High-pressure Raman and infrared spectroscopic studies of ZrP2O7

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP₂O₇ to 23.2 and 13 GPa respectively. In the pressure range 0.7–4.3 GPa the lattice mode at 248 cm^?1 disappears, new modes appear around 380 and 1111 cm^?1 and the strong symmetric stretching mode at 476 cm^?1 softens, possibly indicating a subtle phase transition. Above 8 GPa all the modes broaden, and all of the Raman modes disappear beyond 18 GPa. On decompression from the highest pressure, 23.2, to 0 GPa all of the modes reappear but with larger full width at half maximum. Lattice dynamics of the high temperature phase of ZrP₂O₇ were studied using first principles method and compared with experimental values.     

High pressure raman study of Y-124

In this work, a micro-Raman study under high hydrostatic pressure (up to ～5.5 GPa) has been carried on YBa₂Cu₄O₈ and Y(Ba, Sr)₂Cu₄O₈ single crystals at room temperature. In both samples, seven strong modes, of A_g symmetry, and one weak, of B_3g symmetry, have been observed and examined in connection with previously published results concerning YBa₂Cu₄O₈. With the Sr substitution for Ba, the ambient pressure measurements show an upward shift in energy for all modes, except those that involve vibrations of the plane and apex oxygen atoms. With increasing hydrostatic pressure all phonons shift to higher energies. Anomalous nonlinear pressure behaviour has been observed for three phonons, which is correlated with the pressure dependence of T _c of these compounds.     

Spectroscopic study ofβ-Ni(OH)2 under pressure

Infrared absorption and Raman study ofβ-Ni(OH)₂ has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA _2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm⁻1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm⁻¹ around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA _1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.     

Raman spectra of shortite Na2Ca2(CO3)3 compressed up to 8 GPa

A rare mineral shortite, Na₂Ca₂(CO₃)₃, occurs among groundmass minerals in unaltered kimberlites, which suggests its participation in the evolution of kimberlite system. This work presents a high pressure Raman spectroscopic study of natural shortite (Udachnaya east kimberlites) compressed in KBr up to 8?GPa in a diamond anvil cell. At ambient pressure the spectrum contains two strong bands related to symmetric C-O stretching vibrations, four in-plane bending modes, and several low-frequency modes of lattice vibrations. Upon the pressure increase up to 8?GPa, almost all the bands exhibit positive shift with the rate of 1–4?cm^?1/GPa for the lattice modes and 3.6 and 3.9?cm^?1/GPa for the C-O stretching modes. The shifts of Raman modes are rather regular, which implies the absence of reconstructive phase transitions within the studied pressure range, similarly to the behavior of nyerereite, a related carbonate mineral. However, minor anomalies in the ν/P and FWHM/P dependences, observed at about 2?GPa, suggest some rearrangement and disordering of carbonate groups. The obtained data can be used for the estimation of residual pressure in shortite-bearing inclusions in deep-seated minerals.     

High pressure study of the 2D polymeric phase of C60 by means of raman spectroscopy

Abstract

The effect of high hydrostatic pressure, up to 12GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal Cm polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800cm^?1). A number of new Raman modes appear in the spectrum for pressures ～ 1.4 and ～ 5.0 GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P=4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.     

High pressure behavior of nano-crystalline CeO2 up to 35 GPa: a Raman investigation

The present paper reports the results of in situ Raman studies carried out on nano-crystalline CeO₂ up to a pressure of 35 GPa at room temperature. The material was characterized at ambient conditions using X-ray diffraction and Raman spectroscopy and was found to have a cubic structure. We observed the Raman peak at ambient at 465 cm^?1, which is characteristic of the cubic structure of the material. The sample was pressurized using a diamond anvil cell using ruby fluorescence as the pressure monitor, and the phase evolution was tracked by Raman spectroscopy. With an increase in the applied pressure, the cubic band was seen to steadily shift to higher wavenumbers. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa. CeO₂ was found to undergo a phase transition to an orthorhombic α -PbCl₂-type structure at this pressure. With the release of the applied pressure, the observed peaks steadily shift to lower wavenumbers. On decompression, the high pressure phase existed down to a total release of pressure.     

