Room-temperature facile synthesis of hexagonal NaYF4 and NaYF4: Yb, Er powder without any organic additives and its upconversion fluorescence properties

Based on the structure difference between cubic and hexagonal phases, a facile method was designed for the synthesis of pure hexagonal phase beta-NaYF4 and beta-NaYF4: Yb, Er powder at room temperature and ambient pressure. In the whole process, neither any organic additives, nor high temperature and high pressure were involved that indicated an environmentally friendly technology. The optimal synthesis conditions for hexagonal phase at room temperature is the pH value between 5.0 and 6.5, the amorphous Y (OH)(3) as Y precursor and the F/Y3+ molar ratio equal to 6:1. A possible preparation mechanism based on the difference between the coordination structure of the hexagonal and the cubic phases was also proposed. It is found that the as prepared powder of the hexagonal phase can emit red upconversion luminescence with a high R/G ratio, which is significantly different from the light emitted by the usually synthesized hexagonal phase. This method can also provide a new perspective for studying the sustainable synthesis of rare earth compounds. (C) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.