High‐pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd₂(MoO₄)₃] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure‐transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure‐transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO₄ stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short‐range and long‐range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure‐transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A₁ symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm⁻¹, soften continuously through the first three phases when increasing pressure. The strong A₂ mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm⁻¹ at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the Mo O stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

Pressure‐induced phase transitions in L‐leucine crystal

Raman spectra of a crystal of L ‐leucine, an essential amino acid, were obtained for pressures between 0 and 6 GPa. The results show anomalies at three pressure values, one between 0 and 0.46 GPa, another between 0.8 and 1.46 GPa, and a third at P ∼ 3.6 GPa. The first two anomalies are characterized by the disappearance of lattice modes (which can indicate occurrence of phase transitions), the appearance of several internal modes, or the splitting of modes of high wavenumbers. The changes of internal modes are related to CH and CH₃ unit motions as well as hydrogen bonds, as can be inferred from the behavior of bands associated with CO₂⁻ moieties. The third anomaly is a discrete change of the slopes of the wavenumber versus pressure plots for most modes observed. Further, decompression to ambient pressure generates the original Raman spectrum, showing that the pressure‐induced anomalies undergone by L ‐leucine crystals are reversible. Copyright © 2008 John Wiley & Sons, Ltd.     

High pressure and low-temperature polymorphism in cyclopentanol

The polymorphism of cyclopentanol (C₅H₁₀O) has been investigated as a function of temperature at ambient pressure and as a function of hydrostatic pressures to 3.7?GPa at room temperature. Differential scanning calorimetry (DSC) and Raman spectra reveal that two plastic phases and two fully ordered crystalline phases are formed during cooling. High pressure Raman and infrared spectra show that cyclopentanol undergoes two-phase transformations. At around 0.6?GPa, the liquid cyclopentanol transforms to a solid plastic structure. On further compression to 1.9?GPa, one fully ordered crystalline phase is observed. Based on pieces of evidence such as peak splitting and emergence of new peaks, it can be concluded that the ordered crystalline structure has a lower symmetry. In addition, the decrease in the wavenumber of the O–H stretching modes at low temperature and high pressure suggests the ordered crystalline phases are characterized by the formation of hydrogen-bonded molecular chains.     

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.     

High‐pressure Raman spectra of deuterated L‐alanine crystal

Raman spectra of deuterated L ‐alanine have been obtained at high‐pressure conditions. A phase transition at ∼1.5 GPa associated with the splitting of some internal modes and increase of the wavenumber of the external modes was observed. Similarly to the hydrogenated L ‐alanine crystal, this first transition was related to a symmetry change. Moreover, further modifications of the Raman spectra were observed at 4.4 GPa, which may be associated to conformational changes of the molecule. To give further support to such a hypothesis, neutron powder diffraction measurements were performed. Information about the cell parameter at atmospheric pressure gave valuable information about the N D distances, shedding light on the behavior of the torsional vibration of ND₃⁺. Copyright © 2009 John Wiley & Sons, Ltd.     

In situ Raman and photoluminescence study on pressure‐induced phase transition in C60 nanotubes

Single crystalline C₆₀ nanotubes having face‐centered‐cubic structure with diameters in the nanometer range were synthesized by a solution method. In situ Raman and photoluminescence spectroscopy under high pressure were employed to study the structural stabilities and transitions of the pristine C₆₀ nanotubes. A phase transition, probably because of the orientational ordering of C₆₀ molecules, from face‐centered‐cubic structure to simple cubic structure occurred at the pressure between 1.46 and 2.26 GPa. At above 20.41 GPa, the Raman spectrum became very diffuse and lost its fine structure in all wavenumber regions, and only two broad and asymmetry peaks initially centered at 1469 and 1570 cm^–1 were observed, indicating an occurrence of amorphization. This amorphous phase remained to be reversible until 31.1 GPa, and it became irreversible to the ambient pressure after the pressure cycle of 34.3 GPa was applied. Copyright © 2012 John Wiley & Sons, Ltd.     

