Millimeter-scale Na2SiF6:Mn4+ red-emitting crystals with intense zero phonon line at 617 nm and enhanced hydrolysis resistance

Macroscopic Mn 4+ -activated fluoride crystals can exhibit improved resistance to hydrolysis and higher quantum efficiency compared to their micro-sized powder counterparts. Among various Mn 4+ -doped fluorides, Na 2 SiF 6 :Mn 4+ is exceptional for its dominant photoluminescence at 627 nm, a notably short wavelength for this class of materials. While traditionally available only as micro-sized powder, this paper presents, for the first time, the growth of Na 2 SiF 6 :Mn 4+ in the form of millimeter-sized crystals. Utilizing a cooling-induced crystallization technique, we have obtained Na 2 SiF 6 :Mn 4+ crystals that exhibit columnar morphology with single-crystal-like characteristics and pronounced zero phonon line emission at 617 nm. The synthesis was meticulously optimized regarding the cooling protocol, cooling rate, and Mn 4+ doping concentration. With increasing dosage of K 2 MnF 6 in the precursor, a phase transformation from Na 2 SiF 6 :Mn 4+ to (Na,K) 2 SiF 6 :Mn 4+ and K 2 SiF 6 :Mn 4+ was observed for the product crystals. The incorporation of Mn 4+ was hindered during the cooling-induced crystallization of the crystals as the nominal Si/Mn concentration ratios significantly exceed the measured values. Comparison of the deterioration behavior in water suggests that the as-grown Na 2 SiF 6 :Mn 4+ crystals displayed significantly improved stability against water attack compared to the Na 2 SiF 6 :Mn 4+ powder phosphor. This work may inspire the development of more durable phosphor crystals for long-lasting white light LED technologies.