A Raman spectroscopic study of a hydrated molybdate mineral ferrimolybdite, Fe2(MoO4)3·7–8H2O

Raman spectra of two well-defined ferrimolybdite samples, Fe23+(Mo6+O4)3·7–8H2O, from the Krupka deposit (northern Bohemia, Czech Republic) and Hůrky near Rakovník occurrence (central Bohemia, Czech Republic) were studied and tentatively interpreted. Observed bands were assigned to the stretching and bending vibrations of molybdate anions, Fe–O units and water molecules. Number of Raman and infrared bands assigned to (MoO4)2− units and water molecules proved that symmetrically (structurally) nonequivalent (MoO4)2− and H2O are present in the crystal structure of ferrimolybdite. Approximate O–H⋯O hydrogen bond lengths (2.80–2.73 Å) were inferred from the published infrared spectra.

Raman spectroscopy of pyrite in marble from Chillagoe, Queensland

Samples of marble from Chillagoe, North Queensland have been analysed using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. Different types of marble were studied including soft white marble, hard white marble and a black marble. In this work, we try to ascertain why the black marble has this colour. Chemical analyses provide evidence for the presence of minerals other calcite in the marble, including the pyrite mineral. Some of these chemical analyses correspond to pyrite minerals in the black marble. The Raman spectra of these crystals were obtained and the Raman spectrum corresponds to that of pyrite from the RRUFF data base. The combination of SEM with EDS and Raman spectroscopy enables the characterisation of the mineral pyrite in Chillagoe black marble.

The O-H Raman Frequency in Inorganic Acids

The band characteristic of the OH group has been recorded in the Raman spectra of many hydroxides and alcohols. It has not so far been observed, however, in the case of the stronger acids. Using the improved technique of complementary filters recently developed by Ananthakrishnan1, and giving long exposures varying from six to twelve days, I have succeeded in obtaining spectra with sulphuric acid and crystals of iodic, selenious and telluric acids, in which the band is clearly seen in the 4046 A. excitation. Table 1 gives the frequency shifts. The value for boric acid is taken from Ananthakrishnan's paper2.

Laser Raman spectroscopic study of ferroelastic LiCsSO4

Lithium caesium sulphate has been reported to undergo a phase transition from the room temperature orthorhombic phase with space groupP cmn to a final phase with space groupP 22/n. Though a sharp anomaly in its physical properties has been found at 202.0;K, it was found that there was a need for careful investigations in the vicinity of 240 and 210.0;K. Since the changes in the crystal structure involve primarily a rotation of the SO4 tetrahedron about thec-axis and as this may be reflected both in the intensity and polarisation of the internal as well as external phonon modes, the laser Raman spectra of oriented single crystals of LiCsSO4 at different temperatures were investigated. For correlation and definite identification of the spectral features, its infrared absorption spectrum was also studied. An analysis of the intensities and polarizations of the internal modes of the sulphate ions reveals the change in symmetry of the crystal. The integrated intensity and peak height of thev 1 line, plotted against temperature show anomalous peaks in the region of the phase transition. Differential scanning calorimetric study gives the enthalpy change ΔH across the phase transition to be 0.213 kJ/mol.

Vibration spectra of ferroelectric alkali trihydrogen selenites and the nature of the hydrogen bond potential in these crystals

Raman spectra of the ferroelectric LiH3 (SeO3)2 and NaH3(SeO3)2 and the anti-ferroelectric KH3 (SeO3)2 have been recorded at room temperature using a He-Ne and also an Ar-ion laser source. The infrared absorption spectra of these crystals and their deuterated analogues have been recorded in the region 400–4000 cm−1 both below and above the Curie temperature. From an analysis of the spectrum in the region 400–900 cm−1 it is concluded that (i) in LiH3 (SeO3)2 the protons are ordered in an asymmetric double minimum potential with a low barrier and the spectrum can be interpreted in terms of HSeO3− and H2SeO3 vibrations, (ii) in NaH3 (SeO3)2 all three protons occupy a single minimum potential at room temperature and below the transition temperature the groups HSeO3− and H2SeO3 are present, (iii) the proton at the inversion centre in KH3(SeO3)2 is in a broad troughed potential well and the low temperature spectrum is more likely to be due to H3SeO3+ and SeO32− species. This deviation of the spectrum from that of the previous two crystals is attributed to the difference in H-bond scheme and hence the absence of any cooperative motion of protons in this crystal.

