splittings and associated frequency shifts were analyzed using a phenomenological model based on lattice dynamics calculations. The three parameters in the model, related to the anharmonic contributions to the crystal potential, were determined unambiguously from the [110] data. Our results differ from previous values reported in the literature. Subsequent predictions of the [100] response using these model parameters are in excellent agreement with the measured results. Weighted centroids of the shifted frequencies are compared with static high-pressure measurements of the degenerate Raman line. Good agreement is demonstrated for compression to 4%, suggesting that the centroid location is not influenced by nonhydrostaticity. Nonlinear elastic stress-strain relations are presented for shock compression along different crystallographic orientations in diamond. Present analysis permits determination of diamond Raman shifts for any arbitrary deformation.

Temperature‐dependent Raman study of L‐arginine hydrochloride monohydrate single crystal

AbstractPolarized Raman spectra of L‐arginine hydrochloride monohydrate single crystal were studied at temperatures ranging from 295 to 10 K. The behavior of the linewidth, wavenumber and intensity of several bands is discussed and related to some particular vibrational modes associated with hydrogen bonds or even to other vibrations, such as CN, CCC, CCN and CH vibrations. The disappearance of some bands in the lattice vibrational region was associated with a phase transition undergone by the crystal between 100 and 110 K. Copyright © 2002 John Wiley &amp; Sons, Ltd.