Nanoparticles of high entropy alloys (HEAs) have distinct properties that result from their high surface-tovolume ratios coupled with synergistic interactions among their five or more constituent elements, which are randomly distributed throughout a crystalline lattice. Methods to synthesize HEA nanoparticles are emerging, including solution approaches that yield colloidal products. However, the complex multielement compositions of HEA nanoparticles make it challenging to identify and understand their reaction chemistry and the pathways by which they form, which hinders their rational synthesis. Here, we demonstrate the synthesis and elucidate the reaction pathways of seven colloidal HEA nanoparticle systems that contain various combinations of noble metals (Pd, Pt, Rh, Ir), 3d transition metals (Ni, Fe, Co), and a p-block element (Sn). The nanoparticles were synthesized by slowly injecting a solution containing all five constituent metal salts into oleylamine and octadecene at 275 °C. Using NiPdPtRhIr as a lead system, we confirmed the homogeneous colocalization of all five elements and achieved tunable compositions by varying their ratios. We also observed heterogeneities, including Pd-rich regions, in a subpopulation of the NiPdPtRhIr sample. Halting the reaction at early time points and characterizing the isolated products revealed a timedependent composition evolution from Pd-rich NiPd seeds to the final NiPdPtRhIr HEA. Similar reactions applied to FePdPtRhIr, CoPdPtRhIr, NiFePdPtIr, and NiFeCoPdPt, with modified conditions to most efficiently incorporate all five elements into each HEA, also revealed similar Pd-rich seeds with system-dependent differences in the rates and sequences of element uptake into the nanoparticles. When moving to SnPdPtRhIr and NiSnPdPtIr, the time-dependent formation pathway was more consistent with simultaneous coreduction rather than through formation of reactive seeds. These studies reveal important similarities and differences among the pathways by which different colloidal HEA nanoparticles form using the same synthetic method, as well as establish generality. The results provide guidelines for incorporating a range of different elements into HEA nanoparticles, ultimately providing fundamental knowledge about how to define and optimize synthetic protocols, expand into different HEA nanoparticle systems, and achieve high phase purity.

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