Millimeter-SizedK2MnF6:Si4+, NH4+ Red-Luminescent Crystals withHigh Absorption Efficiency (AEmax of 93.5%) and ExternalQuantum Efficiency (EQEmax of 68.9%) Grown by Cooling-InducedCrystallization

Luminescent bulk crystals exhibit fewer grain boundaries and defects compared with conventional microsized powdery ones. Herein, targeting Mn⁴⁺-activated fluoride crystals with a sharp line-type red luminescence spectrum, we propose a new cooling-induced crystallization method to grow the fluoride crystals. By this new method, we successfully grew millimeter-sized K₂MnF₆:Si⁴⁺, NH₄⁺ crystals, featuring an AE_max (absorption efficiency) of 93.5% and an EQE_max (external quantum efficiency) of 68.9%, which are among the best values for Mn⁴⁺-activated fluoride red phosphors. The influence of doping Si⁴⁺ and/or NH₄⁺ in K₂MnF₆ on the local coordination structure and luminescence properties was studied. The anomalous thermal quenching behaviors were discussed, the luminescence decay from the excited state was compared, and the origin for the high quantum efficiencies was analyzed.