High Frequency Raman Band Research Articles

Structural and dynamic properties of MgO–Al2O3–SiO2 glasses from molecular dynamics simulations and NMR

Magnesium aluminosilicate glasses have a wide range of applications in composite fiber reinforcements, aircraft windshields, due to their excellent chemical stability and mechanical properties. In this work, molecular dynamics simulations were used to obtain the short- and medium-range structural evolution of MgO/Al2O3 in MgO–Al2O3–SiO2 glass. The structural information was verified using a combination of combining Raman and Nuclear Magnetic Resonance (NMR) spectroscopy. The composition-structure-property relationship of the MgO–Al2O3–SiO2 glasses was constructed, using high-temperature viscosity results calculated by the Einstein-Stoke equation. The results showed that with the increase of MgO/Al2O3 ratio, Mg2+ changed from charge compensator to network modifier, which led to the gradual depolymerization of the glass network. Moreover, with the gradual replacement of Si by Al in the glass network, the disorder of the glass system increases, evidenced by the high-frequency Raman bands and the average chemical shifts of [AlOn] (n = 4, 5) shift linearly. In addition, we found that the de-polymerization led to a decrease in viscosity and an increase in fragility. The viscosity is negatively correlated with both the fragility and the atomic diffusion coefficient. It provides a theoretical basis for improving the preparation process of MgO–Al2O3–SiO2 glass and optimizing the glass properties.

Coupled phonons, magnetic excitations, and ferroelectricity in AlFeO3: Raman and first-principles studies

We determine the nature of coupled phonons and magnetic excitations in AlFeO3 using inelastic light scattering from 5 K to 315 K covering a spectral range from 100-2200 cm-1 and complementary first-principles density functional theory-based calculations. A strong spin-phonon coupling and magnetic ordering induced phonon renormalization are evident in (a) anomalous temperature dependence of many modes with frequencies below 850 cm-1, particularly near the magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band positions of high frequency Raman bands between 1100-1800 cm-1, in particular a broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies at ~ 100 K, due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of phonons spectrum and their coupling with spins.

Structural investigation in the TiB 2–(Na 2O·B 2O 3·Al 2O 3) system

Composites in the TiB 2–Na 2O·B 2O 3·Al 2O 3 systems, TiB 2–MBA (MB stands for sodium metaborate and A is Al 2O 3), were prepared by self-propagating high-temperature synthesis (SHS), in simultaneous mode. Selection of these compositions was ruled by the interesting properties of both TiB 2 and double borates of alkali metal and aluminum. The structure of the obtained materials was evaluated by micro-Raman spectroscopy, from room temperature up to 600 °C, and X-ray photoelectron spectroscopy (XPS). Formation of the TiB 2 and TiO 2− x B x phases along with TiO 2 as rutile were identified as titanium speciation in the grain phase embedded in a sodium aluminum borate matrix. Integration of the Raman spectra of the grain phases revealed a TiB 2 content of 16.99% and 23.32% for the two composite investigated 2TiB 2·2MBA and 3TiB 2·5MBA. A constrained-width model for the spectral deconvolution of the high-frequency Raman band was forwarded to calculate the proportion of tetrahedral boron atoms (7.424%) in the blank borate matrix Na 2B 2O 4·Al 2O 3 in solid phase.

Raman scattering boson peak and differential scanning calorimetry studies of the glasstransition in tellurium–zinc oxide glasses

Raman scattering and differential scanning calorimetry (DSC)measurements have been carried out on four mixed tellurium–zinc oxide(TeO2)1 − x(ZnO)x (x = 0.1, 0.2, 0.3, 0.4) glasses under variable temperature, with particular attention being given to therespective glass transition region. From the DSC measurements, the glass transition temperatureTg has been determined for each glass, showing a monotonous decrease ofTg with increasing ZnO content. The Raman study is focused on the low-frequency band of the glasses,the so-called boson peak (BP), whose frequency undergoes an abrupt decrease at a temperatureTd very close tothe respective Tg values obtained by DSC. These results show that the BP is highly sensitive to dynamical effectsover the glass transition and provides a means for an equally reliable (to DSC) determination ofTg in tellurite glasses and other network glasses. The discontinuous temperaturedependence of the BP frequency at the glass transition, along with the absenceof such a behaviour by the high-frequency Raman bands (due to local atomicvibrations), indicates that marked changes of the medium range order (MRO) occur atTg and confirms the correlation between the BP and the MRO of glasses.

Raman evidence for orbiton-mediated multiphonon scattering in multiferroicTbMnO3

Temperature-dependent Raman spectra ofTbMnO3 from 5 to 300 K in the spectral range of 200–1525 cm − 1 show five first-order Raman allowed modes and two high frequencymodes. The intensity ratio of the high frequency Raman band to thecorresponding first-order Raman mode is nearly constant and high (∼0.6) at all temperatures, suggesting an orbiton–phonon mixed nature of the high frequencymode. One of the first-order phonon modes shows anomalous softening belowTN (∼46 K), suggesting a strong spin–phonon coupling.

