Interdependence of structure and chemical order in high symmetry (PdAu)N nanoclusters

Abstract

In this work we study the stabilities of high-symmetry Au_N, Pd_N and (AuPd)_N clusters, for N < 1500, using mathematical constructs, a semi-empirical potential with two different parameter sets, and a quasi-Newtonian minimisation technique. For Pd_N clusters, both parameter sets tested result in preferences for icosahedral (Ih) structures for N < 1000 over other high-symmetry 12-vertex geometries; for Au_N clusters we find a tendency towards face-centred cubic (FCC) structures at values of N lower than seen for Pd_N: parameter set I of Cleri and Rosato [Cleri and Rosato, Phys. Rev. B, 1993, 48, 22] gave a transition at N ≈ 650 to the I-Dh, whilst for parameter set II of Baletto et al. [Baletto et al., J. Chem. Phys., 2002, 116, 3856] this value was lower still. For (AuPd)_N clusters we found that the preferred arrangement is (Pd_coreAu_shell)_N, with thin (monolayer) surface coverings of Au being most energetically favourable compared to the homogeneous clusters; however for parameter set II multiple layers of Au lead to energetic instability. (Au_corePd_shell)_N clusters are not energetically favourable with thin coatings of Pd, however as the shell coating thickens so the stability improves. Ih structures are unfavourable compared to the Ino-decahedron and cuboctahedron for (Au_corePd_shell)_N, whereas the FCC-type structures are strongly preferred for (Pd_coreAu_shell)_N. Overall, the strong tendency towards core–shell segregation is emphasised for parameter set I more than II, agreeing with previous work on smaller (AuPd)_N clusters.