Shock wave induced decomposition of RDX: time-resolved spectroscopy

Time-resolved optical spectroscopy was used to examine chemical decomposition of RDX crystals shocked along the [111] orientation to peak stresses between 7 and 20 GPa. Shock-induced emission, produced by decomposition intermediates, was observed over a broad spectral range from 350 to 850 nm. A threshold in the emission response of RDX was found at about 10 GPa peak stress. Below this threshold, the emission spectrum remained unchanged during shock compression. Above 10 GPa, the emission spectrum changed with a long wavelength component dominating the spectrum. The long wavelength emission is attributed to the formation of NO2 radicals. Above the 10 GPa threshold, the spectrally integrated intensity increased significantly, suggesting the acceleration of chemical decomposition. This acceleration is attributed to bimolecular reactions between unreacted RDX and free radicals. These results provide a significant experimental foundation for further development of a decomposition mechanism for shocked RDX (following paper in this issue).     

Raman spectra of shock compressed pentaerythritol tetranitrate single crystals: anisotropic response

To gain insight into the anisotropic sensitivity of shocked pentaerythritol tetranitrate (PETN) single crystals, single-pulse Raman spectroscopy was used to examine the response of crystals shocked along the [100] (insensitive) and [110] (sensitive) orientations. High-resolution Raman spectra revealed several orientation-dependent features under shock compression: (i) substantially different stress dependence of the Raman shift for the CH(2) and NO(2) stretching modes for the two orientations, (ii) discontinuity in the stress dependence of the Raman shift for the CH(2) stretching modes above 4 GPa for the [110] orientation, and (iii) large broadening for the CH(2) and NO(2) asymmetric stretching modes for stresses above 4 GPa for the [110] orientation. The present data in combination with previous static pressure results provide support for conformational changes in PETN molecules for shock compression along the [110] (sensitive) orientation. Implications of the present results for the anisotropic sensitivity of shocked PETN are discussed.     

High pressure Raman spectroscopy of single crystals of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Several distinct changes in the spectra were found at 4.0 +/- 0.3 GPa, confirming the alpha-gamma phase transition previously observed in polycrystalline samples. Detailed analyses of pressure-induced changes in the internal and external (lattice) modes revealed several features above 4 GPa: (i) splitting of both the A' and A' ' internal modes, (ii) a significant increase in the pressure dependence of the Raman shift for NO2 modes, and (iii) no apparent change in the number of external modes. It is proposed that the alpha-gamma phase transition leads to a rearrangement between the RDX molecules, which in turn significantly changes the intermolecular interaction experienced by the N-O bonds. Symmetry correlation analyses indicate that the gamma-polymorph may assume one of the three orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor group is favored over the other structures, and it is proposed that gamma-phase RDX has a space group isomorphous with a point group D2h with eight molecules occupying the C1 symmetry sites, similar to the alpha-phase. It is believed that the factor group splitting can account for the observed increase in the number of modes in the gamma-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression.     

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.     

Time-resolved spectroscopic measurements of shock-wave induced decomposition in cyclotrimethylene trinitramine (RDX) crystals: anisotropic response

Plate impact experiments on the (210), (100), and (111) planes were performed to examine the role of crystalline anisotropy on the shock-induced decomposition of cyclotrimethylenetrinitramine (RDX) crystals. Time-resolved emission spectroscopy was used to probe the decomposition of single crystals shocked to peak stresses ranging between 7 and 20 GPa. Emission produced by decomposition intermediates was analyzed in terms of induction time to emission, emission intensity, and the emission spectra shapes as a function of stress and time. Utilizing these features, we found that the shock-induced decomposition of RDX crystals exhibits considerable anisotropy. Crystals shocked on the (210) and (100) planes were more sensitive to decomposition than crystals shocked on the (111) plane. The possible sources of the observed anisotropy are discussed with regard to the inelastic deformation mechanisms of shocked RDX. Our results suggest that, despite the anisotropy observed for shock initiation, decomposition pathways for all three orientations are similar.     

