The synthesis conditions of the Yb³⁺ and Tb³⁺ co doped NaYF₄ were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post annealing of the as prepared materials resulted in the enhancement of the otherwise rather weak up conversion from Tb³⁺ by 1-2 orders of magnitude. At the same Lime, the temperature of formation of the hexagonal NaRF₄ phase with high up conversion could be lowered by 100 °C down to 350 °C. This improvement in up conversion was concluded to result from the better stoichiometry of the material without washing with wafer. The deficit of Na⁺ would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up conversion intensify. A further enhancement in the up conversion luminescence was observed to be due to the Er³⁺ ion impurity frequently associated with high concentration Yb³⁺ materials. The mechanism involving the unintentional Er³⁺ sensitizer and the resonance energy transfer in the Yb³⁺-Er³⁺-Tb³⁺ co-doped NaYF₄ were discussed based on the energy level schemes of the Yb³⁺, Er³⁺, and Tb³⁺ ions in NaYF₄. (C) 2013 Elsevier B.V. All rights reserved.