First-principles study on superconductivity of solid oxygen

The superconductivity of solid oxygen in ζ phase was investigated by first-principles calculations based on the density functional theory. Using a monoclinic C2/m structure, we calculated the superconducting transition temperature by the Allen–Dynes formula and obtained 2.4 K at 100 GPa for the effective screened Coulomb repulsion constant μ* of 0.13. The transition temperature slowly decreases with increasing pressure and becomes 1.3 K at 200 GPa. The phonon analysis shows that the electron–phonon coupling is dominantly enhanced by the intermolecular vibrations of O₂ rather than the intramolecular ones. The phonon modes showing the strong electron–phonon coupling were found to be concentrated in the phonon frequency range of 100–150 cm^?1 at around the M-point in the Brillouin zone.     

Phonon spectrum of a naphthalene crystal at a high pressure: Influence of shortened distances on the lattice and intramolecular vibrations

The Raman spectra of a naphthalene crystal have been measured at room temperature in the pressure range up to 20 GPa. The pressure shift and Grüneisen parameters for intermolecular and intramolecular phonons have been determined. The maximum rate of the pressure shift for intermolecular phonons is 44 cm^?1/GPa, and the rate of the pressure shift for intramolecular phonons lies in the range from 1 to 11 cm^?1/GPa for different modes. The pressure dependence of the phonon frequencies for direct and inverse pressure variations has a hysteresis in the pressure range from 2.5 to 16.5 GPa. It has been shown that the linear dependence of the intermolecular phonon frequency on the crystal density has a peculiarity, which indicates a possible phase transition at a pressure of 3.5 GPa. The pressure dependence of intramolecular phonons related to the stretching vibrations of hydrogen atoms exhibits features that are characteristic of intermolecular phonons, which is associated with the influence of shortened distances between the hydrogen atoms of the neighboring molecules on the intermolecular interaction potential.     

Raman spectroscopy of melamine at high pressures up to 60 GPa

In this work, the Raman scattering of melamine was studied under high pressure up to 60 GPa. The behavior of the most intensive peaks of the Raman spectrum of melamine, 677 cm^?1 and 985 cm^?1 modes, and their line widths do not show any phase transition or indication of formation of sp ³ bonds. Comparing the behavior of the line width of the Raman peaks of graphite under pressure and that of melamine leads us to conclude that the s-triasine (C–N) ring is more rigid than the C–C graphite ring. High pressure results with melamine suggest that the direct phase transition g-C₃N₄ to dense C₃N₄ phase should occur above 60 GPa.     

Synthesis of Ge-Sn at high pressure and high temperature in a laser-heated diamond anvil cell

Ge–Sn compound is predicted to be a direct band gap semiconductor with a tunable band gap. However, the bulk synthesis of this material by conventional methods at ambient pressure is unsuccessful due to the poor solubility of Sn in Ge. We report the successful synthesis of Ge–Sn in a laser-heated diamond anvil cell (LHDAC) at ~7.6 GPa &; ~2000 K. In situ Raman spectroscopy of the sample showed, apart from the characteristic Raman modes of Ge TO (Г) and β-Sn TO (Г), two additional Raman modes at ~225 cm^?1 (named Ge–Sn₁) and ~133 cm^?1 (named Ge–Sn₂). When the sample was quenched, the Ge–Sn₁ mode remained stable at ~215 cm^?1, whereas the Ge–Sn₂ mode had diminished in intensity. Comparing the Ge–Sn Raman mode at ~225 cm^?1 with the one observed in thin film studies, we interpret that the observed phonon mode may be formed due to Sn-rich Ge–Sn system. The additional Raman mode seen at ~133 cm^?1 suggested the formation of low symmetry phase under high P–T conditions. The results are compared with Ge–Si binary system.     

