DFT study on crystalline 1,1-diamino-2,2-dintroethylene under high pressures

DFT calculations have been performed to study the structural, electronic, absorption, and thermodynamic properties of crystalline 1,1-diamino-2,2-dintroethylene (α-FOX-7) in the pressure range of 0-40 GPa. A comprehensive analysis of the variation trends of the lattice constants, bond lengths, bond angles, and twist angles under compression shows that six structural transformations occur in α-FOX-7 at 2, 5, 11, 19, 29, and 35 GPa, respectively. The C1-N1 and C1-N2 bond lengths decrease much faster than any other bonds under compression, indicating that the C-NO2 cleavage is possible to trigger the decomposition of α-FOX-7. The intra-molecular H-bonding interaction weakens at 2 and 5 GPa, which may be caused by the structural transformations, but it then strengthens with the increasing pressure up to 40 GPa. The inter-molecular H-bonding interaction strengthens with the increasing pressure. The band gap of α-FOX-7 increases at 11 GPa suddenly and decreases obviously at 19, 29, and 35 GPa, which are caused by the structural transformations. α-FOX-7 has relatively high optical activity at high pressure. All the structural transformations are endothermic and not spontaneous at room temperature.