Structural Transformation and Chemical Stability of a Shock-Compressed Insensitive High Explosive Single Crystal: Time-Resolved Raman Spectroscopy.

Despite the considerable interest in insensitive high explosives (IHE) as a safer alternative to conventional high explosives, a good understanding of the low sensitivity of IHEs to shock initiation is lacking. In particular, real-time measurements to directly probe the molecular-level response of shock-compressed IHE single crystals constitute an important need. To address this need, plate impact experiments were conducted to determine time-resolved changes in the Raman spectra of 1,1-diamino-2,2-dinitroethene (FOX-7) single crystals-a representative IHE crystal-shock-compressed up to 20 GPa longitudinal stress. The Raman measurements examined vibrational frequencies from 800 to 1500 cm-1 with 15 ns time resolution and were conducted at several peak stresses. At 4-6 GPa, two new Raman peaks appeared, in addition to the original peaks, consistent with onset of the α'-ε structural transformation reported previously in static compression work. The measured spectra indicated completion of the transformation at 10 GPa. Raman data to 20 GPa showed neither additional transformations nor any indication of chemical decomposition. This finding, though consistent with recent continuum measurements, is in marked contrast to the chemical decomposition observed at lower stresses in shock-compressed conventional high explosive single crystals. Our Raman results support the previous suggestion that strengthening of intra- and intermolecular bonds, because of the α'-ε structural transformation, plays a significant role in the insensitivity of FOX-7 single crystals to shock initiation. The present work, in conjunction with previous static compression studies, provides the first experimental insight into the molecular-level response of a shock-compressed IHE single crystal and can serve as a bench mark for theoretical studies.

High-pressure vibrational and polymorphic response of 1,1-diamino-2,2-dinitroethene single crystals: Raman spectroscopy.

Raman spectroscopy was used to examine the vibrational and polymorphic behavior of 1,1-diamino-2,2-dinitroethene (FOX-7) to elucidate its structural and chemical stability under high pressure. Measurements were performed on single crystals compressed in a diamond anvil cell, and data were obtained over the entire frequency range of FOX-7 Raman activity. Several new features were observed with increase of pressure: (i) new vibrational peaks and discontinuity in the shifts of the peaks at 2 and 4.5 GPa, (ii) apparent coupling or mixing of several modes, and (iii) changes in the NH2 stretching spectral shape and modes shift. The spectral changes at 2 GPa, in contrast to previous reports, involved only a few peaks and likely resulted from a small molecular transformation. In contrast, changes at 4.5 GPa involved most of the modes, and the pressure for the onset and completion of the changes depended on the pressure medium. A large pressure hysteresis regarding the changes at 4.5 GPa implies a reconstructive transformation. We suggest that this transformation reflects a change in the balance between interlayer (van der Waals) and in-layer (H-bonding) interactions. Despite these transformations, further compression to 40 GPa and subsequent release of pressure did not cause any irreversible changes. This finding implies that FOX-7 has remarkable chemical stability under high pressures. The observed coupling between the various modes with increasing pressure was analyzed within the Fermi resonance model. The potential implication of the coupling of modes for shock insensitivity of FOX-7 is briefly discussed.

High pressure-high temperature polymorphism and decomposition of pentaerythritol tetranitrate (PETN).

To elucidate the behavior of PETN at thermo-mechanical conditions relevant for shock initiation, Raman spectroscopy, and optical imaging were used to examine its static high pressure and high temperature (HP-HT) response. Experiments were performed on single crystals in a heated diamond anvil cell at pressures to 14 GPa and temperatures ranging from room temperatures to 550 K. Regarding the pressure-induced PETN-I transition to PETN-III at room temperature, our results show that nonhydrostaticity plays an important role in driving this transition. Furthermore, we found that PETN-III transforms to PETN-IV at high temperatures, and this transformation can involve lowering of molecular symmetry from C2 to C1. The HP-HT phase diagram for PETN presented here includes the melting/decomposition curve and boundaries between three PETN phases: PETN-I, PETN-III, and PETN-IV. The relevance of static compression results for shock initiation of PETN is discussed.

