(NH4)2MnF6: A highly soluble, K+-free Mn4+ precursor for red fluoride phosphors

Abstract

To address the limitations of low solubility and K+ impurity introduction in traditional Mn4+ sources (e.g., K2MnF6) for synthesizing Mn4+-activated fluoride phosphors, we synthesize a new K+-free compound, (NH4)2MnF6, which exhibits approximately two times higher solubility than K2MnF6. (NH4)2MnF6 was successfully synthesized via an optimized reduction-precipitation-purification process using a KMnO4/NH4HF2/HF system with controlled gradient H2O2 addition. Utilizing this Mn4⁺ source, red-emitting (NH4)2SiF6:Mn4+ crystals with high absorption efficiency were synthesized by cooling-induced crystallization. The doping efficiency of Mn4+ in (NH4)2SiF6 using (NH4)2MnF6 as the Mn4+-precursor significantly surpasses that achievable with K2MnF6. Furthermore, distinct doping behavior was observed: (NH4)2MnF6 facilitated a phase transition from cubic to hexagonal (NH4)2SiF6:Mn4+ with increasing Mn4⁺ concentration, while K2MnF6 led to K2SiF6:Mn4+ formation. The hexagonal phase induced reduced lattice symmetry around Mn4+, resulting in intense and tunable zero phonon line emission. Under 455 nm blue excitation, the crystals exhibited narrow-band red emission peaking at 633 nm. The successful synthesis of highly soluble (NH4)2MnF6 paves the way for developing K⁺-free red phosphors with high doping levels, tunable luminescence properties, and reduced HF consumption in scalable crystal growth.