The NaYF4:Yb3+,Tb3+(xYb: 0.20, xTb: 0.04) materials were prepared using the co-precipitation method. The as-prepared material was washed either with or without water in addition to ethanol and thereafter annealed for 5 h at 500 °C. This resulted in materials with moderate or very high up-converson luminescence intensity, respectively. The structural study carried out with X-ray powder diffraction revealed microstrains in the rare earth(R) sublattice that were relaxed for the material with very high up-conversion intensity thus decreasing energy losses. The local structural details were investigated with R LIII and Y K edge extended X-ray absorption fine structure(EXAFS) using synchrotron radiation. Around 10 mol.% of the Yb3+ ions were found to occupy the Na site in the material with very high up-conversion intensity. These Yb species formed clusters with the Tb3+ ions occupying the regular Na/R sites. Such clustering enhanced the energy transfer between Yb3+ and Tb3+ thus intensifying the up-conversion emission. The NaYF4: Yb3 +, Tb3 + (xYb: 0.20, xTb: 0.04) materials were prepared using the co-precipitation method. The as-prepared material was washed either with or without water in addition to ethanol and thereafter annealed for 5 h at 500 ° C. This resulted in materials with moderate or very high up-converson luminescence intensity, respectively. The structural study carried out with X-ray powder rendering revealed microstrains in the rare earth (R) sublattice that were relaxed for the material with very The up structural characteristics were investigated with R LIII and YK edge extended X-ray absorption fine structure (EXAFS) using synchrotron radiation. Around 10 mol.% of the Yb3 + ions were found to occupy the Na site in the material with very high up-conversion intensity. These Yb species formed clusters with the Tb3 + ions occupying the regular Na / R sites. Such clustering enhanced the energy transfer between Yb3 + and Tb3 + intens intensif ying the up-conversion emission.