The pressure-induced phase transition of the solid β–HMX

Abstract

The structural, vibrational and thermodynamic properties of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β–HMX) crystal have been studied using the isothermal-isobaric molecular dynamics (NPT-MD) simulations. The variations of cell volume, lattice constants and molecular geometry of solid β–HMX are presented and discussed at different pressure and temperature. It was found that the N–N bond is significantly lengthened with increasing temperature, which suggests that it is relevant to the initial decomposition. An abrupt change at 27 Gpa for the volume and internal geometrical parameters was observed. This is in good accord with the experimental observation that there is a phase transition at 27 GPa, which is clearly due to conformational change, not chemical reaction. The vibrational frequencies at ambient conditions agree well with experimental results, and the pressure/temperature-induced frequency shifts of these modes are discussed. Frequency discontinuity was also observed at pressure when the phase transition occurred. The Grüneisen parameter was obtained using the vibrational frequency.