Phase Separation in AgCu and AgNi Core–Shell Icosahedral Nanoparticles: A Harmonic Thermodynamics Study

The temperature dependence of equilibrium chemical ordering in Ag-rich AgCu and AgNi icosahedral nanoalloys is numerically studied within the harmonic superposition approximation to the partition function. The nanoalloys are modeled by an atomistic force field derived from the second-moment approximation to the tight-binding model. It is found that the equilibrium chemical ordering consists of phase-separated arrangements of the core–shell type, at least up to room temperature for AgCu and up to 600 K for AgNi. This result is rationalized in terms of the concept of preferred nucleation sites for phase separation, such as the central site in the icosahedron.