Dependence of the structures and chemical ordering of Pd-Pt nanoalloys on potential parameters

The structures and chemical ordering (segregation properties) of Pd-Pt clusters (1 : 1 compositions for N = 2-20 atoms and all compositions for 34 atoms) have been studied using a combination of a genetic algorithm global optimization technique (GA) coupled with the Gupta semi-empirical potential and density functional theory (DFT) calculations. An initial DFT energetic analysis of small Pd-Pt clusters (N = 2-20) showed that their corresponding binding energies are slightly biased towards the stronger metal-metal bonding interactions (i. e. Pt-Pt). This led to a detailed analysis of Pd-Pt structural motifs and segregation effects, where the heteronuclear Pd-Pt parameters in the Gupta potential are derived as weighted averages of the Pd-Pd and Pt-Pt parameters, with the weighting factor (w) ranging from 0 (Pt-biased) to 1 (Pd-biased). The introduction of the weighting factor allowed us to identify three main types of segregation: core-shell; spherical cap; and ball-and-cup (intermediate between the first two types). The structural motifs predicted by the Gupta potential, as a function of composition and potential weighting factor, have been compared to our previous published Gupta and DFT calculations for 34-atom Pt-Pt clusters. From this study, we have found that a slightly Pd-biased weighting factor (w = 0.6) stabilises the mixed decahedral close packed structural motif, previously reported as the DFT global minimum in the region of most exothermic mixing for 34-atom Pd-Pt clusters. Our results show that by finely tuning the Gupta potential, one can qualitatively reproduce structural and chemical ordering patterns observed at higher levels of theory (e. g. DFT).