In Situ X‐Ray Scattering Reveals the Nucleation, Growth, and Self‐Assembly of CsBr Nanocrystals in a Heterogeneous Medium

ABSTRACT Understanding how nanocrystals form is central to the rational synthesis of nanomaterials, yet the formation mechanisms of ionic nanocrystals remain poorly resolved. Here, we use synchrotron‐based in situ small‐ and wide‐angle X‐ray scattering to track the formation and assembly of CsBr nanocrystals in solution. We find that the precursors spontaneously organize into dense clusters before reaction, generating a heterogeneous reaction medium that dictates the subsequent formation pathway. Following injection of Cs‐oleate, these clusters act as metastable precursor reservoirs that regulate precursor availability, thereby sustaining extended nucleation and promoting effective size focusing. At later stages, the same clusters function as depletants that drive the self‐assembly of CsBr nanocrystals into face‐centered cubic superlattices. These superlattices then undergo partial disassembly through Ostwald ripening, governed by the interplay between structural ordering and kinetic evolution. Together, our results identify multiple roles of precursor clusters during nanocrystal formation and assembly and provide mechanistic insights into ionic nanocrystal synthesis in heterogeneous media.