RDX: Vibrational properties of four phases from first-principles calculation

Understanding the initiation mechanism of energetic materials (EMs) is the core of their safety research, and it is closely related to the basic physical properties of crystal. The vibration properties of crystals play an important role in the study of macroscopic properties and microscopic processes of materials, which are related to many fundamental physical properties. Cyclotrimethylene trinitramine (RDX), as one of the most widely used EMs, has received extensive attention since its inception. In this paper, the vibrational properties of α, β, γ, ε-RDX under their respective stable pressure were mainly studied. By comparing the Raman peaks and phonon densities of states of the four phases, it is concluded that the RDX exhibits methylene vibrations in the high-frequency region, coupled nitro and methylene vibrations in the mid-frequency region, and the ring vibration in the low-frequency region. The characteristic Raman peaks will disappear and new peaks will appear during the transition from α-RDX to β, γ, ε-RDX. Moreover, the Raman peak of RDX has a blue shift phenomenon under pressure.