Optical microscopy and in situ Raman scattering of single crystalline ethylene hydrate and binary methane-ethylene hydrate at high pressures

Visual observations through a microscope and in situ Raman measurements have been made for single crystalline ethylene hydrate (EH) and binary methane-ethylene hydrate (MEH) at pressures up to 3.7 GPa and room temperature. Both hydrates showed pressure-induced phase transitions at 1.6, 2.0, and 3.0 GPa for EH and at 1.7, 2.1, and 3.3 GPa for MEH. The cubic sI phase of EH and MEH remains stable up to 1.6 and 1.7 GPa, respectively, which are more widely ranging values than the values for the methane hydrate sI phase. In this sI phase of binary MEH, the cage occupancies by methane and ethylene molecules are investigated from Raman spectra. Above P = 3.0 GPa for EH and 3.3 GPa for MEH, they decomposed by associating with the formation of the polyethylene.     

Shock wave-induced phase transition in RDX single crystals

The real-time, molecular-level response of oriented single crystals of hexahydro-1,3,5-trinitro-s-triazine (RDX) to shock compression was examined using Raman spectroscopy. Single crystals of [111], [210], or [100] orientation were shocked under stepwise loading to peak stresses from 3.0 to 5.5 GPa. Two types of measurements were performed: (i) high-resolution Raman spectroscopy to probe the material at peak stress and (ii) time-resolved Raman spectroscopy to monitor the evolution of molecular changes as the shock wave reverberated through the material. The frequency shift of the CH stretching modes under shock loading appeared to be similar for all three crystal orientations below 3.5 GPa. Significant spectral changes were observed in crystals shocked above 4.5 GPa. These changes were similar to those observed in static pressure measurements, indicating the occurrence of the alpha-gamma phase transition in shocked RDX crystals. No apparent orientation dependence in the molecular response of RDX to shock compression up to 5.5 GPa was observed. The phase transition had an incubation time of approximately 100 ns when RDX was shocked to 5.5 GPa peak stress. The observation of the alpha-gamma phase transition under shock wave loading is briefly discussed in connection with the onset of chemical decomposition in shocked RDX.     

Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy

We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.     

Experimental determination and modeling of structure II hydrates in mixtures of methane+water+1,4-dioxane

Hydrate phase equilibrium conditions were measured with a Cailletet apparatus in the pressure range 2<14 MPa. The investigated 1,4-dioxane concentrations were 1, 2, 5, 7, 10, 20 and 30 mol% relative to water. The results show that adding 1,4-dioxane up to concentrations of 6 mol%, about the stoichiometric ratio of large sII cages to water (1/17), reduced the equilibrium pressure of hydrate formation. Adding 1,4-dioxane beyond 6 mol% caused a slow increase of the equilibrium pressures. The hydrate phase equilibria data were modeled as equilibrium between a liquid phase of water and 1,4-dioxane, with a small amount of methane, and a sII hydrate of 1,4-dioxane and methane. The chemical potential of the hydrate phase was described using the van der Waals and Platteeuw theory. Activity coefficients of the liquid phase were calculated by a van Laar relation, based on literature 1,4-dioxane+water VLE data. The predicted equilibrium pressures calculated were within 5% of the data up to a concentration of 20 mol% 1,4-dioxane relative to water.     

Microscopic observation and in-situ Raman scattering studies on high-pressure phase transformations of Kr hydrate

Direct observations through a microscope and in-situ Raman scattering measurements of synthesized single-crystalline Kr hydrate have been performed at pressures up to 5.2 GPa and 296 K. We have observed that the initial cubic structure II (sII) of Kr hydrate successively transforms to a cubic structure I (sI), a hexagonal structure, and an orthorhombic structure (sO) called "filled ice" at 0.45, 0.75, and 1.8 GPa, respectively. The sO phase exists at least up to 5.2 GPa. In addition to these transformations, we have also found the new phase behavior at 1.0 GPa, which is most likely caused by the change of cage occupancy of host water cages by guest Kr atoms without structural change. Raman scattering measurements for observed phases have shown that the lattice vibrational peak at around 130 cm(-1) disappears in the pressure region of sI, which enables us to distinguish the sI phase from sII and sH phases.     

Structural, electronic, and dynamical properties of methane under high pressure

The electronic structure and lattice dynamical properties of solid methane under high pressure have been studied based on density functional theory. We identify a cubic structure with space group of I43m below 14 GPa, the Pmn2(1) structure in the range of 14-21 GPa, and the P2(1)/c structure from 21 to 65 GPa. Our obtained Raman spectra of the P2(1)/c structure agree well with the typical Raman active modes in the available experimental data. At 65 GPa, methane undergoes a phase transition from P2(1)/c to Pnma. The structures with P2(1)/c and Pnma symmetries are insulating, and under any pressure studied methane always remains in molecular form. For Pnma phase, the orientational ordering of CH(4) molecules varies significantly at 79, 88, and 92 GPa, and by further increasing pressure the rotation of the molecules freezes and orientational ordering remains unchanged.     