Real-time (nanoseconds) determination of liquid phase growth during shock-induced melting.

Melting of solids is a fundamental natural phenomenon whose pressure dependence has been of interest for nearly a century. However, the temporal evolution of the molten phase under pressure has eluded measurements because of experimental challenges. By using the shock front as a fiducial, we investigated the time-dependent growth of the molten phase in shock-compressed germanium. In situ x-ray diffraction measurements at different times (1 to 6 nanoseconds) behind the shock front quantified the real-time growth of the liquid phase at several peak stresses. These results show that the characteristic time for melting in shock-compressed germanium decreases from ~7.2 nanoseconds at 35 gigapascals to less than 1 nanosecond at 42 gigapascals. Our melting kinetics results suggest the need to consider heterogeneous nucleation as a mechanism for shock-induced melting and provide an approach to measuring melting kinetics in shock-compressed solids.

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.

High pressure-high temperature decomposition of γ-cyclotrimethylene trinitramine.

Decomposition of γ-cyclotrimethylene trinitramine (γ-RDX) under high pressure-high temperature conditions was examined to elucidate the reactive behavior of RDX crystals. Vibrational spectroscopy measurements were obtained for single crystals in a diamond anvil cell (DAC) at pressures from 6 to 12 GPa and temperatures up to 600 K. Global decomposition rates, activation energies, and activation volumes at several pressures and temperatures below the P-T locus for the γ-RDX decomposition were obtained. Similar to ε-RDX, but in contrast to α-RDX, we found that pressure decelerates the decomposition of γ-RDX. The decomposition deceleration with pressure in the γ-phase can be attributed to pressure-inhibiting bond homolysis step(s). The main decomposition species were identified as N(2)O, CO(2), and H(2)O, in accord with the species reported for the α-phase decomposition at high pressures. This work complements previous studies on RDX at HP-HT conditions and provides comprehensive results on the reactive behavior of γ-RDX; the γ-phase plays a key role in RDX decomposition at P-T conditions relevant to shock wave initiation.

Phase diagram of hexahydro-1,3,5-trinitro-1,3,5-triazine crystals at high pressures and temperatures.

Raman spectroscopy and optical imaging were used to determine the phase boundaries between various hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) polymorphs. Experiments were performed on single crystals at pressures up to 8.0 GPa and temperatures ranging from room temperature to 550 K. Several distinct pressure regions were found in the RDX response at elevated temperatures: (i) melting of alpha-RDX followed by decomposition, below 2.0 GPa, (ii) decomposition of alpha-RDX between 2.0 and 2.8 GPa, (iii) irreversible transformation of alpha- and gamma-RDX to epsilon-RDX between 2.8 and 6.0 GPa, and (iv) decomposition of gamma-RDX above 6.0 GPa. A triple point between the alpha-, gamma-, and epsilon-RDX was found at 3.7 GPa and 466 K. The alpha-gamma phase transition was confirmed to occur at the same pressure, approximately 3.7 GPa, regardless of temperature, in the range of 295-460 K. Furthermore, it was determined that epsilon-RDX (i) has limited chemical stability under the pressure and temperature conditions where it is produced and (ii) decomposes according to the autocatalytic rate law. The findings reported here have provided new information about the response of RDX crystals at high pressures and temperatures.

Raman spectroscopy of high-pressure-high-temperature polymorph of hexahydro-1,3,5-trinitro-1,3,5-triazine (epsilon-RDX).

Raman spectroscopy was used to determine the vibrational structure and the stability of the high-pressure-high-temperature (HP-HT) polymorph of RDX after it had been quenched to room temperature. Although this polymorph has limited chemical stability under high pressure and temperature, we show that it is chemically and structurally stable from 0.6 GPa to at least 20 GPa at room temperature. Below 0.6 GPa, it readily converts to the alpha-polymorph. Pressure dependence of the vibrational structure of the HP-HT polymorph was measured and compared with the vibrational structures of other known RDX polymorphs: alpha, beta, and gamma. In contrast with previous suggestions, our data indicate that the HP-HT polymorph can have a different structure than the beta-polymorph. This finding supports the recent suggestion that the HP-HT polymorph should be given a separate designation, epsilon-RDX. Furthermore, symmetry correlation analyses of Raman spectra indicate that the HP-HT polymorph (epsilon-RDX) may assume the space group isomorphous with the C(2v)[C(1)(4)] point group and with molecules adopting the pseudo-AAA conformation.

