Highly Regular, Uniform K₃ScF₆:Mn⁴⁺ Phosphors: Facile Synthesis, Microstructures, Photoluminescence Properties, and Application in Light-Emitting Diode Devices

Abstract

A new generation of red phosphors of complex fluoride matrices activated with Mn⁴⁺ has gained a broad interest in getting high color quality and low color temperature of solid-state white light-emitting diodes (WLEDs). However, besides their instability toward moisture, the extremely irregular and nonuniform morphologies of these phosphors have limited their practical industry applications. In the present study, a novel type of K₃ScF₆:Mn⁴⁺ red phosphor with highly regular, uniform, and high color purity was obtained successfully through a facile coprecipitation route under mild conditions. The crystal structure was identified with aids of the powder X-ray diffraction, Rietveld refinement, and density functional theory calculations. The prototype crystallizes in the space group Fm3 m with a cubic structure, and the lattice parameters are fitted well to be a = b = c = 8.4859(8) Å and V = 611.074(2) ų. The Mn⁴⁺ ions occupy Sc³⁺ sites and locate at the centers of the distorted ScF₆ octahedrons. A wide band gap of approximately 6.15 eV can provide sufficient space to accommodate impurity energy levels. Unlike most other Mn⁴⁺ ion-activated fluoride phosphors, the as-prepared K₃ScF₆:Mn⁴⁺ phosphors demonstrate highly uniform and regular morphologies with shapes transforming from cube to octahedron with increasing Mn⁴⁺ ion concentration. Under blue light excitation, the as-prepared K₃ScF₆:Mn⁴⁺ sample exhibits intense sharp-line red fluorescence (the strongest peak located at 631 nm) with high color purity. An excellent recovery in luminescence upon heating and cooling processes implies high stability of K₃ScF₆:Mn⁴⁺. Furthermore, a warm WLED fabricated with blue GaN chips merged with the mixture of K₃ScF₆:Mn⁴⁺ and the well-known commercial YAG:Ce³⁺ yellow phosphors exhibits wonderful color quality with lower correlated color temperature (3250 K) and higher color-rendering index ( R_a = 86.4). These results suggest that the K₃ScF₆:Mn⁴⁺ phosphor possesses stupendous potentiality for practical applications.