γ-Surfaces for molecular crystal cyclotetramethylene-tetranitramine (β-HMX)

The gamma-surface represents the energetic cost associated with relative, rigid body sliding of crystal planes and contains useful information related to plastic deformation of the respective crystal. Here, we present gamma-surfaces for the most active glide planes of the energetic molecular crystal cyclotetramethylene-tetranitramine in the monoclinic beta phase, i.e., (101) and (011), at pressures up to 15GPa. We observe the existence of stable staking faults in both planes and at all pressures and report the increase in the stacking fault energy with pressure. We also report the energetic barriers for sliding along minimum energy paths in various directions contained in these planes as well as the critical resolved shear stress at which the crystal becomes unstable in the absence of crystal defects. [100] traces of the gamma-surface for multiple planes such as (001), (010), and (021) are further evaluated in view of the previously reported importance of this slip direction for dislocation cross-slip. It is observed that increasing the pressure does not modify the topology of the gamma-surface in an essential way, which implies that although barriers for slip increase, the general phenomenology of dislocation motion is not modified qualitatively by the pressure. The energy barriers increase faster with pressure in the (011) plane, and hence, it is implied that the (101) plane is the most active glide plane at high pressures. The results are generally relevant for studies of plastic deformation in this molecular crystal.