The electronic and optical properties of a narrow-band red-emitting nanophosphor K₂NaGaF₆: Mn⁴⁺ for warm white light-emitting diodes

Recently, as a key red component in the development of warm white light-emitting diodes (WLEDs), Mn⁴⁺-doped fluorides with narrow red emission have sparked rapidly growing interest because they improve color rendition and enhance the visual energy efficiency. Herein, a red nanophosphor, K₂NaGaF₆:Mn⁴⁺, with a diameter of 150-250 nm has been synthesized using a simple co-precipitation method. Rietveld refinement reveals that it crystallizes in the space group Fm3m with the cell parameter a = 8.25320(4) Å. The exchange charge model (ECM) has been used to calculate the energy levels of Mn⁴⁺ ions in K₂NaGaF₆, which match well with the experimental spectra. The as-synthesized phosphor exhibits a narrow red emission at around 630 nm (spin-forbidden ²E_g → ⁴A₂ transition of Mn⁴⁺ ions) when excited at 365 nm (⁴A_2g → ⁴T_1g) and 467 nm (⁴A_2g → ⁴T_2g), with a quantum efficiency (QE) of 61% and good resistance to thermal quenching. Based on the structure, the formation mechanism of ZPL has been discussed. In addition, the concentration-dependent decay curves of Mn⁴⁺ in K₂NaGaF₆ were fitted using the Inokuti-Hirayama model, suggesting that the dipole-dipole interactions determine the concentration quenching. Finally, encouraged by the good performance, a warm LED with a CRI of 89.4 and CCT of 3779 K was fabricated by employing the title nanophosphor as the red component. Our findings suggest that K₂NaGaF₆:Mn⁴⁺ can be a viable candidate for the red phosphor used in warm WLEDs. © 2018 The Royal Society of Chemistry.