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

## TL;DR

This paper introduces a scalable method to produce high-quality upconversion particles at a lower temperature and in less time.

## Contribution

A novel synthesis method using fluorine excess and inverted crucibles for large-scale production of β-NaLnF4 upconversion particles.

## Key findings

- Sintering temperature was reduced from over 550 to 350 °C.
- Heating time was decreased from several hours to 30 minutes.
- The method enables quantitative yields in the tens-of-grams scale.

## Abstract

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.