Decomposition of γ-Cyclotrimethylene Trinitramine (γ-RDX): Relevance for Shock Wave Initiation

To elucidate the reactive behavior of RDX crystals at pressures and temperatures relevant to shock wave initiation, Raman spectroscopy and optical imaging were used to determine the pressure-temperature (P-T) stability and the decomposition of γ-RDX, the high pressure phase of RDX. Experiments were performed on single crystals in a diamond anvil cell at pressures from 6 to 12 GPa and at temperatures up to 600 K. Evidence for the direct decomposition of γ-RDX above 6 GPa, without the involvement of other phases, is provided. The upper limit of the P-T locus for the γ-RDX thermal decomposition was determined. A refined P-T phase diagram of RDX is presented that includes the current findings for γ-RDX. The static compression results are used to gain key insight into the shock initiation of RDX, including a determination of the RDX phase at decomposition and understanding the role of pressure and temperature in accelerating shock induced decomposition. This study has established the important role that γ-RDX plays in decomposition of RDX under static and shock compression conditions; thus theoretical modeling of RDX decomposition at high pressures and temperatures needs to incorporate the γ-phase response.     

Melting curve and fluid equation of state of carbon dioxide at high pressure and high temperature

The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively heated diamond anvil cell. The melting line was determined from room temperature up to 11.1+/-0.1 GPa and 800+/-5 K by visual observation of the solid-fluid equilibrium and in situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2. An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P-T range of 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.     

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO₂F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO₂F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4 GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4 GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO₂F. Disorder-induced first order Raman modes at 176, 212, 381 and 485 cm⁻¹ soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K⁻¹) is in fair agreement with the reported values (−1 to +1 ppm K⁻¹). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.     

Oxygen phase equilibria near 298 K

Raman spectra of fluid and solid oxygen have been measured at temperatures near 298 K to pressure greater than 180 kbar (18 GPa). At 298 K, fluid oxygen freezes at 59.1±0.5 kbar which is 2 kbar higher than the freezing pressure of n-H₂ at this temperature. Solid—solid phase transitions are observed near 96 and 99 kbar. The phase boundaries near room temperature and the intense visible absorption spectra of the very high pressure phase are described.     

High temperature structures and orientational disorder in compressed solid nitrogen

An experimental study of the phase transitions at high temperature in compressed solid nitrogen has been performed using Raman spectroscopy. Knowledge of the equilibrium phase diagram in the region of the ordered epsilon phase and the two disordered delta and deltaloc phases, at pressures between 10 and 20 GPa, has been extended up to 500 K. The Raman scattering line shape and line width of the active vibrons has been measured accurately, along isobaric scans, across the phase transitions. Analysis of the width and of its different behavior with increasing temperature in the three phases led to more precise conclusions about the nature of the disorder in the different phases. Observation of an evident shoulder in the nu2 band of the deltaloc phase suggests the possibility that sites of two different symmetries may be occupied by the disk molecules in this structure.     

Static compression of tetramethylammonium borohydride

Raman spectroscopy and synchrotron X-ray diffraction are used to examine the high-pressure behavior of tetramethylammonium borohydride (TMAB) to 40 GPa at room temperature. The measurements reveal weak pressure-induced structural transitions around 5 and 20 GPa. Rietveld analysis and Le Bail fits of the powder diffraction data based on known structures of tetramethylammonium salts indicate that the transitions are mediated by orientational ordering of the BH(4)(-) tetrahedra followed by tilting of the (CH(3))(4)N(+) groups. X-ray diffraction patterns obtained during pressure release suggest reversibility with a degree of hysteresis. Changes in the Raman spectrum confirm that these transitions are not accompanied by bonding changes between the two ionic species. At ambient conditions, TMAB does not possess dihydrogen bonding, and Raman data confirms that this feature is not activated upon compression. The pressure-volume equation of state obtained from the diffraction data gives a bulk modulus [K(0) = 5.9(6) GPa, K(0)' = 9.6(4)] slightly lower than that observed for ammonia borane. Raman spectra obtained over the entire pressure range (spanning over 40% densification) indicate that the intramolecular vibrational modes are largely coupled.     

