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

Abstract

In this work we study the stabilities of high-symmetry AuN, PdN 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 PdN clusters, both parameter sets tested result in preferences for icosahedral (Ih) structures for N < 1000 over other high-symmetry 12-vertex geometries; for AuN clusters we find a tendency towards face-centred cubic (FCC) structures at values of N lower than seen for PdN: 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 (PdcoreAushell)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. (AucorePdshell)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 (AucorePdshell)N, whereas the FCC-type structures are strongly preferred for (PdcoreAushell)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.