Abstract
A new coarse-grained layer model (CGLM) for efficient computation of axially symmetric elemental equilibrium configurations in alloy nanoparticles (NPs) is introduced and applied to chemical-order transitions in Pt-Ir truncated octahedra (TOs) comprising up to tens of thousands of atoms. The model is based on adaptation of the free energy concentration expansion method (FCEM) using coordination-dependent bond-energy variations (CBEV) as input extracted from DFT-computed elemental bulk and surface energies. Thermally induced quite sharp transitions from low-T asymmetric quasi-Janus and quasi ball-and-cup configurations to symmetric multi-shells furnish unparalleled nanophase composite diagrams for 1289-, 2406- and 4033-atom NPs. At even higher temperatures entropic atomic mixing in the multi-shells gradually intensifies, as reflected in broad heat-capacity Schottky humps, which become sharper for much larger TOs (e.g., ∼10 nm, ∼30,000 atoms), due to transformation to solid-solution-like cores.
Highlights
In the extensive theoretical and experimental research devoted to alloy nanoparticles (NPs), considerable attention has been given recently to bi-metallic phase-separated systems.[1]
Atomic site-specific chemical order in fcc-based 201- and 586-atom Pt–Ir truncated-octahedron (TO) nanoparticles was studied[11] by us using near-surface coordinationdependent bond-energy variations (CBEV)[12] as input to the statistical–mechanical free-energy concentration expansion method (FCEM),[13] which takes into account analytically short-range order
The coarse-grained layer model (CGLM)-free energy concentration expansion method (FCEM)/CBEV is applied to a single-composition 29 881-atom Pt–Ir TO revealing additional features
Summary
In the extensive theoretical and experimental research devoted to alloy nanoparticles (NPs), considerable attention has been given recently to bi-metallic phase-separated (demixed) systems.[1]. Atomic site-specific chemical order in fcc-based 201- and 586-atom Pt–Ir truncated-octahedron (TO) nanoparticles was studied[11] by us using near-surface coordinationdependent bond-energy variations (CBEV)[12] as input to the statistical–mechanical free-energy concentration expansion method (FCEM),[13] which takes into account analytically short-range order. In this weakly miscible bi-metallic alloy having a relatively small atomic-size mismatch, axially-symmetric quasi-Janus configurations were found to be stabilized at low temperatures mainly due to CBEV-induced preferential strengthening of Pt-surface/Ir-subsurface bonds, and the greatly reduced number of energetically unfavorable hetero-atomic bonds. The latter turn out to be highly involved and substantially distinct from our above-mentioned previous study for small-medium NPs, as well as from the Pt–Ir bulk diagram
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