HF-Free Synthesis of Narrow-Band Cs2GeF6: Mn4+ Red Phosphors via a Molten Salt Method

Mn4+-activated fluoride phosphors possess outstanding luminescent properties, making them highly suitable for applications in lighting and display technologies. However, the synthesis of such phosphors generally requires the use of large amounts of highly toxic aqueous HF, leading to serious environmental pollution. To eliminate the use of hazardous HF solution, a low-temperature molten salt method employing NH4HF2 was developed to synthesize the narrow-band red emitter Cs2GeF6: Mn4+ phosphor. Following the reaction, the product was washed with a dilute H2O2 solution to remove residual NH4HF2 and other impurities. The phase purity and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the luminescence properties were examined via photoluminescence (PL) spectroscopy. The obtained phosphors exhibit bright red emission characteristics of Mn4+ under blue-violet excitation. Among them, Cs2GeF6: 0.08 Mn4+ shows the highest emission intensity, with an internal quantum efficiency (IQE) of 78%. A white light-emitting diode (WLED) fabricated by combining this phosphor with a blue chip and commercial Y3Al5O12: Ce3+ (YAG) phosphor achieved a high luminous efficacy (LE) of ~146 lm/W, a correlated color temperature (CCT) of ~4396 K, and a color rendering index (Ra) of ~83, alongside excellent operational color stability.