A low-temperature co-precipitation approach to synthesize fluoride phosphors K2MF6:Mn4+ (M = Ge, Si) for white LED applications

Abstract

A new class of Mn⁴⁺ activated alkali-metal hexafluoride red phosphors are emerging for white light-emitting diodes because of their sharp red line ²E_g → ⁴A_2g emissions (600–650 nm) excited by irradiation of ⁴A_2g → ⁴T_1g (320–380 nm) and ⁴A_2g → ⁴T_2g (380–500 nm) transitions. However, these phosphors have the drawbacks of difficult control of the Mn valence state during synthesis and lack of underlying mechanisms for structure–photoluminescence relationships. In this study, we explore a novel, highly productive route to the quantifiable synthesis of K₂GeF₆:Mn⁴⁺ by the chemical co-precipitation method at room temperature. The prepared yellowish K₂GeF₆:Mn⁴⁺ powders exhibit a hexagonal shape and high crystallinity without significant defects. The photoluminescence thermal stability and white light-emitting diodes applicability of K₂GeF₆:Mn⁴⁺ suggest that it is a promising commercial red phosphor because of its efficient emission intensity, high color purity and excellent thermal stability. Structural analyses and theoretical calculations reveal that the red shift of the K₂GeF₆:Mn⁴⁺ red phosphor compared with K₂SiF₆:Mn⁴⁺ is due to the longer Ge–F distance and lower effective Mulliken charge of F ions in coordination environments of the MnF₆²⁻ octahedron. The split feature in K₂GeF₆:Mn⁴⁺ is due to the hexagonal distortion in the host. The structure–photoluminescence mechanism is predicted to be general in hexafluoride red phosphors to tune the optical properties through cationic substitutions and crystal structure adjustments.