Computational Study of Structural and Energetic Properties of Ammonium Perchlorate at Interfaces

We investigated the structural and energetic properties of the ammonium perchlorate (AP) crystal in bulk and at interfaces using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Thermal rotations of ammonium ions were observed in the bulk AP crystal at room temperature, in agreement with experimental findings. We investigated atomic density distributions near six crystallographic surfaces of the AP crystal. We observed significant thermal rotations of perchlorate ions near surfaces in contrast to their behavior in the bulk AP crystal. We also calculated the surface energies of these six surfaces of the AP crystal and found that the three surfaces (210), (010), and (001) have the lowest surface energies. We performed dynamic debonding and tensile deformation simulations to create (100), (010), and (001) surfaces. Clean (001) surfaces were formed with the debonding process, and a void bounded by (001) surfaces was formed during tensile deformations. However, clean (100) or (010) surfaces were not formed with either debonding or tensile deformation simulations even though the (010) surface has a lower surface energy estimated from MD simulations than the (001) surface. This is in agreement with the frequently observed (001) surface in the AP crystal. Our results suggest that the investigation of the surface energy and dynamic formation of surfaces with MD simulations can be a useful approach to predict the morphology of molecular crystals to complement accurate DFT calculations.