SrB4O7:Sm2+: an optical sensor reflecting non-hydrostatic pressure at high-temperature and/or high pressure in a diamond anvil cell

A systematic investigation on the fluorescent spectra of SrB₄O₇:Sm²⁺ was performed in detail at high-temperature up to 623?K and/or high pressure up to 23.2?GPa with different pressure-transmitting media (PTMs), respectively. Combined with experiment data of previous research, the change of the ⁷D₀–⁵F₀ line (0–0 line) full width at half maximum (FWHM) of SrB₄O₇:Sm²⁺ under different pressure environments was specifically discussed. The results indicate that the FWHM of 0–0 line is sensitive to the non-hydrostatic pressure environment in 2-propanol, and methanol and ethanol mixture (ME) PTMs at ambient temperature. The first-order and the second-order derivation of the temperature dependence of 0–0 line FWHM at ambient pressure are 1.48(±0.21)?×?10^?4?nm/K and 9.63(±0.63)?×?10^?7?nm²/K² below 623?K. The 0–0 line FWHM is also sensitive to the non-hydrostatic pressure environment in ME at high-temperature and high pressure simultaneous, the non-hydrostatic transition pressures are 9.6?GPa at 323?K, 11.0?GPa at 373?K, 14.4?GPa at 423?K, respectively. SrB₄O₇:Sm²⁺ is recommended as an optical sensor to reflect the change of pressure environment in liquid media at high-temperature and/or high pressure.     

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.     

Terahertz Cyclotron Photoconductivity in a Highly Unbalanced Two-Dimensional Electron–Hole System

The magnetic properties of the α-Fe₂O₃ hematite at a high hydrostatic pressure have been studied by synchrotron Mössbauer spectroscopy (nuclear forward scattering (NFS)) on iron nuclei. Time-domain NFS spectra of hematite have been measured in a diamond anvil cell in the pressure range of 0–72 GPa and the temperature range of 36–300 K in order to study the magnetic properties at a phase transition near a critical pressure of ~50 GPa. In addition, Raman spectra at room temperature have been studied in the pressure range of 0–77 GPa. Neon has been used as a pressure-transmitting medium. The appearance of an intermediate electronic state has been revealed at a pressure of ~48 GPa. This state is probably related to the spin crossover in Fe³⁺ ions at their transition from the high-spin state (HS, S = 5/2) to a low-spin one (LS, S = 1/2). It has been found that the transient pressure range of the HS–LS crossover is extended from 48 to 55 GPa and is almost independent of the temperature. This surprising result differs fundamentally from other cases of the spin crossover in Fe³⁺ ions observed in other crystals based on iron oxides. The transition region of spin crossover appears because of thermal fluctuations between HS and LS states in the critical pressure range and is significantly narrowed at cooling because of the suppression of thermal excitations. The magnetic P–T phase diagram of α-Fe₂O₃ at high pressures and low temperatures in the spin crossover region has been constructed according to the results of measurements.     

Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature

Using an in situ method of Raman spectroscopy and resistance‐heated diamond anvil cell, the system datolite CaBSiO₄(OH) – water has been investigated at simultaneously high pressure and temperature (up to Р ~5 GPa and Т ~250 °С). Two polymorphic transitions have been observed: (1) pressure‐induced phase transition or the feature in pressure dependence of Raman band wavenumbers at P = 2 GPа and constant T = 22 °С and (2) heating‐induced phase transition at T ~90 °С and P ~5 GPа. The number of Raman bands is retained at the first transition but changed at the second transition. The first transition is mainly distinguished by the changes in the slopes of pressure dependence of Raman peaks at 2 GPa. The second transition is characterized by several strong changes: the wavenumber jumps of major bands, the merging of strong doublets at 378 and 391 cm⁻¹ (values for ambient conditions), the splitting of the intermediate‐intensity band at 292 cm⁻¹, and the transformation of some low‐wavenumber bands at 160–190 cm⁻¹. No spectral and visual signs of overhydration and amorphization have been observed. No noticeable dissolution of datolite in the water medium occurred at 5 GPa and 250 °С after 3 h, which corresponds to typical conditions of the ‘cold’ zones of slab subduction. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the phase transitions in the (NH4)3WO3F3 oxyfluoride

The Raman spectra of (NH₄)₃WO₃F₃ perovskite-like oxyfluorides are measured in the frequency range 70–3600 cm^?1. The anomalies observed in the spectral parameters upon phase transitions at a temperature of 200 K under atmospheric pressure and at room temperature under a pressure of 1.4 GPa are interpreted.     

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.     

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the⁵⁷Fe Mössbauer spectra of pure iron foil and α-Fe₂O₃ powder under high pressure have been measured at room temperature.⁵⁷Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.⁵⁷Fe Mössbauer spectra of Fe₂O₃ under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.     

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.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Photo-and pressure-induced transformations in the linear orthorhombic polymer of C<Subscript>60</Subscript>

Stability of the linear orthorhombic polymer of C₆₀ under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm² of the 514.5 nm line of an Ar⁺ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show similar irreversible changes even at the laser intensity as low as 470 W/cm². The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from those of the other known polymeric phases of C₆₀. The Raman spectra of the phototransformed linear orthorhombic polymer of C₆₀ were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near 4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C₆₀, after pressure release and exposure to ambient conditions for 30 h. The text was submitted by the authors in English.     

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.     

