Effects of Surface-Subsurface Bond-Energy Variations on Equilibrium Compositional Structures Evaluated for Pt-Ir Nanoparticles

The compositional structures of equilibrated Pt-Ir 923-atom cuboctahedron nanoparticles (NPs) are predicted by employing the recently introduced approach for deriving DFT-based coordination dependence of bond-energy variations (CBEVs) combined with the highly efficient statistical-mechanical free-energy concentration expansion method (FCEM). The roles played by preferential strengthening of intrasurface and surface-subsurface bonds in site-specific segregation are elucidated. While CBEV effects enhance Pt surface segregation to all NP surface sites, the driving force obtained for certain (111) sites exceeds the (100) values, in contrast to the simple bond-breaking model. Moreover, the CBEV induces Ir segregation to subsurface sites except the NP subvertexes. Computation of the complete temperature dependence of the compositional structure for low Pt content NPs indicates the occurrence of several smoothly varying atomic exchange processes between surface sites, which are reflected as distinct Schottky-type peaks in the configurational heat-capacity curve. Preliminary results, computed for high Pt content NPs, having onion-like structure at low temperatures, reveal a sharp intracore separation-like phase transition.