Pressure-induced structure, vibrational properties, and initial decomposition mechanisms of δ-HMX crystal: A periodic...

A systematical theoretical study of structure, vibrational properties, and initial decomposition mechanisms of δ-HMX in the pressure range of 0-30 GPa was performed by using density functional theory (DFT). The results show that HMX presents a structural transition at 8 GPa, which has not been found in experiments yet. As the pressure increases, the intra-molecular hydrogen bonding interactions enhance gradually, but they have a sudden decrease at 8 GPa. A comprehensive analysis of density of states and population analysis indicates that the N-N bonds are prone to cleave upon impact or stimulation and the N-NO₂ group may act as the active site of the initial decomposition of δ-HMX under the compression. The pressure-dependent frequency shifts indicate that all the modes move towards higher vibrational frequencies. Between the two competition decomposition mechanisms of the δ-HMX crystal, the N-NO₂ homolysis becomes more preferential than the HONO elimination as the pressure increases. Our studies may shed a light on a new understanding of the initial decomposition mechanisms of energetic materials under high pressures.