Mechanisms of Plastic Deformation of Metal-Organic Framework-5

We use large-scale molecular dynamics simulations to investigate the mechanisms responsible for plastic deformation in metal-organic framework-5 (MOF-5). Simulations of uniaxial compression along [001], [101], and [111] directions reveal that structural collapse of {001} planes is responsible for irreversible deformation. The process involves slip along either one of the two < 100 > directions on the collapsing plane; this local shear process is due to the flexibility of the connection between of Zn-O clusters and 1,4-benzenedicarboxylate ligands. Thus, the collapse is driven both by compressive and shear stresses, and this fact explains the anisotropy in the mechanical response of this cubic crystal. The development of shear-collapse bands follows a nucleation and growth process with nuclei elongated along the slip direction and their subsequent growth in the directions normal to the slip and at much slower rates. This process is reminiscent of the glide of screw dislocations. Compression along the [101] and [111] directions led to intersection of active shear-collapse bands and the activation of multiple < 001 >{100} systems. We also find that partially collapsed planes reduce the stiffness of the structures, an observation that can explain discrepancies between experimental and theoretical stiffness predictions.