A Raman study of multiferroic LuMnO3

Magnetic and dielectric measurements confirm the multiferroic nature of LuMnO3. Raman spectra of LuMnO3 have been recorded in the 77-800 K range covering both the antiferromagnetic transition at 90 K and the ferroelectric-paraelectric transition at 750 K. The changes in the phonon modes frequencies and band-widths indicate the presence of phonon-spin coupling in the antiferromagnetically ordered phase. The ferroelectric-paraelectric transition is accompanied by the broadening and disappearance of many of the phonon modes. Some of the phonon modes also show anomalies at the ferroelectric transition.

Raman evidence for orbiton-mediated multiphonon scattering in multiferroic TbMnO3

Temperature-dependent Raman spectra of TbMnO3 from 5 to 300 K in the spectral range of 200-1525 cm(-1) show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first-order Raman mode is nearly constant and high (similar to 0.6) at all temperatures, suggesting an orbiton-phonon mixed nature of the high frequency mode. One of the first-order phonon modes shows anomalous softening below T-N (similar to 46 K), suggesting a strong spin-phonon coupling.

Studies of electronic structure by x-ray Raman and emission spectroscopy: applications to MgB2 superconductors and high pressure physics

X-ray Raman scattering and x-ray emission spectroscopies were used to study the electronic properties and phase transitions in several condensed matter systems. The experimental work, carried out at the European Synchrotron Radiation Facility, was complemented by theoretical calculations of the x-ray spectra and of the electronic structure. The electronic structure of MgB2 at the Fermi level is dominated by the boron σ and π bands. The high density of states provided by these bands is the key feature of the electronic structure contributing to the high critical temperature of superconductivity in MgB2. The electronic structure of MgB2 can be modified by atomic substitutions, which introduce extra electrons or holes into the bands. X ray Raman scattering was used to probe the interesting σ and π band hole states in pure and aluminum substituted MgB2. A method for determining the final state density of electron states from experimental x-ray Raman scattering spectra was examined and applied to the experimental data on both pure MgB2 and on Mg(0.83)Al(0.17)B2. The extracted final state density of electron states for the pure and aluminum substituted samples revealed clear substitution induced changes in the σ and π bands. The experimental work was supported by theoretical calculations of the electronic structure and x-ray Raman spectra. X-ray emission at the metal Kβ line was applied to the studies of pressure and temperature induced spin state transitions in transition metal oxides. The experimental studies were complemented by cluster multiplet calculations of the electronic structure and emission spectra. In LaCoO3 evidence for the appearance of an intermediate spin state was found and the presence of a pressure induced spin transition was confirmed. Pressure induced changes in the electronic structure of transition metal monoxides were studied experimentally and were analyzed using the cluster multiplet approach. The effects of hybridization, bandwidth and crystal field splitting in stabilizing the high pressure spin state were discussed. Emission spectroscopy at the Kβ line was also applied to FeCO3 and a pressure induced iron spin state transition was discovered.

Anomalous Raman scattering from phonons and electrons of superconducting FeSe0.82

We report interesting anomalies in the temperature dependent Raman spectra of FeSe0.82 measured from 3 K to 300 K in the spectral range from 60 to 1800 cm(-1) and determine their origin using complementary first-principles density functional calculations. A phonon mode near 100 cm-1 exhibits a sharp increase by similar to 5% in the frequency below a temperature T-s (similar to 100 K) attributed to strong spin-phonon coupling and onset of short-range antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm-1 and 1600 cm-1, attributed to electronic Raman scattering from (x(2)-y(2)) to xz/yz d-orbitals of Fe. (C) 2010 Elsevier Ltd. All rights reserved.

