High-Pressure Polymorphism of 1,1-Diamino-2,2-dinitroethene (FOX-7)

Understanding the polymorphic response of energetic crystals is important for understanding their initiation and reactive behavior. Here, we report on the high-pressure polymorphism of the energetic crystal FOX-7 [C$_{2}$(NO$_{2}$)$_{2}$(NH$_{2}$)$_{2}$]; the low sensitivity to initiation has attracted considerable research interest in this crystal. Micro-Raman spectroscopy and synchrotron x-ray diffraction measurements were used to gain insight into the mechanisms of polymorphic transformations, and their role in the high-pressure structural stability of FOX-7. Experiments were performed on single crystals compressed statically to 40 GPa (Raman) and to 12 GPa (x-ray). Two instances of spectral changes were detected at 2 and 4.5 GPa with Raman spectroscopy. Different experimental approaches, including isotope substitution (H/D), nonhydrostatic compression and laser radiation were used to understand the molecular processes associated with the observed spectral changes. The x-ray diffraction results demonstrated that the same space group, P\textit{2}$_{\mathrm{1}}$/n, is maintained to 4.5 GPa, the $\beta $ angle reduces to almost 90$^{\mathrm{0}}$, and the crystal shows anisotropic compression. Preliminary structure refinement results indicate that changes at 2 GPa can result from the amino groups twist out of the molecular plane. The structural changes at 4.5 GPa indicate the reconstructive character of the phase transition at this pressure.