Calibration of berlinite (AlPO4) as Raman spectroscopic pressure sensor for diamond‐anvil cell experiments at elevated temperatures

Changes in the band position of the 462 and the 1111 cm^–1 A₁ modes of berlinite (AlPO₄) with temperature and pressure were determined in situ to 500°C and to 10 GPa using Raman spectroscopy and diamond‐anvil cells. These bands shift in opposite directions with pressure and, likewise, with temperature. At a known temperature, the relative difference of both band positions (Δν)_P,T can therefore be used as a pressure gauge that does not require calibration of the spectrometer. At ambient pressure, the observed temperature dependence of this relative difference of the line positions is very close to linear and can be described by (Δν)_{T, 0.1 MPa} (cm^–1) = 0.0181 T – 0.46 where 23 ≤ T (°C) ≤ 500. Along the 23°C isotherm to 10 GPa, pressure and relative wavenumber difference (Δν)_{P, 23°C} are related by the equation P (GPa) = 0.00083 [(Δν)_{P, 23°C}]² – 0.062 (Δν)_{P, 23°C}. Both equations can be combined to determine pressures at higher temperatures under the assumption that the change in (Δν)_P,T with pressure is insensitive to temperature. Copyright © 2011 John Wiley & Sons, Ltd.     

High-precision measurements of the compressibility and the electrical resistivity of bulk <Emphasis Type="Italic">g</Emphasis>-As<Subscript>2</Subscript>Te<Subscript>3</Subscript> glasses at a hydrostatic pressure up to 8.5 GPa

High-precision studies of the volume and the electrical resistivity of g-As₂Te₃ glasses at a high hydrostatic pressure up to 8.5 GPa at room temperature are performed. The glasses exhibit elastic behavior in compression only at a pressure up to 1 GPa, and a diffuse structural transformation and inelastic density relaxation (logarithmic in time) begin at higher pressures. When the pressure increases further, the relaxation rate passes through a sharp maximum at 2.5 GPa, which is accompanied by softening the relaxing bulk modulus, and then decreases, being noticeable up to the maximum pressure. When pressure is relieved, an unusual inflection point is observed in the baric dependence of the bulk modulus near 4 GPa. The polyamorphic transformation is only partly reversible and the residual densification after pressure release is 2%. In compression, the electrical resistivity of the g-As₂Te₃ glasses decreases exponentially with increasing pressure (at a pressure up to 2 GPa); then, it decreases faster by almost three orders of magnitude in the pressure range 2–3.5 GPa. At a pressure of 5 GPa, the electrical resistivity reaches 10^–3 Ω cm, which is characteristic of a metallic state; this resistivity continues to decrease with increasing pressure and reaches 1.7 × 10^–4 Ω cm at 8.1 GPa. The reverse metal–semiconductor transition occurs at a pressure of 3 GPa when pressure is relieved. When the pressure is decreased to atmospheric pressure, the electrical resistivity of the glasses is below the initial pressure by two–three orders of magnitude. Under normal conditions, both the volume and the electrical resistivity relax to quasi-equilibrium values in several months. Comparative structural and Raman spectroscopy investigations demonstrate that the glasses subjected to high pressure have the maximum chemical order. The glasses with a higher order have a lower electrical resistivity. The polyamorphism in the As₂Te₃ glasses is caused by both structural changes and chemical ordering. The g-As₂Te₃ compound is the first example of glasses, where the reversible metallization under pressure has been studied under hydrostatic conditions.     

Vibrational properties of Cs4W11O35 and Rb4W11O35 systems: high pressure and polarized Raman spectra

Cs₄ W₁₁O₃₅ (CW) and Rb₄ W₁₁O₃₅ (RW) belong to the class of hexagonal bronzes whose structure originates from the K_xWO₃ superconductor hexatungstate. Charge‐imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second‐harmonic generation. In this work, we report the polarized Raman spectra results for both CW and RW, as well as results of high‐pressure Raman scattering experiments (0.0–11.0 GPa) for the Cs₄ W₁₁O₃₅ system, in which we have observed two structural phase transitions at ∼4 and 7.5 GPa. We discuss these transformations and polarized Raman spectra on the basis of lattice dynamics calculation in the related system KNbW₂O₉. Polarized Raman spectra provide strong indication that the highest wavenumber modes observed in these systems originate from tungsten or oxygen vacancies. The observation of a soft‐like mode indicates that the observed phase transitions exhibit a displacive‐type behavior, thus further indicating that these transformations are likely related to reorientations of the octahedral units. The soft mode nature is discussed as well. PACS: 77.80.Bh; 78.30.Hv; 78.30.‐j. Copyright © 2010 John Wiley & Sons, Ltd.