The ground state α-phase of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) undergoes phase transition to γ-phase around 3.8 GPa under hydrostatic loading. γ-RDX plays an important role in the detonation mechanism of RDX.1 While the two polymorphs have been extensively studied in their respective stability range, the atomistic modeling of α-γ phase transition under hydrostatic loading has received much less consideration. In this work, we report α-γ phase transformation under hydrostatic loading using molecular dynamics (MD) with a non-reactive fully flexible Smith-Bharadwaj (SB) molecular potential.
The simulation box consists of α-RDX molecules with non-orthogonal periodic boundary conditions. Initially, the system is equilibrated at 0Pa and 300K using isothermal-isostress (NσT) ensemble and then equilibrated till 450K in increments of 25K at 0Pa. At each temperature increment, the system is equilibrated from 0-2GPa and 2-4GPa in increments of 0.5GPa and 0.1GPa, respectively. At each equilibration point, the pressure-volume-temperature data, the individual energy terms in SB potential, and the atomic positions of all molecules in the system are collected and analyzed to predict the α-γ phase transition.
We observe a sharp decrease in the volume around 2.5GPa for temperature beyond 350K, which is indicative of α-γ phase transition. Though the transition pressure is below the experimentally observed pressure of 4GPa, it is consistent with other phase transformation studies of RDX using MD simulations with SB potential.2 We also observe an abrupt decrease in the dihedral and improper dihedral energy terms of the SB potential, and a sudden increase in the angle energy around the transition point, which is due to the change in the conformation of RDX during phase transition. After the phase transition, the angle and dihedral energies approximately saturate to the respective energies at 0Pa, which is also predicted in previous MD simulations. There is no change in the bond energy since the bond distance and type do not change between the two polymorphs. The wag angles for two nitro groups of all the molecules remain axial throughout the phase transformation process. For the third nitro group, the wag angle for half of the molecules remains equatorial, whereas, that for the other half evolves into intermediate orientation. These are consistent with the experimental crystal structures of the two polymorphs.
Our simulations indicate that the SB potential is able to capture the α-γ phase transition in RDX given appropriate thermal excitation to induce changes in the molecular conformation. It elucidates the efficacy of SB potential in investigating problems involving α-γ phase transition in RDX.
References
1. Dreger and Gupta, J. Phys. Chem. A 116, 8713 (2012).
2. Munday et al., J. Phys. Chem. B 115, 4378 (2011).
2016 Materials Research Society Fall Meeting, Boston, Massachusetts (2016)