In situ Raman spectroscopic study of the pressure induced structural changes in ammonia borane

The effect of static compression up to 65 GPa at ambient temperature on ammonia borane, BH(3)NH(3), has been investigated using in situ Raman spectroscopy in a diamond anvil cells. Two phase transitions were observed at approximately 12 GPa and previously not reported transition at 27 GPa. It was demonstrated that ammonia borane behaves differently under compression at quasi-hydrostatic and non-hydrostatic conditions. The ability of BH(3)NH(3) to generate second harmonic of the laser light observed up to 130 GPa suggests that the non-centrosymmetric point group symmetry is preserved in the material up to very high pressures.     

High-pressure vibrational properties of polyethylene

The pressure evolution of the vibrational spectrum of polyethylene was investigated up to 50 GPa along different isotherms by Fourier-transform infrared and Raman spectroscopy and at 0 K by density-functional theory calculations. The infrared data allow for the detection of the orthorhombic Pnam to monoclinic P2(1)∕m phase transition which is characterized by a strong hysteresis both on compression and decompression experiments. However, an upper and lower boundary for the transition pressure are identified. An even more pronounced hysteresis is observed for the higher-pressure transition to the monoclinic A2/m phase. The hysteresis does not allow in this case the determination of a well defined P-T transition line. The ambient structural properties of polyethylene are fully recovered after compression/decompression cycles indicating that the polymer is structurally and chemically stable up to 50 GPa. A phase diagram of polyethylene up to 50 GPa and 650 K is proposed. Analysis of the pressure evolution of the Davydov splittings and of the anomalous intensification with pressure of the IR active wagging mode provides insight about the nature of the intermolecular interactions in crystalline polyethylene.     

High-pressure study of tetramethylsilane by Raman spectroscopy

High-pressure behavior of tetramethylsilane, one of the Group IVa hydrides, was investigated by Raman scattering measurements at pressures up to 142 GPa and room temperature. Our results revealed the phase transitions at 0.6, 9, and 16 GPa from both the mode frequency shifts with pressure and the changes of the full width half maxima of these modes. These transitions were suggested to result from the changes in the inter- and intra-molecular bonding of this material. We also observed two other possible phase transitions at 49-69 GPa and 96 GPa. No indication of metallization in tetramethylsilane was found with stepwise compression to 142 GPa.     

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.     

Polymorphism of dense, hot oxygen

The phase diagram and polymorphism of oxygen at high pressures and temperatures are of great interest to condensed matter and earth science. X-ray diffraction and Raman spectroscopy of oxygen using laser and resistively heated diamond anvil cells reveal that the molecular high-pressure phase ε-O(2), which consists of (O(2))(4) clusters, reversibly transforms in the pressure range of 44 to 90 GPa and temperatures near 1000 K to a new phase with higher symmetry. The data suggest that this new phase (η') is isostructural to a phase η reported previously at lower pressures and temperatures, but differs from it in the P-T range of stability and type of intermolecular association. The melting curve increases monotonically up to the maximum pressures studied (～60 GPa). The structure factor of the fluid measured as a function of pressure to 58 GPa shows continuous changes toward molecular dissociation.     

High-pressure vibrational spectroscopy of sulfur dioxide

Solid sulfur dioxide was investigated by vibrational spectroscopy over a broad pressure and temperature range, extending to 32.5 GPa at 75-300 K in diamond anvil cells. Synchrotron infrared spectra provided the first measurements of the pressure dependence of the lattice modes in the far-IR region. Below 17.5 GPa, two fundamentals exhibit splittings enhanced by pressure. The asymmetric stretching mode of SO(2) exhibits a remarkable pressure-induced softening. The observations are consistent with the ambient pressure Raman measurements indicating that SO(2) crystallizes in an acentric cell, but are inconsistent with a previously proposed interpretation that the structure of the high-pressure phase consists of (SO(2))(3) clusters. Dramatic changes in the Raman spectra are found above 17.5 GPa at room temperature. These indicate major changes in structure and possible formation of SO(2) clustering with an enlarged unit cell. The behavior at low temperature differs from that at room temperature. These findings provide constraints on the phase diagram of sulfur dioxide.     

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.     