Spectroscopic study of shock-induced decomposition in ammonium perchlorate single crystals

Time-resolved Raman scattering measurements were performed on ammonium perchlorate (AP) single crystals under stepwise shock loading. For particular temperature and pressure conditions, the intensity of the Raman spectra in shocked AP decayed exponentially with time. This decay is attributed to shock-induced chemical decomposition in AP. A series of shock experiments, reaching peak stresses from 10-18 GPa, demonstrated that higher stresses inhibit decomposition while higher temperatures promote it. No orientation dependence was found when AP crystals were shocked normal to the (210) and (001) crystallographic planes. VISAR (velocity interferometer system for any reflector) particle velocity measurements and time-resolved optical extinction measurements carried out to verify these observations are consistent with the Raman data. The combined kinetic and spectroscopic results are consistent with a proton-transfer reaction as the first decomposition step in shocked AP.     

Shock wave induced decomposition of RDX: quantum chemistry calculations

Quantum chemical calculations on single molecules were performed to provide insight into the decomposition mechanism of shocked RDX. These calculations complement time-resolved spectroscopy measurements on shock wave compressed RDX crystals (previous paper, this issue). It is proposed that unimolecular decomposition is the primary pathway for RDX decomposition in its early stages and at stresses lower than approximately 10 GPa. This decomposition leads to the generation of broadband emission from 350 to 850 nm. Chemiluminescence from (2)B1 and (2)B2 excited states of NO2 radicals is associated with a major portion of the experimentally observed emission spectrum (>400 nm). The remaining portion (<400 nm) of the emission spectrum primarily results from excited HONO intermediates. It is proposed that for stresses higher than 10 GPa, bimolecular reactions between radical decomposition products and unreacted RDX molecules become the dominant pathway. This radical assisted homolysis pathway is cyclic and leads to the acceleration of decomposition, with increased production of low energy NO2 radicals. These radicals produce emission that is stronger in the long wavelength portion of the spectrum. Finally, a comprehensive chemical decomposition mechanism is put forward that is consistent with the experimental observations of shock-induced emission in RDX crystals.     

Nanosecond rapid freezing of liquid benzene under shock compression studied by time-resolved coherent anti-Stokes Raman spectroscopy

Nanosecond time-resolved coherent anti-Stokes Raman spectroscopy is used to investigate the shock-induced liquid-solid phase transition and crystallization of liquid benzene. Temporal evolution of the Raman shift of the ring-breathing and C-H stretching modes is investigated. A metastable supercompressed state and a liquid-solid phase transition are observed under shock compression. Time-resolved Raman spectra reveal that the liquid state is initially a metastable state and rapidly transforms to the solid state within 25 ns under shock compression at 4.2 GPa.     

High-pressure vibrational spectroscopy of energetic materials: hexahydro-1,3,5-trinitro-1,3,5-triazine

Vibrational spectroscopy has been used to investigate the room-temperature high-pressure phases of the energetic material hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The pressure-induced alterations in the spectral profiles were studied in a compression sequence to 30.2 GPa using Raman spectroscopy and to 26.6 GPa using far-infrared spectroscopy. At pressures near 4.0 GPa, several changes become immediately apparent in the Raman spectrum, such as large frequency shifts, mode splittings, and intensity changes, which are associated with a phase transition from alpha-RDX to gamma-RDX. Our study extends the kinetic stability of gamma-RDX to pressures near 18.0 GPa. Evidence for a new phase was found at pressures between 17.8 and 18.8 GPa and is based on the appearance of new vibrational bands and associated changes in intensity patterns. The new phase has vibrational characteristics that are similar to those of beta-RDX, suggesting the two polymorphs share a related crystal structure.     

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.     

High-pressure effects in pyrene crystals: vibrational spectroscopy

The response of pyrene crystals to high pressure was examined using Raman and FTIR spectroscopies. Raman spectra of external and internal modes were measured up to 11 GPa. Changes in the external modes were observed at approximately 0.3 GPa, indicating the onset of a phase transition. We demonstrated that at this pressure pyrene I (P2(1)/a, 4 mol/unit cell) transforms to pyrene III (P2(1)/a, 2 mol/unit cell). Further increase of pressure produced a gradual broadening of the internal modes and an increase of fluorescence background, indicating the formation of another phase above 2.0 GPa. Irreversible chemical changes were observed upon gradual compression to 40 GPa. FTIR spectroscopy of the recovered product indicated a transformation of pyrene into an amorphous hydrogenated carbon (a-C:H) structure.     

