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.