Raman spectroscopy quantifying the composition of stuffed β-quartz derivative phases in lithium aluminosilicate glass-ceramics

Using Raman spectroscopy method, the attempt was made to quantify the SiO2 content in stuffed β-quartz derivative phases precipitated in lithium aluminosilicate glass-ceramics. The initial glasses were prepared by a conventional melt quenching technique, annealed and heat-treated in a temperature range of 660–850°C. The unit-cell parameters were determined by X-ray diffraction (XRD) analysis. In Raman spectra, the one-mode behavior of complex mixed crystals of a series of stuffed β-quartz derivative phases x(LiAl)(1−x)SiO2 was determined. It was demonstrated that the linear dependence of the frequency of the high-frequency Raman band with SiO2 content can be used for quantifying the SiO2 concentration in the crystalline phase with β-quartz structure. It was found that the linear dependence of the unit cell’s volume of β-quartz derivative phases on the SiO2 content can be used for quantifying the SiO2 content in the crystalline phase with β-quartz structure as well. The results obtained by both independent methods are in a good agreement.

Radiation damage and Raman vibrational modes of single-walled carbon nanotubes

We report on the behavior of Raman scattering for the radial and tangential vibrational modes of single-walled carbon nanotubes (CNT) with different absorption doses of high-energy electron irradiation ( E e = 1.8 MeV). We find that the atomic displacement of carbon from the CNT walls leads to the decrease of their diameter. In the case of the tangential optical vibrations the translational symmetry of CNT breaks down because of the radiation defects. Thus, new vibrational modes both for the armchair and zigzag CNT appear. Substantial changes in the intensities and broadening are observed for the high-frequency Raman bands with a 1.0 MGy absorption dose.

Novel Nanocarbons: Global Topology and Curvature Perspectives

ABSTRACTCarbon nanotubes (both the single- and multi-walled), in the family of nanostructured carbons, are of great interest because of several unsurpassable physical properties and it needs to be shown that they are physically stable and structurally unaltered when subjected to radiation. In addition to testing them for space applications, when exposed to high energy electron beam from transmission electron microscopy, the results seem quite promising in terms of nano-engineering/ nano-manufacturing for producing novel nanocarbons [1-3]. Experimental studies of effects of electron beam irradiation on carbon nanotubes show that multi-walled ones tend to be relatively more robust than their single-walled kins. The increased exposure on an individual bundle of single-wall nanotubes promoted graphitization, pinching, and cross-linking analogous to polymers forming an intra-molecular junction (IMJ) within the area of electron beam focus, possibly through aggregates of amorphous carbon [2,3]. Formation of novel nanostructures (nano-ring and helix-like) due to irradiation are observed. These studies shed light on the dynamics of nanomanufacturing and a regime of possible relevance of these materials for: (i) short-term space missions; (ii) radiation hard programmable logic circuits; and (iii) radiation pressure sensors. It is suggestive that a local reorganization occurs. Through resonance Raman spectroscopy and related techniques we also elucidate an important notion of global topology and curvature at nanoscale which points to an emergent paradigm of Curvature/Topology → Property → Functionality in these technologically important geometries of carbons: nanotubes, fullerenes, nanorings, nanocones, nanohorns and nanodisks. To this end, we have determined the variation in first order high frequency Raman band which indicates a strong electron-phonon coupling. These concepts also apply to nanostructures of other “topological materials” such as BN nanotubes and nanotori, helical gold nanotubes as well as Möbius conjugated polymers.

Advanced Carbon-based Material as Space Radiation Shields

ABSTRACTCarbon-based materials including microcrystalline diamond, nanocrystalline diamond, and carbon nanotubes films were prepared by microwave plasma-assisted chemical vapor deposition (MWCVD) technique. While the former were submitted to gamma radiation doses of 1, 5, and 20 Mrad, the latter to low energy electron beam of 30 keV or to 30 GeV/cm2) to study the radiation-induced structural transformation. The characterizations were performed prior to and after irradiation using Raman spectroscopy, scanning electron microscopy, and X-ray diffraction, techniques. Microcrystalline diamond showed a dramatic modification in the structural properties only after a cumulative dose of 26 Mrad (2 Grad/cm2), while nanocrystalline carbon showed a relatively small but systematic transformation with increasing gamma radiation dose. The results indicate that nanocrystalline carbon tends to reach a state of saturation when submitted to 26 Mrad doses of gamma radiation, suggesting the possibility of fabricating radiation buffer materials that would undergo internal sp3 ⇔ sp2 inter-conversion while absorbing ionizing radiation without changing their average microstructure and protecting the device/material underneath. Single- and multi-walled nanotubes exhibited structural modifications after 5.5–8 hrs of continuous exposure to electron beam. The variation in the characteristic X-ray peaks for multi-walled and single-walled corresponding to intertube spacing and the high frequency Raman band around 1580 cm−1 (G band) are reflected in their corresponding spectra. The results indicate that there is an increase in the intertube spacing for multi-walled nanotubes due to electron irradiation. While single-wall nanotubes tends to ‘collapse’ after > 8 hours of exposure forming multi-wall nanotubes analyzed using scanning electron microscopy and Raman spectroscopy. These C materials can be employed for preventing space radiation from reaching sensitive materials and electronic devices at least for short term experiments and entitled them as ‘space radiation shields’.

