High pressure raman study of PbMoO4

Abstract

Raman spectra of PbMoO₄ have been measured up to 31 GPa in a diamond anvil cell (DAC). Two new phases were found at 10 and 16 GPa pressures at room temperature.     

Elemental sulfur under high hydrostatic pressure. An up-to-date Raman study

We report a high pressure Raman study of orthorhombic elemental sulfur from ambient pressure to ～ 25 GPa. Using a near infrared laser and low laser intensity on the scattering volume, we achieve off-resonant conditions up to larger pressures in comparison with previous studies. Raman spectra were recorded over the full spectral range including external (librational, translational) and internal (bond bending and bond stretching) modes. Drastic changes are observed as regards the peak frequencies, relative intensities and band splitting of degenerate modes. The main outcome of the present study is the observation of a “structural” transition at ～ 16 GPa manifested as slope changes of certain frequencies and sudden relative intensities changes. The present findings are discussed in the context of previous pressure Raman studies and comparison with existing X-ray diffraction as well as ab initio molecular dynamics results is attempted.     

Graphite at high pressures: Pseudomelting at 44 GPa

Abstract

The Raman spectra comparative study of graphite-like modifications of carbon: monocrystalline graphite, pyrolytic graphite and amorphous (glassy) carbon was performed at high pressures and room temperature. At P = 44 GPa an abrupt broadening of Raman bands of pyrolytic graphite and glassy carbon was observed. The same process in monocrystalline graphite occures continuously from 23 to 44 GPa and for higher pressures the Raman spectra of all three substances under study become practically indistinguishable from one another and from that of amorphous carbon (a-C) at the same pressure. This result is ascribed to the formation at these pressures of the dence amorphous carbon modification.     

Carbonization in boron-ion-implanted polymethylmethacrylate as revealed from Raman spectroscopy and electrical measurements

ABSTRACT

The results of Raman spectroscopy and electrical measurements of 40 keV boron-ion-implanted polymethylmethacrylate with ion doses from 6.25 × 10¹⁴ to 5.0 × 10¹⁶ ions/cm² are reported for the first time. The Raman spectra recorded in the 400–3800 cm^?1 range, showing the formation of new carbon–carbon bands for the as-implanted samples at higher ion doses (>10¹⁶ ions/cm²), are found to be an additional support for carbonization processes earlier revealed by slow positrons. The current–voltage dependences at 360 K testify also that the as-implanted samples examined with higher fluences (3.75 × 10¹⁶ and 5.0 × 10¹⁶ ions/cm²) have created a very thin conductive layer or conductive joints due to carbonization.     

Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO<Subscript>3</Subscript> perovskite

Raman and Mössbauer spectra from ¹¹⁹Sn nuclei in CaSnO₃ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn–O–Sn bonds in chains of oxygen octahedra SnO₆ along the c axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO₃ samples with the ilmenite structure have been obtained for the first time.     

Photoluminescence and Raman spectroscopy studies of Eu3+-doped rutile TiO2 nanocrystals at high pressures

Nanocrystal samples (particle size about 90 nm) of Eu³⁺-doped rutile titanium dioxide (TiO₂) nanocrystals (rutile Eu³⁺/TiO₂ nanocrystals) were synthesized by the sol–gel method with hydrothermal treatment. The pressure effect on photoluminescence (PL) and Raman spectra of the rutile Eu³⁺/TiO₂ nanocrystals was investigated with a diamond anvil cell under hydrostatic pressure condition. Raman spectra of the samples at high pressures indicated that the critical pressure for the transition from the rutile phase to a new baddeleyite-type phase was between 10 and 14.2 GPa. The position of Raman bands shifted to high wavenumbers and the PL intensity of ⁵D ₀→⁷F ₂ transition of Eu³⁺ decreased down to zero with the increase of pressure before the phase transition occurred. After releasing the pressure, the rutile phase was not recovered and a α-PbO₂-type phase was observed at ambient pressure.     

