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.

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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.

The decomposition mechanism in shocked pentaerythritol tetranitrate (PETN) was examined using timeresolved emission spectroscopy. PETN single crystals were subjected to stepwise loading along [100] and [110] to peak stresses between 2 and 13 GPa. Due to concurrent changes in the optical transmission of PETN, emission spectra were analyzed using the absorption data acquired separately under the same loading conditions. Analyses of the corrected emission data revealed two bands in the spectra at ∼3.0 and ∼2.4 eV. Both bands were observed in every experiment regardless of stress or crystal orientation. However, their relative and absolute intensities, and temporal behavior revealed stress and orientation dependence. The emission was identified as chemiluminescence from the nitronium ion, NO 2 + , on the basis of its electronic structure and properties. NO 2 + electronic structure was analyzed using ab initio calculations, which showed transition energies matching those of the emitting intermediate observed experimentally. Several chemical pathways compatible with the formation of NO 2 + are considered and evaluated. Finally, a four-step chemical initiation mechanism in shocked crystalline PETN is proposed and discussed in detail.

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.

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).

Geometry optimizations and normal-mode analyses of the pentaerythritol tetranitrate (PETN) conformer belonging to the S 4 molecular point group and comprising the crystalline solid were performed using density functional theory methods (B3LYP and B3PW91). The basis sets used in this study were 6-31G(d) and 6-311+G(d,p). The structural results are in good agreement with experimental X-ray diffraction data. The predicted bond lengths and bond angles are accurate to within ∼2.5% and ∼1.2%, respectively. Raman and infrared spectra of crystalline PETN were measured and compared with the calculated spectra. The calculated and measured spectra agree very well in the spectral region below 1100 cm -1 ; the agreement is satisfactory for frequencies higher than 1100 cm -1 . On the basis of the calculations and analyses, normal mode assignments were made and mode symmetries determined.

A chemical mechanism to explain the observed anisotropy in the shock wave initiation of pentaerythritol tetranitrate (PETN) single crystals is proposed on the basis of semiempirical quantum chemical calculations. Building on the previously proposed model of steric hindrance to shear, the molecular mechanics of shear deformation at the lattice level is correlated with rotational conformations of PETN. The numerous stable conformations of PETN differ in symmetry and dipole moment values. The initial conformation belongs to the S 4 molecular point group and possesses no dipole moment. Because of shear deformations, the molecules change conformations. The [110] shocks result in sterically hindered shear and generate polar conformations. In contrast, the [100] shocks result in little or no polarization. Because the decomposition chemistry of PETN at 5−10 GPa is likely dominated by ionic reactions, local polarity of the lattice plays a crucial role in reactivity. The polar lattice stabilizes the transition state due to dipole−dipole interactions and, thus, facilitates the ionic dissociation. In contrast, the nonpolar lattice results in no stabilization and low reaction rates. Plausible ionic reactions are briefly discussed and experiments are suggested to verify the mechanism proposed.