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 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 MnF₆^2- 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.