Ultrafast Aqueous Synthesis of Lanthanide‐Doped Nanocrystals With Composition‐Dependent Phase Evolution at Room Temperature

Due to their unique optical properties, lanthanide‐doped inorganic nanocrystals are promising building blocks for optoelectronic devices, optical sensing, and bioimaging. However, their widespread use is limited due to the lack of environmentally benign, rapid, and scalable synthetic methods. Herein, we present a versatile and ultrafast aqueous coprecipitation strategy that enables the preparation of lanthanide‐doped nanocrystals within 1 min at room temperature. Using the K‐Ln‐F (Ln = Ce–Lu) system as a model, we demonstrate a distinct structural evolution from the cubic KLn 3 F 10 ‐type structure to the orthorhombic KLn 2 F 7 ‐type structure, primarily driven by the lanthanide contraction effect. This approach can be extended to various host matrices by tuning alkali‐metal cations (Li + –Cs + ) and diverse inorganic anions (e.g., VO 4 3− , PO 4 3− , CrO 4 2− , and SO 4 2− ). The as‐synthesized KCe 3 F 10 :20%Tb 3+ nanocrystals exhibit efficient green downshifting emission with an absolute photoluminescence quantum yield (PLQY) of 89.1%. Moreover, tunable multicolor upconversion luminescence spanning the visible region is achieved by modulating dopant concentrations and host matrices. These findings provide new possibilities for the scalable and potentially commercial production of lanthanide‐doped luminescent materials and their practical applications in related luminescent systems.