Millimeter-Sized K2MnF6:Si4+, NH4+ Red-Luminescent Crystals with High Absorption Efficiency (AEmax of 93.5%) and External Quantum Efficiency (EQEmax of 68.9%) Grown by Cooling-Induced Crystallization

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 AEmax (absorption efficiency) of 93.5% and an EQEmax (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.