Abstract

Abstract High entropy alloy nanoparticles bring hope to developing more efficient nanomaterials for high‐temperature applications. Nevertheless, the enhanced thermal stability of nearly equiatomic nanoalloys containing at least 5 metals is nothing more than theoretical speculation about the impact of thermodynamic contributions on their structural properties and remains to be proven. Here, in situ aberration‐corrected scanning transmission electron microscopy (STEM) and molecular dynamics simulations are combined to investigate at the atomic scale the thermal behavior of AuCoCuNiPt nanoparticles (NPs) from 298 to 973 K. Both in situ STEM heating and atomistic simulations reveal strong structural and chemical evolutions in the NPs with the formation and melting of an AuCu layer at the surface of NPs at high temperature. This phase separation that appears progressively with temperature is driven by pronounced atomic diffusion that is surprisingly more active in these quinary nanoalloys than in monometallic and bimetallic subsystems. Besides ruling out the existence of sluggish diffusion in AuCoCuNiPt nanoalloys and lowering their temperature range of application, the study allows distinguishing kinetic and thermodynamic effects on their structural properties, which is an essential prerequisite to better control the synthesis of complex nanomaterials.