Molecular simulation of gas hydrate nanoclusters in water shell: Structure and phase transitions

Gas hydrate nanoclusters surrounded by water shells are studied by the molecular dynamic method. Hydrates of methane (sI structures) and krypton (sII structures), as well an ice nanocluster in a supercooled water shell, are considered. The main attention was focused on studying the local structure and phase transitions. Variations in local partial densities with an increase in temperature are monitored. Melting points of nanosized samples of gas hydrates are determined using caloric curves. Additional information on the behavior of the considered systems is obtained from the temperature dependences of diffusion coefficients and the Lindemann criterion. Two-phase transitions are revealed for gas hydrate nanoclusters. The first phase transition at 210 K can be assigned to the melting of the ice shell. The second transition at 230–235 K is identified as the phase transition in the hydrate core. The melting of ice cluster is observed at 215 K, which corresponds to the melting point of bulk crystal upon the use of the SPC/E water model.     

Dissociation of methane under high pressure

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2(1)2(1)2(1), Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C(2)H(6)) at 95 GPa, butane (C(4)H(10)) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.     

High-pressure phase transitions of Cu2O

In the present study, we extensively explored the crystal structures of Cu₂O on increasing the pressure from 0 GPa to 24 GPa using the first-principles density functional calculations. A series of pressure-induced structure phase transitions of Cu₂O are examined. The calculated results show that the phase transitions (Pn-3m phase → R-3m phase → P-3m1 phase) occur at 5 GPa and 12 GPa, respectively. The P-3m1 phase is found to be the metallic phase via band-gap closure under high pressure.     

Characterization of the high-pressure structures and phase transformations in SnO2. A density functional theory study

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.     

"Stubborn" triaminotrinitrobenzene: unusually high chemical stability of a molecular solid to 150 GPa

We report an unexpectedly high chemical stability of molecular solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under static high pressures. In contrast to the high-pressure behavior of the majority of molecular solids, TATB remains both chemically stable and an insulator to 150 GPa--well above the predicted metallization pressure of 120 GPa. Single crystal studies have shown that TATB exhibits pressure-induced Raman changes associated with two subtle structural phase transitions at 28 and 56 GPa. These phase transitions are accompanied by remarkable color changes, from yellow to orange and to dark red with increasing pressure. We suggest that the high-stability of TATB arises as a result of its hydrogen-bonded aromatic two-dimensional (2D) layered structure and highly repulsive interlayer interaction, hindering the formation of 3D networks or metallic states.     

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.     

Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO(2)N(2) structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275 K and 30 MPa) which were considered suitable for CO(2) sequestration. Substitution of methane guests in both the small and large sI cages by CO(2) and N(2) fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO(2) and N(2) have positive free energies. Substitution of methane with CO(2) in the large cages has a large negative free energy and substitution of the methane in the large cages with N(2) has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO(2) sequestration is to be attempted. The calculations also indicate that N(2) does not directly compete with CO(2) during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO(2) in structure II (sII) cages also has a negative free energy. In cases where sII CO(2) clathrates are formed, only single occupancy of the large cages will be observed.     

Self-diffusion of methane in single-walled carbon nanotubes at sub- and supercritical conditions

The diffusivities of methane in single-walled carbon nanotubes (SWNTs) are investigated at various temperatures and pressures using classical molecular dynamics (MD) simulations complemented with grand canonical Monte Carlo (GCMC) simulations. The carbon atoms at the nanotubes are structured according to the (m, m) armchair arrangement and the interactions between each methane molecule and all atoms of the confining surface are explicitly considered. It is found that the parallel self-diffusion coefficient of methane in an infinitely long, defect-free SWNT decreases dramatically as the temperature falls, especially at subcritical temperatures and high loading of gas molecules when the adsorbed gas forms a solidlike structure. With the increase in pressure, the diffusion coefficient first declines rapidly and then exhibits a nonmonotonic behavior due to the layering transitions of the adsorbed gas molecules as seen in the equilibrium density profiles. At a subcritical temperature, the diffusion of methane in a fully loaded SWNT follows a solidlike behavior, and the value of the diffusion coefficient varies drastically with the nanotube diameter. At a supercritical temperature, however, the diffusion coefficient at high pressure reaches a plateau, with the limiting value essentially independent of the nanotube size. For SWNTs with the radius larger than approximately 2 nm, capillary condensation occurs when the temperature is sufficiently low, following the layer-by-layer adsorption of gas molecules on the nanotube surface. For SWNTs with a diameter less than about 2 nm, no condensation is observed because the system becomes essentially one-dimensional.     

