Abstract
Model asymmetric decahedra with incoherent twin boundaries have been proposed to account for microscopic images of small gold and silver particles. To test this hypothesis we compare the energetics of such asymmetric structures with the corresponding structures that contain only coherent twin boundaries. Geometry optimisations were performed using Lennard-Jones and Sutton-Chen potentials; the latter contain many-body terms and should therefore describe metal bonding more realistically than the former. The results are relatively insensitive to the potential employed, suggesting that the simple pairwise Lennard-Jones potential can be used as a qualitative guide to the structural features of small noble metal particles. The symmetrical starting configurations converged to local minima without significant structural changes for all sizes and potentials, whereas the asymmetric starting configurations converged to higher-order stationary points or to distorted structures where the original incoherent boundaries were converted into approximately coherent boundaries. The optimized asymmetric structures were always higher in energy than the corresponding symmetric structures. We suggest that the experimental observations may be accounted for by more distorted asymmetric particles, which maintain a pseudo-fivefold symmetry along the most commonly observed ⟨110⟩ direction.