High-Pressure Crystal Structures of an Insensitive Energetic Crystal: 1,1-Diamino-2,2-dinitroethene

Understanding the insensitivity/stability of insensitive high explosive crystals requires detailed structural information at high pressures and high temperatures of interest. Synchrotron single-crystal X-ray diffraction experiments were used to determine the high-pressure structures of 1,1-diamino-2,2-dinitroethene (FOX-7), a prototypical insensitive high explosive. The phase transition around 4.5 GPa was investigated and the structures were determined at 4.27 GPa (alpha' phase) and 5.9 GPa (e phase). The alpha' phase (monoclinic, P2(1)/n), structurally indistinguishable from the ambient a phase, transforms to the new e phase (triclinic, P1). The most notable features of the e phase, compared to those of the a' phase, are formation of planar layers and flattening of molecules. Density functional theory (DFT-D2) calculations complemented the experimental results. The results presented here are important for understanding the molecular and crystalline attributes governing the high-pressure insensitivity/stability of insensitive high explosive crystals.