Molecular dynamics study of martensitic phase transformations in HMX: β-HMX twinning and β-ε phase transition

We study the response of β-1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (β-HMX, P2₁/n space group setting) to uniaxial compression using molecular dynamics simulations. Compressing β-HMX along the c* = a × b direction, where a and b are the crystallographic unit-cell vectors, at T=300 K leads to a twinning transformation when the normal stress component parallel to c* reaches approximately 0.9 GPa. The energetic barrier to the transformation occurs when the β-HMX lattice parameters a and c become equal, and the overall process corresponds to a martensitic transformation. The mechanism can be approximately separated into two stages: glide of the essentially intact {101} crystal planes along〈10-1〉crystal directions followed by molecular rotations within the unit cells. The pathway is symmetric in the sense that the initial and final structures are equivalent in the β-HMX crystal frame but with switching of the original a and c lattice vectors in the Cartesian frame. If the uniaxial compression along c* is applied to a β-HMX crystal which is already subject to a hydrostatic pressure ≳ 10 GPa, the transformation described above proceeds through the crystal-plane gliding stage but no molecular rotation occurs. This results in a high-pressure phase of HMX belonging to the P2₁/n space group, which we tentatively associate with ε-HMX. The coexistence curve for β- and ε-HMX is constructed using the harmonic approximation for the crystal Hamiltonians.