Quasi-seeded growth, phase transformation, and size tuning of multifunctional hexagonal NaLnF₄(Ln = Y, Gd, Yb) nanocrystalsvia in situcation-exchange reaction

In situ cation exchange between alkali ions in ternary alkali lanthanide (Ln3+) fluoride on the nanoscale is reported. Experimental results reveal that the differences in the solubility constants and thermodynamic stability of the reactants and products affect the irreversible cation exchange between potassium and sodium, resulting in the phase transformation of cubic KLnF4 to hexagonal NaLnF4. The unusual cation-exchange reaction can be used as an efficient tool to produce seed nuclei and control the nucleation and final size of hexagonal NaLnF4 nanocrystals (NCs). During the in situ cation exchange between potassium and sodium and corresponding cubic to hexagonal phase transformation, the stable hexagonal NaLnF4 particles serving as the seed nuclei play a key role in the final size of the NCs. The in situ cation exchange approach provides not only valuable insights into the growth dynamics of Ln3+ doping-induced size tuning and phase transformation in NaYF4 and alkaline-earth fluoride NCs, but also a method to prepare high-quality multifunctional NaLnF4 NCs with tunable paramagnetism and multi-color upconversion emission.