Chemical ordering in icosahedral and truncated octahedral CoPt@Ag nanoalloys

Abstract In this study, the ternary alloying effects on chemical ordering and structural properties of CoPt@Ag nanoalloys were investigated. The optimization of the chemical ordering was performed for icosahedral (Ih) structures with 309 and 561 atoms, and truncated octahedral (TO) structures with 314 and 586 atoms, as they compete in size. The results show that the most stable compositions of the Ih and TO structures for each size have almost the same Co/Pt ratio. The best chemical ordering structures with smaller cores in the Ih configurations of 309 and 561 atoms generally exhibit irregular and asymmetric cores, while those with larger cores tend to have centered cores. Some compositions present a quasi-Janus arrangement in the icosahedral CoPt@Ag nanoalloys. For the TO compositions of 314 and 586 atoms, when the number of Co atoms exceeds a critical value, structural deformations begin at the surface, and the shell becomes asymmetric. With the further increasing in the number of Co atoms, the symmetries of the cores are broken, leading to deformed structures. Additionally, it was found that Co atoms show a greater preference for mixing with Pt atoms than with Ag atoms.