Compression behaviors of binary skutterudite CoP3 in noble gases up to 40 GPa at room temperature

The binary skutterudite CoP(3) has a large void at the body-centered site of each cubic unit cell and is, therefore, called a nonfilled skutterudite. We investigated its room-temperature compression behavior up to 40.4 GPa in helium and argon using a diamond-anvil cell. High-pressure in situ X-ray diffraction and Raman scattering measurements found no phase transition and a stable cubic structure up to the maximum pressure in both media. A fitting of the present pressure-volume data to the third-order Birch-Murnaghan equation of state yields a zero-pressure bulk modulus K(0) of 147(3) GPa [pressure derivative K(0)' of 4.4(2)] and 171(5) GPa [where K(0)' = 4.2(4)] in helium and argon, respectively. The Gru?neisen parameter was determined to be 1.4 from the Raman scattering measurements. Thus, CoP(3) is stiffer than other binary skutterudites and could therefore be used as a host cage to accommodate large atoms under high pressure without structural collapse.     

Pressure-induced structural changes of the tetragonal Bi2CuO4

The tetragonal compound Bi₂CuO₄ was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.     

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.     

X-ray diffraction and Raman studies of the CuGeO3(III)-(IV) transformation under high pressures

Both X-ray diffraction and Raman spectroscopy measurement were carried out on the same powder sample of CuGeO(3)(III) in a diamond anvil cell to high pressures at room temperature. The phase transformation of (III)-(IV) phase was observed at about 7GPa with both methods and the results were also in accord with previous powder diffraction and Raman measurements, respectively. However, the powder diffraction data were strikingly different from those reported in a recent single-crystal study on the phase (III). It is, therefore, evident that the phase transformations in CuGeO(3)(III) would be as complicated as those in CuGeO(3)(I) and that the monoclinic phase obtained from single-crystal phase (III) at approximately 7GPa is not the phase (IV) previously observed but rather a new phase (IVa) in CuGeO(3).     

岩盐结构氧化锌物态方程的分子动力学模拟

The equation of state of ZnO with rocksalt phase under high pressure and high temperature was calculated by using the molecular dynamics method with effective pair potentials which consist of the Coulomb, dispersion, and repulsion interaction. It was shown that molecular dynamics simulation is very successful in accurately reproducing the measured molar volumes of the rocksalt phase of ZnO over a wide range of temperatures and pressures. The simulated P-V -T data matched experimental results up to 10.4 GPa and 1273 K. In addition, the linear thermal expansion coe±cient, isothermal bulk modulus and its pressure derivative were also calculated and compared with available experimental data and the latest theoretical results at ambient condition. At extended temperature and pressure ranges, the P-V -T relationship, linear thermal expansion coe±cient, and isothermal bulk modulus were predicted up to 2273 K and 50 GPa. The detailed knowledge of thermodynamic behavior and equations of state at extreme conditions are of fundamental importance to the understanding of the physical properties of ZnO.     

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.     

Structure-property relationships in the crystals of the smallest amino acid: an incoherent inelastic neutron scattering study of the glycine polymorphs

Incoherent inelastic neutron scattering spectra for the three crystalline polymorphs (alpha- P2(1)/n, beta- P2(1), gamma- P3(1)) of glycine (C2H5NO2) at temperatures between 5 and 300 K (using the time-of-flight (ToF) spectrometer NEAT at HMI) and at pressures from ambient up to 1 GPa (using the ToF spectrometer IN6 at the ILL) were measured. Significant differences in the band positions and their relative intensities in the density of states (DoS) were observed for the three polymorphs, which can be related to the different intermolecular interactions. The mean-squared displacement, (T), dependence reveals a change in dynamic properties at about the same temperature (150 K) for all the three forms, which can be related to the reorientation of the NH3 group. Besides, a dynamic transition in beta-glycine at about 230-250 K on cooling was also observed, supporting previously obtained adiabatic calorimetry data. This behavior is similar to that already observed in amorphous solids, on approaching the glass transition temperatures, as well as in biological systems. It suggests the onset of degrees of freedom most likely related to transitions between slightly different conformational orientations. The DoS obtained as a function of pressure has confirmed the stability of the alpha-form with respect to pressure and also depicted a sign of the previously reported reversible beta-beta' glycine phase transition in between 0.6 and 0.8 GPa. Moreover, a remarkable kinetic effect in the pressure-induced phase transition in gamma-glycine was revealed. After the sample was kept at 0.8 GPa for an hour in the neutron beam, an irreversible transition into a high-pressure form (different from the beta'-form) occurred, although previously in X-Ray diffraction and Raman spectroscopy experiments a gamma- to delta-glycine phase transition was observed above 3.5 GPa only.