Quasi-seeded growth, phase transformation, and size tuning of multifunctional hexagonal NaLnF ₄ (Ln = Y, Gd, Yb) nanocrystals via in situ cation-exchange reaction

In situ cation exchange between alkali ions in ternary alkali lanthanide (Ln ³⁺) 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 KLnF ₄ to hexagonal NaLnF ₄. 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 NaLnF ₄ nanocrystals (NCs). During the in situ cation exchange between potassium and sodium and corresponding cubic to hexagonal phase transformation, the stable hexagonal NaLnF ₄ 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 Ln ³⁺ doping-induced size tuning and phase transformation in NaYF ₄ and alkaline-earth fluoride NCs, but also a method to prepare high-quality multifunctional NaLnF ₄ NCs with tunable paramagnetism and multi-color upconversion emission. © 2012 The Royal Society of Chemistry.