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

A new class of Mn4+ activated alkali-metal hexafluoride red phosphors are emerging for white light-emitting diodes because of their sharp red line E-2(g) -> (4)A(2g) emissions (600-650 nm) excited by irradiation of (4)A(2g) -> T-4(1g) (320-380 nm) and (4)A(2g) -> T-4(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.