Tens-of-Grams Synthesis of β-NaLnF₄ Upconversion Particles Using Fluorine Excess and Inverted Crucibles as the Sintering Device

This work presents a robust method for the synthesis of pure β-NaLnF₄ upconversion particles (Ln = rare-earths) on a tens-of-grams scale. The sintering process was improved by incorporating NH₄HF₂ as a fluorinating agent and by arranging three inverted crucibles in increasing sizes to mitigate heat dissipation. This synergistically reduced the sintering temperature from over 550 to 350 °C and decreased the heating time from several hours to just 30 min. This method offers several advantages: (i) prevents impurities and surface oxygen defects that disrupt upconversion frequency due to multiphoton relaxation; (ii) requires a simple experimental setup, eliminating the need for inert atmospheres, furnaces, or special reactors; (iii) avoids the use of organic solvents for separation and washing; (iv) allows modulation of particle size at the submicrometric scale based on sintering temperature and heating time; and (v) provides quantitative yields in the tens-of-grams scale. This strategy could enable the mass production and broad distribution of upconverting materials, which are crucial for developing a wide range of advanced products with enhanced performance, including sensors, contrast agents, solar cells, security printing, and light-emitting devices.