Abstract: As one of highly promising luminescent material, Cs₂NaHoCl₆:Yb³⁺ not only retains the advantages of traditional halide lead perovskite, but also enables easier doping with rare earth ions, thus endowing it with richer luminescent properties. It has wide applications in the field of biomedicine and solid-state lighting, etc. However, the development of green and pollution-free batch synthesis methods is still extremely urgent. In this study, Cs₂NaHoCl₆:Yb³⁺ was synthesized by the mechanochemical method, which only required grinding the metal chlorides and then drying them. High temperature or any toxic solvents is not needed throughout the entire synthesis process, and the yield is nearly 100%. Under the excitation of 980 nm laser, Cs₂NaHoCl₆:Yb³⁺ emits red emission at 665 nm and green emission at 550 nm, corresponding to the ⁵F₅→⁵I₈ and ⁵S₂/⁵F₄→⁵I₈ transitions of Ho³⁺ ions. The upconversion intensity gradually increases with the increase of Yb³⁺ doping concentration, and reaches the strongest when 40% (in stoichiometric ratio) Yb³⁺ ions are doped, and secondary phases appear as further increases Yb³⁺ content to 50% (in stoichiometric ratio). Furthermore, anti-thermal quenching upconversion luminescence were observed in the Cs₂NaHoCl₆:Yb³⁺ sample, which may be related to the thermal activation and passivation of trap states. Another possible reason is that the increased temperature leads to the desorption of -OH groups on the surface of Cs₂NaHoCl₆:Yb³⁺, thereby weakening the surface quenching effect and enhancing the upconversion intensity. This study develops a green and pollution-free method for synthesizing lead-free double perovskites, where anti-thermal quenching upconversion luminescence were also observed, laying the foundation for the large-scale production of halide perovskite materials.

Key words: mechanochemical method, double perovskite, Cs₂NaHoCl₆, upconversion luminescence