High-pressure Raman Spectroscopy Research Articles

Identification of inner and outer shells of double-wall carbon nanotubes using high-pressure Raman spectroscopy

The high-pressure Raman response of double-wall carbon nanotubes (DWCNTs) in the radial breathing mode (RBM) frequency region can be used for the identification of both the inner and their respective outer tubes. A simple anharmonic model of this system demonstrates that the pressure response of the inner tube in a DWCNT is uniquely defined by the inner-outer tube spacing. Consequently, a plot of the normalized pressure coefficients of the inner tube RBM frequencies as a function of their ambient pressure frequency serves as a map which allows the identification of unknown DWCNTs. The present experimental results form the basis of such a plot.

Pressure-Induced Phase Transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.

Structural characterization ofLu1.8Y0.2SiO5crystals

The structural and vibrational properties of Lu{sub 1.8}Y{sub 0.2}SiO{sub 5} (LYSO) single crystals were investigated by means of Raman spectroscopy and x-ray diffraction measurements. Unit cell parameters and bond lengths were determined by Rietveld refinement of the collected x-ray diffraction powder spectra. By comparison with the vibrational spectra of the parent compounds Lu{sub 2}SiO{sub 5} and Y{sub 2}SiO{sub 5} and by using polarized Raman measurements, we propose the assignment of the principal vibrational modes of LYSO crystals. The strict connection of Raman spectra of the LYSO solid solution and of the pure lutetium and yttrium crystals, as well as the analysis of the polarized measurements, confirms that LYSO structure adopts the C2/c space group symmetry. The structural analogies of LYSO with the pure compound Lu{sub 2}SiO{sub 5} are further shown by means of high pressure Raman spectroscopy, and the possibility of considering the LYSO crystal analogous to the LSO structure with a tensile stress between 0.25 and 0.80 GPa is discussed.

Hydrogen storage properties of clathrate hydrate materials

The design of hydrogen storage materials that achieve all of the U.S. Department of Energy's 2015 goals (storage capacity, operating conditions, reversibility, cost, etc.) has proved to be a challenge. Recent progress in the area of clathrate hydrates suggests many potential advantages when compared with other “conventional” materials. In this work, the hydrogen storage potential of several clathrate systems (including sII, semi-clathrates, and Jeffrey's structures) has been investigated by high-pressure Raman spectroscopy, by direct gas release measurements, and by theoretical storage considerations. Some key findings of this work include new insights into the spectral properties of hydrogen molecules within hydrate cavities and the maximum possible hydrogen storage capacity of different clathrate structures. Ultimately, these findings will aid in the design and production of new hydrogen storage materials.

Noncentrosymmetric phase of submicron NaNbO3 crystallites

The temperature and pressure characteristics of a noncentrosymmetric crystal modification of NaNbO3 were studied by Raman spectroscopy. A transition towards the bulk-like structure of NaNbO3 occurs in the temperature range from 280 to 360 °C. High-pressure Raman spectroscopy revealed successive pressure-induced phase transitions at around 2, 6.5 and 10 GPa. Raman scattering profiles recorded above 7 GPa correspond to those reported for the bulk. The temperature-induced spectral changes were completely reversible between −150 and 450 °C. Those induced by pressure were almost reversible from ambient pressure up to 15.9 GPa. Piezoresponse force microscopy demonstrated the occurrence of piezoelectric activity for submicron NaNbO3 crystals with particle size ranging from 200 to 400 nm. The noncentrosymmetric crystallographic structure plays a critical role for the enhancement of piezoelectricity.

Slow amorphization of zeolites

The dynamics of amorphization in two zeolites with different densities is investigated using high-pressure Raman spectroscopy. Slow amorphization of the denser zeolite under pressure leads to the formation of a low-density amorphous (LDA) phase that transforms into a more disordered high-density amorphous (HDA) phase with a further increase in the pressure. It is revealed that the LDA-HDA transformation is a first order phase transition occurring with an increase in the silicon coordination.

Comparative high pressure Raman study of individual and bundled single‐wall carbon nanotubes

AbstractIn this work, individual single‐wall carbon nanotubes (i‐SWCNTs) and their parent bundled material (b‐SWCNTs) are investigated by means of high pressure Raman spectroscopy. Pressure application results in the broadening and intensity redistribution of the radial breathing modes (RBMs) in both materials. These changes are more pronounced in the case of bundled SWCNTs as it is also evident by the abrupt increase of the G+ bandwidth at ∼5.5 GPa, in contrast to the smoother pressure response of the individual tubes. Our experimental findings reflect the size dependent cross‐section deformation of carbon nanotubes which evolves differently in the two materials investigated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Conformations of 1-Butyl-3-methylimidazolium Chloride Probed by High Pressure Raman Spectroscopy

