HF-Free Synthesis of Li₂SiF₆:Mn⁴⁺: A Red-Emitting Phosphor

Li₂SiF₆:Mn⁴⁺ was synthesized via a new HF-free synthesis route by a high-pressure/high-temperature doping experiment at 5.5 GPa and 750 °C. It is proven that the phosphor cannot be synthesized by the common wet-chemical precipitation route in aqueous HF. The sample was characterized by powder X-ray diffraction, EDX, and luminescence spectroscopy. At room temperature, Li₂SiF₆:Mn⁴⁺ exhibits seven emission lines with the strongest line at λ_max ≈ 630 nm and a dominant wavelength of λ_dom ≈ 618 nm. The CIE coordinates are 0.688 and 0.312 for x and y, respectively. The compound shows a luminous efficacy of radiation (LER) of 218 lm W_opt^-1, which exceeds the LER of current state-of-the-art red LED phosphor K₂SiF₆:Mn⁴⁺ by 7% due to a blue-shift of the emission. It reveals excellent thermal quenching behavior up to 125 °C.