Atomistic Imaging of Competition between Surface Diffusion and Phase Transition during the Intermetallic Formation of Faceted Particles.

To explore the ordering mechanism of facet alloy nanocrystals with randomly distributed atoms, we investigate kinetic and thermodynamic behaviors of the ordering phase transition from face-centered cubic Pt3Co nanocrystals to L12-Pt3Co intermetallic nanocrystals. It is observed that the ordering occurs from the surface and then gradually into the interior in a layer-by-layer mode, involving the competition between two kinds of phase transition modes: long-range surface diffusion-induced phase transition (SDIPT) and short-range reconstruction-induced body phase transition (RIBPT). The density functional theory calculations demonstrate that the surface status acts as a pivotal part in the thermodynamics and kinetics of the nanoscale ordering transition. With the development of the controllable heating process, both SDIPT and RIBPT modes can be manipulated as well as the morphology of the final product. This in situ work lays the foundations for potentially realizing shape-controlled intermetallic nanostructures by utilizing the thermal annealing method and makes preparations for the rational design of the surface and near-surface atomic configurations at the atomic scale.