High-pressure Raman and infrared study of ZrV 2O7

The room-temperature Raman and infrared spectra of zirconium vanadate (ZrV ₂O₇) were observed up to pressures of 12 GPa and 5.7 GPa, respectively. The frequencies of the optically active modes at ambient pressure were calculated using direct methods and compared with experimental values. Average mode Grüneisen parameters were calculated for the Raman and infrared active modes. Changes in the spectra under pressure indicate a phase transition at ∼1.6 GPa, which is consistent with the previously observed α (cubic) to β (pseudo-tetragonal) phase transition, and changes in the spectra at ∼4 GPa are consistent with an irreversible transformation to an amorphous structure.     

Infrared Spectra of Crystalline,Glassy and Liquid Methanol at High Pressures

Abstract

Mid-infrared spectra in the range 400–1800 cm^?1 of methanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 3.6 GPa, and the spectra of the resulting metastable glass are very similar to those of the liquid. When maintained at high pressure, the glass spontaneously transforms to an ordered crystalline phase which is stable over the range 3.6 to 11 GPa. Small changes in peak wavenumbers for 14 internal modes as a function of pressure are observed, indicating that distortion of the molecules is minimal. A slight decrease for the C-O-H bending mode is attributed to charge transfer from the molecular 0-H bond to the strengthening intermolecular hydrogen bond.     

INFRARED SPECTRA OF BROMOFORM AT HIGH PRESSURES

Infrared spectra in the wavenumber ranges 100–1800 and 2700–4800 cm^?1 are reported for bromoform samples in diamond anvil cells at ambient temperature and at pressures up to 10 GPa. The freezing pressure is estimated to be 0.13 ± 0.02 GPa. The spectra appear to evolve smoothly and no major discontinuities are detected. The dependence on pressure of eleven peak wavenumbers (five fundamentals and six combinations) is presented. All modes show small percentage increases in wavenumber over this pressure range, except for the degenerate bend, ν₆, which exhibits a 20% increase, suggesting that the equilibrium Br-C-Br angles may be slightly changing with increasing pressure.     

Vibrational spectra of barium oxalate hemihydrate

The infrared and Raman spectra of barium oxalate hemihydrate, BaC₂O₄ · 0.5H₂O, were recorded and discussed on the basis of their structural peculiarities and in comparison with the spectra of the previously investigated calcium and strontium oxalates.     

High-Pressure Raman Study of Zincblende,Cinnabar, and Cmcm Phases Of ZnTe

Raman measurements of ZnTe have been performed at pressures up to 15 GPa. Frequencies, line widths, and intensities of first- and second-order Raman features of the zincblende phase (0-9.5 GPa) were studied in detail. In this note, we focus on the Raman spectra of the high-pressure cinnabar and Cmcm phases. In the transition regime from cinnabar to Cmcm (12.2 to 13.7 GPa) the Raman data indicate the possible existence of a new intermediate high-pressure phase.     

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.     

Raman scattering and phase changes of CulnSe2 and LiInSe2 at high pressure

Abstract

We have investigated the effect of pressure on the Raman spectra of the ternary chalcogenides CulnSe₂ (chalcopyrite structure) and LiInSe₂ (β-NaFeO₂ structure) for pressures extending well above their first pressure-induced phase transitions. Sign and magnitude of Griineisen parameters are discussed by comparing to related tetrahedrally coordinated chalcogenides. Discontinuous changes of the Raman frequencies indicate pressure-induced phase transitions at 8.0±0.3 and 4.2±0.2 GPa in CuInSe₂ and in LiInSe₂, respectively. The Raman spectra of the low-pressure phases are not recovered after pressure release but a highly disordered structure is induced. In the case of LiInSe₂, the effect of laser heating on the Raman spectra of the high-pressure phase is investigated and discussed in light of recent high-pressure x-ray investigations.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

Phase diagram of RDX (hexahydro-1,3,5-trinitro-s-triazine) at high pressures and temperatures

Abstract

The phase diagram of RDX-h₆ (hexahydro-1,3,5-trinito-s-triazine) and RDX-d₆ has been studied by Raman spectroscopy to more than 13 GPa at 295 K and 7.5 GPa between 150 and 450 K. Two stable high pressure phases have been found. γ-RDX or RDX-III forms from α-RDX above 3.8 GPa below 380 K. β-RDX forms when α- or γ-RDX are heated, can be retained metastably at low temperatues, and may be related to a very unstable form occasionally recovered at ambient pressure. Deuterium isotopic substitution and shear increase the temperature where β-RDX begins to form on heating.     

