High-pressure structural evolution and intermolecular interactions in CL-20 and the CL-20/HMX cocrystal

Understanding the pressure response of energetic materials (EMs) is crucial for addressing issues pertaining to sensitivity, performance, and safety. In this work, pressure-dependent Raman spectroscopy in conjunction with structure responses and intermolecular interactions is used to probe the high-pressure structural stability of CL-20 and CL-20/HMX. High-pressure Raman spectroscopy shows that pure CL-20 undergoes an initial phase transformation within the 2-6 GPa range, accompanied by a discontinuous shift in Raman wavenumbers. The resulting structure at 6 GPa is significantly distinct from the ambient-pressure γ-phase structure, arising from differences in the orientation of nitro groups relative to adjacent five- and six-membered rings. In contrast, the CL-20/HMX cocrystal maintains its structural stability up to 15 GPa mediated by a robust hydrogen-bonding network between molecular layers. Pressure-induced suppression of O⋯O interactions and the spatial delocalization of the frontier molecular orbitals of CL-20/HMX collectively contribute to its structural integrity. Crucially, symmetry-dependent vibrational mode coupling occurring at 3 and 9 GPa facilitates external energy transfer within the crystal lattice, thereby enhancing structural stability. This work elucidates pressure-induced structural transformation and interaction evolution in pure CL-20, while revealing spectroscopic signatures of structural stability in the CL-20/HMX cocrystal.