K ₂ SiF ₆ :Mn ⁴⁺ Red Phosphor Crystals: Near-Unity Quantum Efficiency, Size-Modulated Thermal Quenching, and Cooling-Crystallization Engineering of Millimeter Sizes for Crystal-to-Die Packaging

Mn⁴⁺-activated K₂SiF₆ red phosphors are commercialized emitters for white LEDs, yet their photophysical properties are conventionally discussed in powder form, with limited insight into how crystallite dimensions modulate the electronic transition of Mn⁴⁺. Herein, we report the size-controlled growth of K₂SiF₆:Mn⁴⁺ crystals with sizes of up to 2 mm via metastable-zone engineering during cooling. Solubility-temperature phase diagram guided the cooling rate and multistep isothermal holds, affording precise control over nucleation density and dopant homogeneity. The 2 mm crystals exhibit an internal quantum efficiency of 98.1%, maintain 78% at 500 K of the room-temperature photoluminescence intensity, and retain 90% intensity after 5 h water immersion─far outperforming the micro powder counterparts. The thermal quenching temperature increases from 450 K (powder) to ∼500 K (0.7-1 mm crystal), and to >500 K (2 mm crystal), strongly correlating with crystal size. The as-grown crystals with edge size of 2 or 1 mm were mounted on high-power LEDs using size-matched crystal-to-die packaging strategy, and the warm-white LEDs achieved high luminous efficacies of 165.7 and 179.5 lm/W, Ra of 81.7 and 81.6, and Tc of 3182 and 4586 K. The study offers a chemical design lever for next-generation thermally robust fluoride phosphors.