Molecular dynamics simulation of FOX-7 decomposition reaction under high temperature and pressure

Context

The decompositions of FOX-7 under high temperatures (2750–3750 K) and high pressures (0–50 GPa) were investigated using the ReaxFF-lg reactive force field molecular dynamics method, revealing its thermodynamic evolution and product formation mechanisms. The decomposition products are all NO₂, NO, N₂, H₂O, CO₂, HNCO, H₂, CO and NH₃, under high-temperatures and high-pressures conditions. Among these products, the intermediate products are NO₂ and NO, and the stabilization products are N₂, H₂O, CO₂, HNCO, H₂, CO and NH₃. And N₂ is consistently the most abundant product, while HNCO is the least abundant substance. In general, the yield of these products shows a positive correlation with temperature and a negative correlation with pressure. However, NH₃ content increases as pressure rises under high pressures. Additionally, FOX-7’s initial decomposition pathways are: C–NO₂ cleavage (yielding NO₂), N–O rupture (releasing O) and N–H dissociation (releasing H). This paper investigates the thermal decomposition behavior of FOX-7 under extreme conditions of high temperature and high pressure, revealing its decomposition pathway and providing support for the study of the decomposition behavior of other similar substances.

Methods

Molecular dynamics simulations of FOX-7 were performed using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) with the ReaxFF-lg force field. A 2 × 4 × 2 supercell was constructed based on X-ray diffraction data, optimized geometrically (0.1 fs time step), equilibrated via NVE ensemble (10 ps, heated from 0 to 300 K) and NPT ensemble (15 ps, 300 K), verifying the applicability of ReaxFF-lg. To study high-temperature and high-pressure effects on FOX-7 thermal decomposition, two approaches were used. First, under NVE ensemble, the system was heated to target temperatures (2750—3750 K.) over 150 ps, then maintained for 150 ps (0.1 fs step, periodic boundaries). Second, initial pressures (0–50 GPa) were applied at 300 K via NPT ensemble for 20 ps, followed by heating to 3500 K over 50 ps under NVE. Atomic trajectories, species, and thermodynamic data were recorded every 10 fs.