Pressure-induced phase transitions in crystalline L- and DL-cysteine

A series of extended reversible phase transitions at approximately 0.1, 1.5, 2.0, and approximately 5 GPa was observed for the first time in the crystals of dl-cysteine by Raman spectroscopy. These are the first examples of the phase transitions induced by increasing pressure in the racemic crystal of an amino acid. In the crystals of the orthorhombic l-cysteine, a sequence of reversible structural changes in the pressure range between 1.1 and 3 GPa could be observed by Raman spectroscopy, instead of a single sharp phase transition at 1.9 GPa reported previously in ( Moggach, et al. Acta Crystallogr. 2006, B62, 296- 309 ). The role of the movements of the side -CH 2SH groups and of the changes in the hydrogen-bonding type in dl- and l-cysteine during the phase transitions with increasing pressure is discussed and compared with that on cooling down to 3 K.     

Conformational and phase transformations of chlorocyclohexane at high pressures by Raman spectroscopy

Pressure induced conformational and phase transformations of chlorocyclohexane (CCH) were investigated in a diamond anvil cell by Raman spectroscopy at room temperature. Pure CCH was compressed up to 20 GPa and then decompressed to ambient pressure. The conformational equilibrium was shifted by pressure from equatorial to axial conformers in the fluid phase below 0.7 GPa, consistent with previous observations. Upon further compression, several solid-to-solid phase transitions were identified by the observation of markedly different Raman patterns as well as different pressure dependences of characteristic Raman modes. The possible structures of these phases were analyzed in correlation with previously observed solid phases at low temperatures. Finally, CCH exhibits pressure hysteresis and partial reversibility upon decompression which result in the formation of the phases with different Raman patterns from those obtained upon compression. The difference can be interpreted as conformational contribution as well as the intrinsic plasticity of CCH crystals.     

Brillouin scattering of H2O ice to megabar pressures

The sound velocity in polycrystalline ice was measured as a function of pressure at room temperature to 100 GPa, through the phase field of ice VII and crossing the ice X transition, by Brillouin scattering in order to examine the elasticity, compression mechanism, and structural transitions in this pressure range. In particular, we focused on previously proposed phase transitions below 60 GPa. Throughout this pressure range, we find no evidence for anomalous changes in compressibility, and the sound velocities and elastic moduli do not exhibit measurable discontinuous shifts with pressure. Subtle changes in the pressure dependence of the bulk modulus at intermediate pressures can be attributed to high shear stresses at these compressions. The C(11) and C(12) moduli are consistent with previously reported results to 40 GPa and increase monotonically at higher pressures.     

Structures of the I-, II- and H-methane clathrates and the ice-methane clathrate phase transition from quantum-chemical modeling with force-field thermal corrections

Methane hydrates with the three clathrate structures I, II, and H are studied by quantum-chemical methods. Hybrid density-functional theory B3LYP computations using periodic boundary conditions are combined with force-field methods for the thermal energy effects to calculate energetic, thermodynamic, and structural properties. The pressure dependencies for the crystal structures, lattice energies, and guest molecule interactions are derived. The quantum-chemical geometry optimizations predict too small cell volumes as compared to experimental data, but by including zero-point energy and thermal energy effects, we find the cell volumes increase and the correct densities are obtained. The phase transition from MH-I to ice Ih and methane was computed and found to occur at about 9.7 MPa.     

Application of principal component analysis and Raman spectroscopy in the analysis of polycrystalline BaTiO3 at high pressure

The principal component analysis (PCA) was applied to Raman spectra of polycrystalline BaTiO(3) under pressure from atmospheric pressure to approximately 6.72 GPa. For the system utilized, PCA was able to distinguish spectral features and to determine the phase transition pressure: tetragonal to cubic at approximately 2.0 GPa. The present study demonstrates the potentialities of the application of PCA to the investigation on phase transitions at high pressure by Raman spectroscopy.     

Raman scattering study of high-pressure phase transition in thiourea

A high-pressure Raman spectroscopic study of phase transitions in thiourea is reported. The changes in the Raman spectra with increasing and decreasing pressure have been followed to a maximum pressure of approximately 11 GPa. We observe several changes in the spectra including splitting of modes, appearance of new modes, and sudden change in the slope of the frequency-pressure curve at several pressures. On the basis of this study, we propose the existence of three more transitions in this system to phases VII, VIII, and IX at approximately 1, 3, and 6.1 GPa, respectively, in addition to the V-VI phase transition at 0.35 GPa reported earlier. All the transitions have been found to be completely reversible. We interpret these changes in terms of symmetry-lowering phase transitions.