Solid-solution precursor to melting in onion-ring Pd-Pt nanoclusters: a case of second-order-like phase change?

The thermodynamical behaviour of icosahedral, multilayer Pd-Pt clusters is addressed using a combination of simulation tools, mainly parallel tempering Monte Carlo. A preferential swapping trial move is introduced to increase the chance of successfully exchanging Pd and Pt atoms in the cluster. The 2-, 3- and 4-shell, Pd-rich clusters have been studied. We generally find that the clusters melt at a temperature significantly below the bulk melting point at the same corresponding composition. More interestingly, for the smaller clusters melting is initiated by a solid-solution intermediate phase in which the overall icosahedral frame remains, but the Pd and Pt atoms can swap sites. The transition to this solid-solution phase is seen to have a continuous, second-order like character, which is interpreted from the similarity between the present system with the ferromagnetic Ising model on the 3D cubic lattice. As the cluster grows, the onion-ring structure becomes thermodynamically unstable. The 4-layer cluster already exhibits a solid-solution in its core at temperatures as low as 100 K. The bulk behaviour is thus recovered at very small scales.