Structures, Elasticity, and Sensitivity Characteristics of ɛ-CL-20 under High Pressure from First-Principles Calculations

First-principles calculations have been conducted for ɛ-CL-20 to study the pressure effects on the structures, elasticity, and sensitivity characteristics under 0–75 GPa. It is found that N–NO₂ possesses smaller populations than other bonds and is verified as the trigger bond for ϵ-CL-20, and the nitrogen atoms on nitro groups (–NO₂) are more sensitive to pressure than that on the rings. A detailed understanding of elasticity has been studied and discussed under pressure. In general, the mechanical properties of ɛ-CL-20 are distinctly enhanced under compression. The obtained elastic constants show better agreement with the experiment at zero pressure. As the pressure increases, the lattice interactions towards different directions present pronounced difference. A relationship of C₁₁ ≈ C₃₃ > C₂₂ is obtained in the lower pressure region. However, a different relationship of C₁₁ > C₂₂ ≈ C₃₃ is predicted under higher pressure. The crystal, ɛ-CL-20, changes from brittleness to plasticity according to the sharp increase of B/G and C₁₂–C₄₄ under compression. And the band gap of ɛ-CL-20 decreases with the increasing pressure, indicating the crystal becomes more sensitive under compression.