The behavior of 1-butyl-3-methylimidazolium chloride has been investigated byRaman spectroscopy as a function of hydrostatic pressure. Under ambient pressure tworotational isomers (GA and AA forms) of the 1-butyl-3-methylimidazolium cation coexist inthe ionic liquid state. As the supercooled liquid was compressed from ambient to 0.9 GPa,the contribution of the GA conformer decreases in intensity as the pressure was elevated. Anew high pressure phase is formed above the pressure of 1.5 GPa. This new high pressurephase of 1-butyl-3-methylimidazolium chloride arises from perturbed GA conformer, i.e.,distorted Crystal 2. Crystal 1 form (AA form) is known as a thermodynamically stable formunder ambient pressure. Nevertheless, crystal 1 form is switched to a metastable state underthe condition of high pressure.

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host–guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A 1 g and E g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host–guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm −1 and 200 cm −1 that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.

The effect of pressure on charge-enhanced C–H⋯O interactions in aqueous triethylamine hydrochloride probed by high pressure Raman spectroscopy

The weak hydrogen bonding structures of protonated triethylamine in aqueous solution were investigated by Raman spectroscopy as a function of hydrostatic pressure. The formation of a weak hydrogen bond was directly evidenced by a low-frequency shift of the hydrogen-bonded C–H stretches in the protonated triethylamine moiety. For pure triethylamine, an increase in pressure leads to a blue frequency shift of the C–H bands. Nevertheless, the high pressure study of the C–H bands for a dilute protonated triethylamine/D 2O solution yielded an unusual nonmonotonic pressure dependence. This result indicates charge-enhanced C–H⋯O hydrogen-bond formation at high pressure.

In situ observation of amorphous-amorphous apparently first-order phase transition in zeolites

In this letter, the authors present the observation of the phase transition between low-density amorphous (LDA) and high-density amorphous (HDA) zeolites using a high pressure Raman spectroscopy. It is found that this transition is apparently of the first order and occurs with a silicon coordination rise. It is shown that the Raman spectra of the LDA-HDA phase transitions in zeolites and in silicon are almost identical, suggesting a generality of amorphous-amorphous transformations both in simple substances and in complex polyatomic materials with tetrahedral configurations.

High pressure Raman spectroscopy of single-walled carbon nanotubes: Effect of chemical environment on individual nanotubes and the nanotube bundle

Abstract The pressure-induced tangential mode Raman peak shifts for single-walled carbon nanotubes (SWNTs) have been studied using a variety of different solvents as hydrostatic pressure-transmitting media. The variation in the nanotube response to hydrostatic pressure with different pressure transmitting media is evidence that the common solvents used are able to penetrate the interstitial spaces in the nanotube bundle. With hexane, we find the surprising result that the individual nanotubes appear unaffected by hydrostatic pressures (i.e. a flat Raman response) up to 0.7 GPa. Qualitatively similar results have been obtained with butanol. Following the approach of Amer et al. [J. Chem. Phys. 121 (2004) 2752], we speculate that this is due to the inability of SWNTs to adsorb some solvents onto their surface at lower pressures. We also find that the role of cohesive energy density in the solvent–nanotube interaction is more complex than previously thought.

HIGH PRESSURE EFFECTS ON ELECTRICAL RESISTIVITY AND DIELECTRIC PROPERTIES OF NANOCRYSTALLINE SnO2

Rutile structured nanocrystalline tin oxide (nano- SnO 2) was prepared by chemical precipitation method with different grain sizes. Its electrical and dielectric properties were studied using complex impedance spectroscopy under different hydrostatic pressures. These studies showed a transition in nano- SnO 2 under high-pressure. The transition pressures obtained from both the resistivity and dielectric measurements agree with each other and increase considerably with decrease in grain size. In order to find whether the transition under pressure is structural related or not, in situ high pressure Raman spectroscopy was done up to 3.38 GPa at room temperature. No structural change was observed and the transition may be due to the co-operative phenomenon of the change in band gap and better connectivity between grain boundaries.

Phase transition of polycrystalline BaTiO3 at high-pressure detected by a pulsed photoacoustic technique

Sound velocity changes of unpolarized polycrystalline BaTiO3, in the diamond anvil cell (DAC) up to 6.8 GPa, across the tetragonal to cubic phase transition at room temperature (RT) have been detected by a photoacoustic technique. The light absorption before, during, and after the structural phase transformation shows changes. This is very clear using a photoacoustic method where a pulsed laser is used as a standard source of ultrasound. The phase transition is recognized by a profile generated through sequence values of the temporal peak positions acquired from sequential measurements of the pulsed photoacoustic signal (PA) obtained for each pressure applied. The results are in agreement with those obtained by high-pressure Raman spectroscopy carried out under similar conditions. In both techniques a 4:1 methanol–ethanol mixture was used as a pressure transmitting medium, and the pressure was calibrated using the ruby fluorescence technique.