The electronic Raman spectrum of cobalt doped MgO

The energies of the ⁴T_1g states of Co²⁺ as a dilute substitutional impurity in MgO relative to the ground Λ_6g doublet have been found by low temperature Raman spectroscopy to be two Λ_8g states at 305 ± 3 cm^?1 and 930 ± 3 cm^?1; the remaining Λ_7g energy is predicted to be in the 980–1010 cm^?1 range the corresponding Λ_6g → Λ_7g Raman transition being weak and buried in the extensive two-phonon background. A second-order perturbation calculation which couples the spin-orbit states to both E_g and T_2g modes of vibration gives a weak but important Jahn-Teller stabilization energy for the Λ_8g states.     

Raman Spectroscopic Study of the Molybdate Mineral Szenicsite and Comparison with Other Paragenetically Related Molybdate Minerals

Abstract

The molybdate‐bearing mineral szenicsite, Cu₃(MoO₄)(OH)₄, has been studied by Raman and infrared spectroscopy. A comparison of the Raman spectra is made with those of the closely related molybdate‐bearing minerals, wulfenite, powellite, lindgrenite, and iriginite, which show common paragenesis. The Raman spectrum of szenicsite displays an intense, sharp band at 898 cm^?1, attributed to the ν₁ symmetric stretching vibration of the MoO₄ units. The position of this particular band may be compared with the values of 871 cm^?1 for wulfenite and scheelite and 879 cm^?1 for powellite. Two Raman bands are observed at 827 and 801 cm^?1 for szenicsite, which are assigned to the ν₃(E _g ) vibrational mode of the molybdate anion. The two MO₄ ν₂ modes are observed at 349 (B _g ) and 308 cm^?1 (A _g ). The Raman band at 408 cm^?1 for szenicsite is assigned to the ν₄(E _g ) band. The Raman spectra are assigned according to a factor group analysis and are related to the structure of the minerals. The various minerals mentioned have characteristically different Raman spectra.     

Pressure‐induced anomalous phase transformation in nano‐crystalline dysprosium sesquioxide

The phase transformation in nano‐crystalline dysprosium sesquioxide (Dy₂O₃) under high pressures is investigated using in situ Raman spectroscopy. The material at ambient was found to be cubic in structure using X‐ray diffraction (XRD) and Raman spectroscopy, while atomic force microscope (AFM) showed the nano‐crystalline nature of the material which was further confirmed using XRD. Under ambient conditions the Raman spectrum showed a predominant cubic phase peak at 374 cm⁻¹, identified as F_g mode. With increase in the applied pressure this band steadily shifts to higher wavenumbers. However, around a pressure of about 14.6 GPa, another broad band is seen to be developing around 530 cm⁻¹ which splits into two distinct peaks as the pressure is further increased. In addition, the cubic phase peak also starts losing intensity significantly, and above a pressure of 17.81 GPa this peak almost completely disappears and is replaced by two strong peaks at about 517 and 553 cm⁻¹. These peaks have been identified as occurring due to the development of hexagonal phase at the expense of cubic phase. Further increase in pressure up to about 25.5 GPa does not lead to any new peaks apart from slight shifting of the hexagonal phase peaks to higher wavenumbers. With release of the applied pressure, these peaks shift to lower wavenumbers and lose their doublet nature. However, the starting cubic phase is not recovered at total release but rather ends up in monoclinic structure. The factors contributing to this anomalous phase evolution would be discussed in detail. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen-bonded OH and OD overtone bands and potential energy curve of methanol

The absorption spectra of CH₃OH, CH₃OD, CD₃OH, and CD₃OD as pure liquids and as carbon tetrachloride solutions were measured in the 3,850 – 16,600cm^?1 region. In addition to the various combination bands, the higher overtone bands of the hydrogen-bonded OH stretching vibration of self-associated methanols were observed at ～6470, 9300–9700, and 12,200 – 12,700 cm^?1 with broad half-widths of ～700, ～1200, and ～1800 cm^?1, respectively, and those of the OD stretching vibration, at ～4900, 7200–7400, and 9200–9600 cm^?1 with half-widths of ～370, ～700, and ～1200 cm^?1, respectively. With the aid of the observed frequencies, we determined the single minimum potential energy curve for the hydrogen-bonded OH and OD stretching vibrations of self-associated methanols. Furthermore, the absorption band due to double excitation of two neighboring OH groups linked together by a hydrogen bond was quantitatively analyzed by using the isotopic isolation technique. The double excitation band of CH₃OH as pure liquid was found to appear at 6730 cm^?1 with an absorbance of 0.08 at 1 mm light path length.     