High pressure Raman studies on solid iodine

Abstract

Raman spectra of solid iodine were measured up to 27 GPa. Additional bands, whch appear above 10 GPa, are discussed with respect to a quasi one dimensional molecular phase existing at intermediate pressures and may indicate a more subtle way of molecular dissociation than previously suggested. Higher order spectra due to two-and three phonon processes involving internal and external modes were observed up to 10 GPa.     

Physical and mechanical properties of C60 under high pressures and high temperatures

The physical and mechanical properties of a C₆₀ fullerene sample have been investigated under high pressure–high temperature conditions using a designer Diamond Anvil Cell. Electrical resistance measurements show evidence of C₆₀ cage collapse at 20 GPa, which leads to the formation of an insulating phase at higher pressure. Energy dispersive X-ray diffraction (EDXD) data indicated that the characteristic fcc reflections gradually decrease in intensity and eventually disappear above 28 GPa. A C₆₀ sample was laser-heated at a pressure of 35 GPa to a temperature of 1910±100 K and, subsequently, decompressed to ambient conditions. The photoluminescence spectra and the Raman spectrum of the pressure–temperature-treated sample were measured at a low temperature of 80 K. Raman peak at 1322.3 cm^?1 with full-width half-maximum of 2.9 cm^?1 was observed from the sample, which is attributed to the hexagonal diamond phase in the sample. The room temperature photoluminescence spectra showed a symmetric emission band centered in the red spectral range with a peak at 690 nm. The structural analysis of the pressure–temperature-processed C₆₀ sample using EDXD method showed strong internal structure orientation and a phase close to hexagonal diamond. Mechanical properties such as hardness and Young’s modulus were measured by nanoindentation technique and the values were found to be 90±7 and 1215±50 GPa, respectively and these values are characteristic of sp³-bonded carbon materials.     

Pressure dependence of the Raman spectra of indium sulphide

Raman spectra of InS single crystals have been studied at different hydrostatic pressures up to 1.2 GPa. Mode-Grüneisen parameters have been obtained for Raman-active normal modes. It is shown that the variations observed in Raman spectra with growing pressure can be interpreted from the standpoint of the structural phase transition D¹²_2h → D¹⁷_4h in InS as the hydrostatic pressure continues to increase. The transition pressure has been evaluated at (7 ± 1) GPa.     

Pressure-Effect on Anatase Titanium Dioxide

Pressure-induced phase transition of anatase titanium dioxide was investigated by Raman, absorption spectroscopy and X-ray diffraction. The change in Raman and absorption spectra with pressure revealed that the transition from anatase to high pressure phase with f -PbO 2 structure (TiO 2 -II) occurred in the pressure range of 4.0-4.6 GPa for a single crystal. The X-ray powder diffraction patterns indicate the presence of superstructural lattice of anatase at pressures more than 3 GPa. The superstructure of anatase disappears on the release of the pressure. A sluggish transition to the high pressure phase is also observed. The anatase coexists with the high pressure phase at 5.2 GPa. The difference in the results between optical spectroscopy (single crystal) and X-ray diffraction (powder) will be due to crystalinity of the sample.     

Infrared Spectra of Liquid and Crystalline Ethanol at High Pressures

Abstract

Mid-infrared spectra in the ranges 400–1800 and 2700–4600 cm^?1 of ethanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 1.8 GPa, and, unlike methanol, the resulting solid is crystalline rather than glassy. No further phase transitions are observed in this pressure range. The wave number shifts of 30 selected peaks with pressure are deduced, and their small magnitudes indicate that only minor distortions of the molecules occur. The effects of the strengthening of the intermolecular hydrogen bonds with pressure on the internal modes are briefly discussed.