Dissociation of ⟨111⟩ dislocations on {11¯0} in pentaerythritol tetranitrate

Plastic deformation in the tetragonal explosive molecular crystal pentaerythritol tetranitrate (PETN) is mediated in part by dislocations with Burgers vector ⟨111⟩ on {11¯0}. This observation is unexpected since ⟨111⟩ is the longest possible Burgers vector in PETN. Moreover, plastic slip on the {101¯} planes has not been observed. The generalised stacking fault energy surface for PETN (11¯0) has been computed using dispersion-corrected density functional theory that identified a metastable stacking fault for the displacement 1/2[111]. The stacking fault energy computed with full relaxation of the atomic positions is 83.5 mJ/m2. This result indicates that dislocations with Burgers vector ⟨111⟩ split into two partial dislocations with Burgers vector 1/2⟨111⟩ on {11¯0} with an equilibrium separation of about 35 and 53 Å for the screw and edge dislocations, respectively. Steric hindrance between molecules render the 1/2⟨111⟩ stacking fault unstable on {101¯}, which prevents the dissociation of ⟨111⟩ dislocations on these planes and accounts for why this slip system is not seen experimentally.