Synthesis and photoluminescence of Mn4+ activated ternary-alkaline fluoride K2NaGaF6 red phosphor for warm-white LED application

Abstract

A novel ternary-alkaline red emitting fluoride phosphor K₂NaGaF₆:Mn⁴⁺ was successfully synthesized through co-precipitation method. The crystal structure, morphology, electronic band structure and luminescence properties of K₂NaGaF₆:Mn⁴⁺ phosphors were investigated in details by using Rietveld refinement of X-ray diffraction data, scanning electron microscopy (SEM), density functional theory (DFT) calculation and different reaction parameters. The K₂NaGaF₆ host has a cubic unit cell with the space group Fmm and lattice parameters of a = 8.2577 (4) Å, Z = 4, V_cell = 563.08 (8) ų. Under blue light excitation, Mn⁴⁺ activated K₂NaGaF₆ exhibits a bright narrow-band red emission. The PL properties of the K₂NaGaF₆:Mn⁴⁺ red phosphors were optimized with different Mn⁴⁺ concentrations and aging times. A warm-white LED device was fabricated using a blue LED chip combined with commercial yellow YAG:Ce³⁺ phosphor and synthesized K₂NaGaF₆:Mn⁴⁺ red phosphor. The color rendering index (CRI, R_a = 81.6) and corresponding color temperature (CCT = 3643 K) easily reached the commercial warm white light LED standards (R_a > 80 and CCT < 4000 K). All these results indicate that K₂NaGaF₆:Mn⁴⁺ phosphor would be a suitable red phosphor candidate for warm-white LED applications.