Atomistic simulations of mechanical properties of LaBr3 single crystals

Abstract

Based upon a many-body La–Br interatomic potential, molecular dynamics simulations have been performed to study mechanical properties of the UCl3 phase, LaBr3 crystal. Both plastic deformation and fracture mechanisms were explored. For plastic deformation, dislocation line energy, core structure, slip mechanism, and mobility were all examined. For fracture mechanism, tensile tests were conducted under different loading directions. We found that the ⟨0001⟩ prism dislocations have the lowest line energies (∼5 eV/Å, compared to >8 eV/Å for the ⟨112¯0⟩ basal dislocations). The ⟨0001⟩ edge dislocation is mobile and its mobility increases with temperature. The ⟨0001⟩ screw dislocation is mobile at 0 K temperature and it becomes immobile as temperature is increased. The ⟨112¯0⟩ edge and screw dislocations are always immobile at any temperatures. The mobile dislocations do not dissociate into partials and they always move in a perfect unit. The immobile dislocations, however, always exhibit nonplanar dissociated core structures. Interestingly, the slip plane of the ⟨0001⟩ edge dislocation differs from the cleavage plane by an one-atomic plane distance, whereas the slip of the ⟨0001⟩ screw dislocation is associated with alternative out-of-plane exchange of Br atoms. The critical shear stress for the onset of ⟨0001⟩ slip was found to be around 1 GPa for the edge dislocation at 300 K, and around 1.5 GPa for both edge and screw dislocations at 0 K. Tensile loading simulations indicated that the theoretical strength of the material is critically determined by the {11¯00} cleavage. The lowest theoretical strength and fracture strain occur when the loading direction is normal to the cleavage plane and the highest theoretical strength and fracture strain occur when the loading direction is parallel to the cleavage plane.