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPS n, n=1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS 4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS 3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS 2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS 2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS 1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (>850 cm −1), dramatic shifts and band splitting were observed in the low-frequency region (<500 cm −1) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the C αC mC α bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPS n molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPS n .

Raman monitoring of the dynamical Jahn-Teller distortions in rhombohedral antiferromagnetic LaMnO 3 and ferromagnetic magnetoresistive La 0.98Mn 0.96O 3

Comparative study of temperature dependences of the Raman spectra of rhombohedral antiferromagnetic LaMnO 3 (T N ≈ 150 K) and ferromagnetic magnetoresistive La 0.98Mn 0.96O 3 (T c ≈ 200 K) confirms the assignment of the two broad high frequency Raman bands to dynamical Jahn-Teller lattice distortions and allows to monitor the variations of distortion amplitudes and polaronic lifetime through the insulator-metal transition in La 0.98Mn 0.96O 3.

Study of the overtones and combination bands in the Raman spectra of polyparaphenylene-based carbons

A detailed study of the second-order Raman spectrum of the polymer polyparaphenylene (PPP) prepared according to the Kovacic method and heat treated at temperatures THT between 650 and 750 °C is presented. The Raman experiments have been performed with five different laser excitation energies in the visible range between 1.92 and 3.05 eV. Several Raman bands in the region between 2400 and 3400 cm−1 have been detected and assigned to the overtones and combination bands of the two conformations of the PPP polymer (benzenoid and quinoid) that co-exist in our samples. Due to the carbonization process, these bands broaden and decrease in intensity with increasing heat treatment temperature, as is also observed for the corresponding first-order Raman features. The complete absence of these high-frequency Raman bands for PPP with heat treatment temperatures in excess of 750 °C indicates complete transformation of the polymer into a disordered carbon material.

In situ Raman study of PPP-based disordered carbon as an anode in a Li ion battery

Electrochemical lithium insertion into polyparaphenylene (PPP)-based disordered carbon was investigated using in situ Raman spectroscopy. The peak position of the high frequency Raman band of 1605 cm −1 indicates characteristically at first no change up to 1.0 V and then the peak downshifts from 1605 cm −1 to 1588 cm −1, and finally disappears presumably because of the decrease in optical skin depth giving rise to the conductivity change. For the charging process, the opposite change was observed with some degree of irreversibility in the peak position, reflecting the irreversible discharge/charge capacity of the electrode. The change in peak position is somewhat similar to that observed in well-ordered graphite, which reflects that charge transfer occurs between the Li and the defective carbon layers, even in such a low heat treatment temperature material.

Formation of fluorozirconate glasses under high pressures

Two ternary and one multicomponent fluorozirconate glasses have been prepared by melt quenching under pressures up to 25 kbar in a piston cylinder apparatus. Infrared and Raman spectra of these glasses were recorded at ambient conditions and were similar to the corresponding spectra of fluorozirconate glasses prepared at ambient pressure. However, a decrease in the frequency of an intense high frequency Raman band indicated that the coordination number of zirconium had increased with increasing quench pressure. In situ Raman spectra at pressures up to 90 kbar were recorded at ambient temperature using a diamond anvil cell. A reversible increase in the frequency of the strong Raman band was observed both for the glass quenched at ambient pressure and that formed at 25 kbar. The initial rate of frequency increase was greater for the high pressure glass, confirming that configurational changes had been quenched in the sample prepared at high pressure.