CuAl 2 revisited: Composition, crystal structure, chemical bonding, compressibility and Raman spectroscopy

The structure of CuAl 2 is usually described as a framework of base condensed tetragonal antiprisms [CuAl 8/4]. The appropriate symmetry governed periodic nodal surface (PNS) divides the space of the structure into two labyrinths. All atoms are located in one labyrinth, whereas the second labyrinth seems to be ‘empty’. The bonding of the CuAl 2 structure was analyzed by the electron localization function (ELF), crystal orbital Hamiltonian population (COHP) analysis and Raman spectroscopy. From the ELF representation it is seen, that the ‘empty’ labyrinth is in fact the place of important covalent interactions. ELF, COHP in combination with high-pressure X-ray diffraction and Raman spectroscopy show that the CuAl 2 structure is described best as a network built of interpenetrating graphite-like nets of three-bonded aluminum atoms with the copper atoms inside the tetragonal-antiprismatic cavities.

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.

High Pressure Raman Spectroscopic Study of the Effects of n‐Ethylamines and Water on the 2‐Nitropropane/Nitric Acid System

AbstractHigh pressure Raman spectroscopy measurements in a diamond anvil cell (0–10 GPa) on 2‐nitropropane/nitric acid/X (X=triethylamine, diethylamine, and water) ternary systems and 2‐nitropropane/nitric acid/water/Y (Y=triethylamine and diethylamine) quaternary systems are reported. The modifications of the chemical behavior of the 2‐nitropropane/nitric acid model system, induced by the presence of triethylamine, diethylamine, and/or water, were studied at ambient and high pressure. At ambient pressure, the ionization of the nitric acid has been observed with each of the additives. Moreover, in the case of ethylamines, new peaks have been observed and the hypothesis of a 2‐nitropropane/ethylamine complex is advanced. At high pressure, the decomposition of the 2‐nitropropane/nitric acid system, with an oxygen balance near zero, has been observed only in presence of triethylamine. The role of each additive to the 2‐nitropropane/nitric acid system in the modification of the respective reducing and oxidizing character of the components, and in the reactivity of the system, is discussed. Several hypotheses are advanced concerning the sensitizing effect of the additives on the 2‐nitropropane/nitric acid system.

Intermolecular Hydrogen Bonding and Structures in 1,3-Dioxane/D2O Mixtures Studied by High-Pressure Raman Spectroscopy

The ability of the equatorial C-H bonds to form C-H—O hydrogen bonds was studied by measuring the high-pressure Raman spectra. We have investigated the effect of pressure versus C-H—O interactions in 1,3-dioxane/D2O mixtures. Dilution of 1,3-dioxane leads to a shift of the C-H signals to higher frequencies under ambient pressure. Based on the pressure-dependence of the Raman spectra, we have found that the equatorial C-H groups of 1,3-dioxane displays a remarkably different ability to serve as the weak hydrogen bond donors in comparison to the axial C-H of 1,3-dioxane. The new Raman spectral features observed under high pressure may arise from the combined effect of hydrogen-bond network changes, hydrogen bond-like C-H—O interactions formation, cluster size changes, etc. This finding may be important to diastereo- and enantioselective synthesis.

Bone Chemical Structure Response to Mechanical Stress Studied by High Pressure Raman Spectroscopy

While the biomechanical properties of bone are reasonably well understood at many levels of structural hierarchy, surprisingly little is known about the response of bone to loading at the ultrastructural and crystal lattice levels. In this study, our aim was to examine the response (i.e., rate of change of the vibrational frequency of mineral and matrix bands as a function of applied pressure) of murine cortical bone subjected to hydrostatic compression. We determined the relative response during loading and unloading of mineral vs. matrix, and within the mineral, phosphate vs. carbonate, as well as proteinated vs. deproteinated bone. For all mineral species, shifts to higher wave numbers were observed as pressure increased. However, the change in vibrational frequency with pressure for the more rigid carbonate was less than for phosphate, and caused primarily by movement of ions within the unit cell. Deformation of phosphate on the other hand, results from both ionic movement as well as distortion. Changes in vibrational frequencies of organic species with pressure are greater than for mineral species, and are consistent with changes in protein secondary structures such as alterations in interfibril cross-links and helix pitch. Changes in vibrational frequency with pressure are similar between loading and unloading, implying reversibility, as a result of the inability to permanently move water out of the lattice. The use of high pressure Raman microspectroscopy enables a deeper understanding of the response of tissue to mechanical stress and demonstrates that individual mineral and matrix constituents respond differently to pressure.

Raman and photoluminescence spectroscopy study of benzoic acid at high pressures

Benzoic acid (C 6H 5COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released.