System-dependent melting behavior of icosahedral anti-Mackay nanoalloys

Abstract

Anti-Mackay icosahedral clusters of composition Ag₃₂M₁₃, where M is either Cu, Ni, or Co, have been recently shown to possess special structural stability both by calculations and experiments. These nanoalloys assume a core–shell arrangement, with an icosahedral core of 13 M atoms surrounded by an Ag shell of anti-Mackay structure. In this paper we study the melting of these three nanoalloys, showing that, despite the close similarity of the structures, melting takes place through quite different mechanisms. In particular, we find that Ag₃₂Co₁₃ and Ag₃₂Ni₁₃ present a premelting phenomenon which involves only the shell of the cluster while the core melts at higher temperatures, in agreement with previous calculations. On the contrary, in Ag₃₂Cu₁₃, melting occurs through stages that involve the shell and the core at the same time. These findings are rationalized in terms of the different features of the energy landscape of these nanoalloys. Our simulations, in which special care has been devoted to avoid non-ergodicity problems, show also that the particles keep their core–shell structures even in the liquid phase, indicating an incomplete miscibility of Ag with Ni, Co or Cu at the nanoscale up to quite high temperatures.