Core-shell vs. multi-shell formation in nanoalloy evolution from disordered configurations.

The evolution towards equilibrium of AuCo, AgNi and AgCu nanoparticles is studied by molecular dynamics simulations. Nanoparticle sizes of about 2.5 nm are considered, in the temperature range from 300 to 700 K. The simulations reveal complex equilibration pathways, in which geometric structure and chemical ordering change with time. These nanoparticles present the same type of strong tendency to phase separation and to surface segregation of either Au or Ag, which lead to the same type of core@shell equilibrium structures. In spite of these similarities, the equilibration pathways of these nanoparticles from chemically disordered configurations present both quantitative and qualitative differences. Quantitative differences are found in the equilibration time scale, which is much longer in AgCu than in AgNi and AuCo. Qualitative differences are found in the presence or absence of geometric structure transformations, and in the formation of different types of three-shell metastable chemical ordering during evolution. It is also shown that surface segregation depends on the geometric structure, being faster in icosahedra than in fcc nanoparticles.