Phase-pure hexagonal lanthanide-doped NaYF4 nanocrystals possess enhanced luminescence properties due to their low phonon energy and chemical stability compared to their cubic phase counterpart. However, the phase-pure hexagonal NaYF4 nanoparticle forms only at high temperature (≥180 °C) and in the presence of certain ligands (e.g., oleic acid), whereas the cubic phase forms at a low reaction temperature. To this end, we report the low temperature (∼48 °C) synthesis of phase-pure hexagonal NaYF4 nanoparticles by converting cubic NaYF4 nanoparticles into the hexagonal phase through surface oxidation of oleic acid (C-18) ligand to azelaic acid (C-9). The mechanism behind the transformation is investigated using density functional theory (DFT) based studies. DFT studies reveal that the phase transformation from cubic → hexagonal phase involves a disorder → order phase transition since all cationic sites are occupied by either Na or Y in the cubic phase, while the hexagonal phase has a more ordered structure ...