High-pressure thermoelectric characteristics of Bi2Te3 semiconductor with different charge carrier densities

The measurements of the absolute values of the thermopower and of the relative electrical resistance have been performed for n type Bi₂Te₃ under hydrostatic pressure up to 9 GPa at room temperature. Under pressures exceeding 5 GPa and up to the phase transition (at 7 GPa), the samples with the charge carrier density below 10^?19 cm^?3 exhibit an anomalous growth of the thermopower. For the purest sample (n = 10^?18 cm^?3), the thermopower is as high as +150 μV/K. The pressure dependence of the electrical resistance for n-Bi₂Te₃ does not exhibit any anomalies up to the pressure corresponding to the phase transition (7 GPa). Thus, the state with the giant thermoelectric efficiency is found in Bi2Te3 under pressure before the phase transition.     

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of MgB₂ ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ∼590 cm⁻¹ related to the E _2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas, beyond this region, the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ∼5 GPa exhibits a change in the slope, as well as a “hysteresis” effect in the frequency vs. pressure behavior. These singularities in the E _2g mode behavior under pressure support the suggestion that MgB₂ may undergo a pressure-induced topological electronic transition.     

Hard mode infrared spectroscopy of CaTiO3—CaFeO2.5 perovskites

Abstract

Powder-absorption infrared (IR) spectra of perovskites CaFe_xTi_1?x O_3?x/2 (0≤x≤1) annealed at different temperatures were investigated at room temperature in the range 135–2000 cm^?1. The spectra change as a function of composition, annealing temperature and structural state (order-disorder of oxygen vacancies). Autocorrelation analysis has been used to determine variations of average line widths of groups of peaks in the primary IR spectra. The band widths increase on increasing Fe content in the region of the structures with disordered oxygen vacancies and they decrease on going through the order-disorder boundary. High degrees of local structural heterogeneity are suggested by the effective line widths of the phases at intermediate compositions. The intensity of bands at ～150 and ～443 cm^?1 decreases with increasing Fe content in the compositional range of the disordered structures. Finally, for every annealing temperature, the frequency of the band at ～600cm^?1 systematically shifts to higher values on increasing Fe content, these values decreasing again for the fully ordered structures.     

Variable-Temperature and High-Pressure Micro-Raman Spectra of Inorganic Artists' Pigments: Crystalline Wulfenite,Lead(II) Molybdate(VI), PbMoO4

Variable-temperature (?150°C to 600°C) and high-pressure (up to ～5 GPa) micro-Raman spectra have been obtained for the mineral wulfenite [lead(II) molybdate(VI), PbMoO₄], a main constituent of the artists' pigment, orange molybdate. The spectra were quite similar in both the temperature and the pressure studies, except for broadening and shifting of some peaks. No phase changes were detected, although there is possibly some amorphization beginning at ～600°C. The photoacoustic IR spectrum in the 1950–450 cm^?1 region is reported for characterization purposes. The long-term stability of PbMoO₄ with respect to extreme changes in both temperature and pressure illustrates the importance of orange molybdate in artwork and protective coatings.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Behavior of natrolite zeolite and fluorapatite at high pressures in a water medium

The hysteresises (～0.3–0.4 GPa) of two transitions in natrolite at 0.9 and 1.45 GPa, considerable changes in the Raman spectra, and the appearance of very intense low-frequency mode at 75 cm^?1 in the overhydrated phase of high pressure of water medium up to 6.2 GPa are observed for the first time. The dependences of the band frequencies of this phase are nonlinear, due clearly to changes in the positions of H₂O in the channels. According to Raman data, fluorapatite placed together with natrolite in a water medium in a diamond anvil cell exhibits no transitions up to 6.2 GPa and displays linear pressure dependences of the band frequencies.