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.

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.

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).

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.

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.

Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds.

The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.

High-pressure-induced phase transitions in pentaerythritol: X-ray and Raman studies.

The high-pressure response of pentaerythritol crystals has been examined to 10 GPa in diamond-anvil cells using angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The results reveal two first-order phase transitions: one at 4.8 GPa from phase I, tetragonal I(), to phase II, orthorhombic Pnn2C2v10, with a small approximately 0.5% volume change, and the other at 7.2 GPa to phase III with an unknown crystal structure. We found that phase I exhibits a large crystallographic anisotropy which rapidly decreases with increasing pressure: the ratio of linear compressibilities between two primary crystal axes decreases from betao= 8.1 at 1 atm to betaP = 2.6 at 4 GPa. We suggest that this apparent decrease in crystal anisotropy is due to the disruption of hydrogen bonding in the (001) plane of phase I and eventually leads to an orthorhombic distortion from a quadrilateral network structure in phase I to a quasi one-dimensional structure in phase II. The crystal structure of phase III exhibits a disordered character, and it is likely a conformational variant of phase II.

High-Pressure Stability of Energetic Crystal of Dihydroxylammonium 5,5'-Bistetrazole-1,1'-diolate: Raman Spectroscopy and DFT Calculations.

The vibrational and structural behavior of a novel, energetic crystal, dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50), was examined over a broad pressure range to elucidate its structural and chemical stability at high pressures. Raman measurements were performed on single crystals compressed to 50 GPa in a diamond anvil cell, and data were obtained over the entire frequency range of TKX-50 Raman activity. The Raman spectroscopy results were complemented by density functional theory (DFT) calculations to provide vibrational mode assignments and to gain insight into pressure effects on the vibrational and crystal response of TKX-50. Several features were observed in Raman spectra measured in the ranges 4-10, 10-13, and 32-36 GPa. We suggest that the changes between 32 and 36 GPa may be associated with a phase transformation. In addition, a number of vibrational modes showed intensity exchange and avoided crossing of vibrational frequency at various pressures, characteristic of the coupling of modes. Despite all these pressure effects, the compression of TKX-50 to 50 GPa and the subsequent release of pressure did not result in any irreversible spectral changes, demonstrating its remarkable chemical stability. DFT calculations, using the PBE functional with an empirical dispersion correction by the Grimme, PBE-D method, were used to calculate pressure effects on Raman frequencies and unit cell parameters. The calculated Raman shifts to 20 GPa are in good overall agreement with the measured shifts over a broad range of frequencies. The calculations also show that TKX-50 exhibits anisotropic compressibility, with a highly incompressible response along the a axis. The calculated bulk modulus, a measure of average stiffness, of TKX-50 is significantly higher than the calculated or measured bulk moduli of other energetic crystals. We suggest that the strong intermolecular interactions and the coupling of vibrational modes may potentially contribute to the shock insensitivity of TKX-50. This work demonstrates the robust high-pressure response of TKX-50, making this crystal attractive for practical applications.

Stiff response of aluminum under ultrafast shockless compression to 110 GPA.

A laser-produced x-ray drive was used to shocklessly compress solid aluminum to a peak longitudinal stress of 110 GPa within 10 ns. Interface velocities versus time for multiple sample thicknesses were measured and converted to stress density (Px-rho) using an iterative Lagrangian analysis. These are the fastest shockless compression Px(rho) results reported to date, and are stiffer than models that have been benchmarked against both static and shock-wave experiments. The present results suggest that at these short time scales there is a higher stress-dependent strength and a stiffer time-dependent inelastic response than had been expected.