Abstract :
Nanoalloys have received much attention in the recent past as they exhibit interesting electronic, optical, and magnetic properties. The properties of nanoalloys are strongly dependent on their geometrical structure and the chemical arrangement. Hence, it is essential to have a comprehensive understanding of the equilibrium structures as a function of size, composition, and temperature. In the current work, the structure of AgCu nanoalloys is studied using a combination of global optimization searches and heating simulations.

Core-shell energetics, where the core and the shell are treated separately, has been used to explain the size-dependent and the composition-dependent structural transitions between the anti-Mackay stacking and the chiral stacking. Results show that the core
and the shell have opposing energy preferences and the interplay between them dictates the structural transitions. A thermal transition to the chiral stacking is observed for the AgCu nanoalloys where the anti-Mackay stacking is energetically favourable. The Cu composition influences the transition temperature and also the “sharpness” of the transition. The thermal stability of the chiral stacking is explained by considering the frequencies of the vibrational modes. The findings from the structural transitions are consolidated in the form of a size-composition map which indicates the preference for the chiral stacking of AgCu nanoalloys. Finally, new chiral motifs have been identified for the AgCu system. These new motifs have chiral Cu cores in contrast to the chiral icosahedra which have achiral Cu cores​