Hexagonal Sodium Lutetium Fluoride Microstructures: Facile and Large-Scale Synthesis, Growth Mechanism and Multicolour Emissions

Highly crystalline β-NaLuF4:Ln3+ microstructures were successfully prepared via a mixed NaCl-KCl flux cooling approach. The chemical reaction process and the formation mechanism have been carefully investigated by XRD, SEM, TEM and PL characterizations. Interestingly, as the reaction time is prolonged, the crystalline structure of NaLuF4 changes from α-phase to β-phase, while the granule shape transfers from nanoparticles to solid microsheets then to porous microsheets. By altering the kinds of doped lanthanide ions, upconverting and downconverting luminescence with multi-colour outputs (red, green and blue) can be realized in β-NaLuF4:Ln3+ (Ln = Yb, Er, Tm, Ce, Tb and Eu). This strategy is not only expected to meet the ever-increasing commercial demand, but also offers an alternative in synthesizing rare earth fluorides.