NaYF₄:Yb:Er nanoparticles (UCNP) were synthesized under mild experimental conditions to obtain a pure cubic lattice. Upon annealing at different temperatures up to T_an = 700 °C phase transitions between hexagonal phase and the cubic phase were induced. Based on the TRANES analysis it was possible to resolve the lattice phase transition in more detail, which was not possible with the standard techniques.