Mechanical Anisotropy of the Energetic Crystal of 1,1-Diamino-2,2-dinitroethylene (FOX-7): A Study by Nanoindentation Experiments and Density Functional Theory Calculations

The mechanical anisotropy of the wavelike π-stacked energetic crystal of 1,1-diamino-2,2-dinitroethylene (FOX-7) is investigated by nanoindentation experiments and density functional theory (DFT) calculations. The FOX-7 crystal exhibits distinct mechanical anisotropy when indented on different faces. The elastic modulus and hardness of the (020), (−101), and (002) faces change in a decreasing order. The indentation on the (020) face induces the largest depth and the highest pile-up around all three edges of the indenter without causing crack formation. By contrast, the indentations on the (−101) and (002) faces are similar and induce a small indentation depth, low pile-up with a small distribution, and crack formation. Mechanical anisotropy is essentially determined by the wavelike π stacking of FOX-7 along the (020) face with the support of intermolecular hydrogen bonds; i.e., the molecular orientations and intermolecular spaces along different faces vary distinctly. This is also supported by the DFT cal...