The ammonia-hydrogen system under pressure

Binary mixtures of hydrogen and ammonia were compressed in diamond anvil cells to 15 GPa at room temperature over a range of compositions. The phase behavior was characterized using optical microscopy, Raman spectroscopy, and synchrotron X-ray diffraction. Below 1.2 GPa we observed two-phase coexistence between liquid ammonia and fluid hydrogen phases with limited solubility of hydrogen within the ammonia-rich phase. Complete immiscibility was observed subsequent to the freezing of ammonia phase III at 1.2 GPa, although hydrogen may become metastably trapped within the disordered face-centered-cubic lattice upon rapid solidification. For all compositions studied, the phase III to phase IV transition of ammonia occurred at ~3.8 GPa and hydrogen solidified at ~5.5 GPa, transition pressures equivalent to those observed for the pure components. A P-x phase diagram for the NH(3)-H(2) system is proposed on the basis of these observations with implications for planetary ices, molecular compound formation, and possible hydrogen storage materials.     

High pressure Raman spectra of DL-lysine hydrochloride

DL-lysine hydrochloride crystals were studied by Raman spectroscopy under hydrostatic pressure using a diamond anvil cell from ambient pressure up to 9.8 GPa in the spectral range from 1150 to 40 cm⁻¹. Changes in the Raman spectrum were observed in all spectral regions analyzed. In particular, modifications in the lattice modes indicate that the crystal undergoes a phase transition. The classification of the vibrational modes, the behavior of their band wavenumber as a function of the pressure and the reversibility of the phase transitions are also discussed.     

Pressure-induced phase transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.     

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.     

High-pressure Raman spectra of racemate dl-alanine crystals

Raman spectroscopy investigations of dl-alanine crystal under high pressures have been carried out up to 18.0 GPa. For instance, around 1.0 GPa and between 1.7 and 2.3 GPa changes in the Raman profile were observed and associated to conformational changes of the molecules in the unit cell or to a phase transition accompanied to slight conformational change of the molecule through CH and CH₃ groups. Moreover, between 6.0 and 7.3 GPa, the appearance of a new low energy lattice modes and to the splitting of a band assigned to the stretching vibration of the CCH₃ moiety were related to a second phase transition. Finally, changes in lattice modes, red shift of the band associated to CCH₃ stretching and increasing of line-width of the band associated to the wagging of CO₂, between 11.6 and 13.2 GPa, are ascribed to a third phase transition. On release of pressure the original phase was obtained again.     

Shock-absorbing and failure mechanisms of WS2 and MoS2 nanoparticles with fullerene-like structures under shock wave pressure

The excellent shock-absorbing performance of WS2 and MoS2 nanoparticles with inorganic fullerene-like structures (IFs) under very high shock wave pressures of 25 GPa is described. The combined techniques of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, thermal analysis, and transmission electron microscopy have been used to evaluate the diverse, intriguing features of shock recovered IFs, of interest for their tribological applications, thereby allowing improved understanding of their antishock behavior and structure-property relationships. Two possible failure mechanisms are proposed and discussed. The supershock-absorbing ability of the IF-WS2 enables them to survive pressures up to 25 GPa accompanied with concurrent temperatures of up to 1000 degrees C without any significant structural degradation or phase change making them probably the strongest cage molecules now known.     

High-pressure induced conformational and phase transformations of 1,2-dichloroethane probed by Raman spectroscopy

1,2-Dichloroethane (DCE) was loaded into diamond anvil cells and compressed up to 30 GPa at room temperature. Pressure-induced transformations were probed using Raman spectroscopy. At pressures below 0.6 GPa, fluid DCE exists in two conformations, gauche and trans in equilibrium, which is shifted to gauche on compression. DCE transforms to a solid phase with exclusive trans conformation upon further compression. All the characteristic Raman shifts remain constant in fluid phase and move to higher frequencies in the solid phase with increasing pressure. At about 4-5 GPa, DCE transforms from a possible disordered phase into a crystalline phase as evidenced by the observation of several lattice modes and peak narrowing. At 8-9 GPa, dramatic changes in Raman patterns of DCE were observed. The splitting of the C-C-Cl bending mode at 325 cm-1, together with the observation of inactive internal mode at 684 cm-1 as well as new lattice modes indicates another pressure-induced phase transformation. All Raman modes exhibit significant changes in pressure dependence at the transformation pressure. The new phase remains crystalline, but likely with a lower symmetry. The observed transformations are reversible in the entire pressure region upon decompression.     

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.