Cystine peptides. Disulfide conformations in fourteen membered loops investigated by Raman spectroscopy and circular dichroism

NHCH3 (X = Gly 1, Ala 2, Aib 3, Leu 4 and D-Ala 5), have been investigated by Raman and circular dichroism (CD) spectroscopy. Solid state Raman spectra are consistent with β-turn conformations in all five peptides. These peptides exhibit similar conformations of the disulfide segment in the solid state with a characteristic disulfide stretching frequency at 519 ± 3 cm-1, indicative of a trans-gauche-gauche arrangement about the Cα—Cβ—S—S—Cβ—Cα bonds. The results correlate well with the solid state conformations determined by X-ray diffraction for peptides 3 and 4. CD studies in chloroform and dimethylsulfoxide establish solvent dependent conformational changes for peptides 1, 3 and 5. Disulfide chirality has been derived using the quadrant rule. CD results together with previously reported nuclear magnetic resonance (n.m.r.) data suggest a conformational coupling between the peptide backbone and the disulfide segment

Vibrational Spectra and Normal Coordinate Analysis of N-Methyl Thiourea

The infrared spectra of N-methylthiourea (NMTU) and its N-deuterated and S-methylated species were measured. Assignment of the infrared and Raman spectra of NMTU has been accomplished by correlation with thiourea and by use of infrared band shifts on N-deuteration as well as S-methylation. Normal coordinate analysis was performed for all the fundamentals of NMTU and NMTU-d3, the assignments obtained from the force field calculations being discussed in relation to those in other related thioureas and thioamides. The potential barriers to the internal rotations for the �NH2, �CH3, and �CN groups were estimated from the force constants.

Raman induced phase conjugation spectroscopy

Raman induced phase conjugation (RIPC) spectroscopy is a relatively new coherent Raman spectroscopic (CRS) technique using optical phase conjugation (OPC), with which complete Raman spectra of transparent media can be obtained. It is a non-degenerate four-wave mixing technique in which two pulsed laser beams at Ω1 and Ω1 ± Δ where A corresponds to a vibrational frequency of a nonlinear medium mix with a third laser beam at Ω1 to generate a fourth beam Ω1 ± Δ, which is nearly phase conjugate to one of the beams at Ω1. With this technique one can measure the ratio of the resonant and nonresonant components of the third-order nonlinear susceptibilities of the nonlinear media. We have used this technique to get Raman spectra of well-known organic solvents like benzene etc., using pulsed Nd: YAG -dye laser systems. We have also studied the effect of delaying one of the interacting beams with respect to the others and the phase conjugate property of RIPC signals.

Pressure-induced phase transitions in LiRbSO4: a Raman spectroscopic study

Room-temperature Raman spectra of LiRbSO4 were studied as a function of pressure up to 170 kbar for two different orientations of the crystal. Four pressure-induced phase transitions at about 2, 17, 32 and 57 kbar were observed. The transitions at 17 and 57 kbar have slow kinetics, taking about 4 h for their completion. These phase transitions are associated with the orientations of the SO4 ions in the unit cell.

Raman spectral studies of borate glasses

Raman spectroscopic measurements in borate glasses have been reviewe. The review shows that the technique is useful in identifying the structural groups present in the borate on the basis of the Krogh-Moe hypothesis. Vitreous B2O3 and alkali borates are extensvvely studied and a satisfactory assignment of bands is possible by a careful consideration of the literature. A cation effect on the borate netwoork is observed. Availaable measurements on binary borates other than alkali borates and on ternary borates are limited and more work is required to identify the structural modifications that take place with composition. Mixed alkali effect is reported only lithium-caesium borade and shows the formation of non-bridging oxygens, destroying the six-membered rings when Li2O is replaced by Cs2O. Fast ionic glasses (alkali borates containing alkali halides) yield the same Raman spectra as the alkali borates, except when the alkali is a fluoride.

Magnetic Raman scattering from 1D antiferromagnets

We study Raman scattering from 1D antiferromagnets within the Fleury-Loudon scheme by applying a finite temperature Lanczos method to a 1D spin-half Heisenberg model with nearest-neighbor (J(1)) and second-neighbor (J(2)) interactions. The low-temperature spectra are analyzed in terms of the known elementary excitations of the system for J(2) = 0 and J(2) = 1/2. We find that the low-T Raman spectra are very broad for \J(2)/J(1)\ less than or equal to 0.3. This broad peak gradually diminishes and shifts with temperature, so that at T > J(1) the spectra are narrower and peaked at low frequencies. The experimental spectra for CuGeO3 are discussed in light of our calculations.