In silico study of α-γ phase transformation in hexahydro-1,3,5-trinitro-1,3,5-triazine

Abstract

Molecular dynamics (MD) simulations are performed using the Smith and Bharadwaj (SB) molecular potential to study the response of α-RDX to hydrostatic compression up to 4 GPa between 300 K and 450 K. We observe a phase transformation to the high-pressure γ polymorph around 2.5 GPa for temperatures greater than or equal to 350 K. The α-γ phase transformation is captured through an abrupt decrease in the ‘b’ lattice parameter and volume while the ‘c’ lattice parameter shows an abrupt increase. The molecular conformation is shown to evolve from AAE to AAE-AAI during the phase transition, consistent with experimental observations. Abrupt changes are also observed in molecular and non-bonded energy terms of the SB potential. The phase at the end of hydrostatic loading is verified to be γ-RDX. The simulations demonstrate that SB potential can be effective in further elucidating the deformation mechanisms in RDX compressed beyond the phase transition pressure. We hypothesize that the difference in angle energy between the two phases plays an important role in the occurrence of α-γ phase transformation in RDX using the SB potential.