Study on phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers by density functional theory method.

The phonon spectra and heat capacities of 2, 4, 6, 8, 10, 12-hexanitrohexaazaisowurtzitane/1-methyl-3, 4, 5-trinitropyrazole (CL-20/MTNP) cocrystal and co-formers were calculated in the framework of DFT. By analyzing the phonon density of states (DOS), the energy flow directions and trigger bonds of cocrystal and co-formers have been obtained and the microscopic physical nature was revealed for thermal decomposition mechanism, detonation performance, and sensitivity. For CL-20/MTNP cocrystal, the phonon number of "doorway" modes and the characteristic vibrational frequencies Δωd are between those of its co-formers, which can provide the microscopic understanding for the ordering of impact sensitivity at experiment, ε-CL-20 > CL-20/MTNP > MTNP. In CL-20/MTNP cocrystal, more phonons and stronger phonon DOS peaks of CL-20 molecules than those of MTNP molecules mean cocrystal's detonation performance is mainly dominated by CL-20 molecules. The heat capacities obtained by the Debye model rise with elevated temperatures at 0-600 K and the order is ε-CL-20 > CL-20/MTNP > MTNP. Graphical abstract The phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers were calculated by density functional theory (DFT). In CL-20/MTNP cocrystal, the detonation performance and impact sensitivity are mainly dominated by CL-20 molecules. The broken bonds caused by energy transfer may undergo a multi-phonon pumping process.