In situ Raman Spectroscopy Characterization of 18O Exchange in Physical Mixtures of Antimony Oxides and Molybdenum Oxide

The interaction of O-18(2) With physical mixtures of MoO3 and Sb2O4 and of MoO3 and Sb2O3 has been investigated by in situ Raman spectroscopy. Signal intensity ratios of the two O-18-related high-frequency Raman bands of MoO3 at 788 and 945 cm(-1) relative to the corresponding O-16-related bands at 820 and 995 cm(-1) lead to the assignment of these bands to the two different terminal Mo=O groups along the a and b axes. At elevated temperatures, the scattering cross section of MoO3 is strongly diminished relative to antimony oxides. MoO3 in the mixture with Sb2O4 does not show enhanced O-18 exchange as compared to pure MoO3, suggesting that efficient spillover of oxygen does not occur in this mixture, probably because of the very slow kinetics of that process. Some spillover during the reexchange with O-16(2) may be indicated by a slightly increased signal intensity of the O-18-related bands. As in the pure compounds, the reexchange with O-16 atoms occurs very slowly and is not influenced by the presence of H2O. Significant reduction of MoO3 in contact with Sb2O3, after evacuation at 648 K as compared to pure MoO3 possibly indicates an intimate cross talk between the two phases, by which MoO3 is reduced via a ''reverse spillover''. Reoxidation of the evacuated sample shows an incorporation of heavy oxygen into the MoO3 lattice. The comparable signal intensities of the O-18-related band and of the line attributed to a suboxide reveal that only oxygen defects are refilled during the O-18 treatment in MoO3. A further oxygen exchange does not seem to take place. In contrast to the reexchange in dry O-16(2), the presence of H2O in the gas phase leads to a distinct decrease of the O-18-related signals. A comparison of the experiments carried out after evaucation at 393 and at 648 K, only shows signals due to O-18 incorporation into MoO3 if MoO3 in the sample had a higher degree of reduction, thus revealing the essential role of lattice defects in the oxygen exchange. The results suggest that in the Sb2O4/MoO3 mixture, surface mobile (spillover) species should be involved to explain the catalytic synergy between the two phases.

Assigning the 625 cm −1 and other high frequency Raman bands of the Z 1 form of DNA using a normal modes analysis and computer graphics

The left-handed zigzag conformation of the ribose phosphate backbone of the double helix of Z-DNA is well established. A normal modes analysis has been performed using the crystal form of d(CpGpCpGpCpG); this is a half turn of double helical Z-DNA. The calculated 625 cm −1 band, which has been shown to be specific for the Z conformation of DNA is discussed with reference to previous work. It would seem that the classical description of vibrational modes in terms of internal coordinates is inadequate for macromolecular systems. The great number of internal coordinates involved in one vibrational mode makes the assignment complex. A new method of interpreting the vibrational modes is described. It is based on the use of Cartesian displacement coordinates to describe the vibrational mode together with computer graphics for its direct visualization.

Cristallochimie des chromites : étude sur des produits de synthèse du système Mg(Al,Cr)2O4

Spectrometries (Raman Laser microprobe, visible spectrometry and microreflectrometry) have been applied to the crystallochemical calibration of members of the Mg(Al,Cr)2O4 series, synthesized by melting in a solar furnace, then analyzed with an electron microprobe and by X-ray diffraction. This study shows that octahedral sites of Cr3+ are more and more distorted when Cr replaces progressively Al, leading possibly to a lowering of the lattice symmetry from O7h, to T2d. The stabilization energy of Cr decreases and the covalent factor increases with the substitution Al → Cr. Two spectral signatures have been shown on polished sections for Cr3+ in the chromite series. First, the reflectance at 600 nm increases linearly from 7 to 12 % with the Cr content increasing. Secondly, the location of the high frequency Raman band varies from 760 cm-1 for the Al-rich end member to 680 cm-1 for the Cr-rich end member, on synthetic members of the quaternay system (Mg,Fe) (Al,Cr)2O4 as well as on natural chromites of different compositions.

Study of aqueous electrolyte solutions by raman and n.m.r. spectroscopy

The ν 5(SO 4 2−) and ν 5(NO 3 −) vibrations, as observed in the Raman spectrum, were studied for several alcali- and earth-alcali sulfates and nitrates in water solution. For the sulfate solutions also the high frequency Raman band of water was observed. The p.m.r. solvent shift for these solutions and for solutions of CdCl 2, MgCl 2, ZnCl 2, CdBr 2 were also investigated. A discussion is presented of possible correlations between the Raman spectral parameters, the ionic contributions to the p.m.r. shifts and the structure of electrolyte solutions.

The high frequency raman band of water in electrolyte solutions

The high frequency Raman band of water is studied in electrolyte solutions. The relative scattering yield as a function of Raman frequency is shown to be a very sensitive parameter. Distinct regions of the band are seen to be influenced specifically by the anions resp. the cations. Depolarization ratio measurements confirm these findings.

The Raman spectrum of boric acid

The Raman spectrum of crystalline boric acid is recorded using mercuryλ2537 excitation. Fifteen Raman lines, three of them belonging to the lattice spectrum, are reported. Satisfactory assignments of all the observed Raman frequencies are made using the available X-ray crystal structure data. From the presence of a new high frequency Raman band at about 3420 cm.−1 it is suggested that there might be a small number of long, weak O-H....O hydrogen bonds in the crystal, in addition to the hydrogen bonds of moderate strength reported from X-ray diffraction data.