Large-sized K2SiF6:Mn4+ phosphor crystals up to 3 mm grown by seeded growth with near-unity quantum efficiency

Large K 2 SiF 6 :Mn 4+ crystals that emit red light surpass conventional micro-powders in both hydrolytic stability and quantum efficiency. However, preparation of K 2 SiF 6 :Mn 4+ crystals to > 1 mm while stabilizing Mn 4+ valence has remained elusive. Herein, seed-assisted crystallization under a controlled cooling ramp from 25°C to –40°C was used which suppresses hydrolysis-driven reduction of [MnF 6 ] 2– and yields K 2 SiF 6 :Mn 4+ crystals with near-unit quantum yield and unprecedented dimensions up to 3 mm—a 200 % size increase over the current record. After 5 h in water the crystals (>2 mm) retain 80.7 % of their initial emission intensity,outperforming sub-millimeter counterparts (58.9 %) and micro-sized powders (3.6 %). The single-crystal like nature enables superior internal/external quantum efficiencies of 97 %/82 % and preserves 82 % of the room-temperature intensity at 500 K. Warm-white LEDs built with these phosphor crystals deliver a high luminous efficacy of 157.3 lm/W and an R 9 value of 79. This work demonstrates that crystal-growth engineering can unlock hydrolysis-resistant fluoride phosphors for high-power optoelectronics.