Optimized photoluminescence of red phosphor K2TiF6:Mn4+ synthesized at room temperature and its formation mechanism

Abstract

A red phosphor K₂TiF₆:Mn⁴⁺ (KTFM) has been synthesized by etching Ti(OC₄H₉)₄ in HF solution with KMnO₄ and KF at room temperature for 30 min. The formation mechanism of red phosphor KTFM has been discussed based on detailed experimental results. We studied the influences of synthetic procedure and KMnO₄ concentration on the powder color and intensity of phosphor luminescence. The actual doping concentration of Mn⁴⁺ in the K₂TiF₆ (KTF) host lattice of the phosphor has been investigated by measuring the concentration of filtrate through ICP-AES analysis. The results showed that about 32.4 mol% of Mn elements was doped into KTF crystals at optimal Mn⁴⁺ concentration (in precursor solution). The presence of HF was found to be essential to doping Mn⁴⁺ into KTF due to the weakly acidic and complexing properties of HF. The red luminescence of Mn⁴⁺ in KTF with a crystal structure matching standard card JCPDs (#28-0825), was first observed in the sample prepared from HF solution concentrations lower than 5 wt%. The dependence of the intensity of the luminescence on HF concentration might be due to the varying of Mn⁴⁺ concentrations in KTF crystals. Higher HF concentration was associated with lower yield, because KTF is soluble in HF at high concentrations. Encapsulation of the red phosphor KTFM with YAG:Ce on a GaN layer produces “warm” white LEDs with color rendering of 86 at 3251 K.