Red-emitting Mn⁴⁺-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn⁴⁺-doped fluorides. Furthermore, concentration quenching of the Mn⁴⁺ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn⁴⁺-doped fluorides by measuring luminescence spectra and decay curves of K₂TiF₆:Mn⁴⁺ between 4 and 600 K and for Mn⁴⁺ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors show that quenching occurs through thermally activated crossover between the ⁴T₂ excited state and ⁴A₂ ground state. The quenching temperature can be optimized by designing host lattices in which Mn⁴⁺ has a high ⁴T₂ state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K₂TiF₆:Mn⁴⁺ up to 5% Mn⁴⁺. This is important, as high Mn⁴⁺ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn⁴⁺d–d transitions. At even higher Mn⁴⁺ concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn⁴⁺ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn⁴⁺ emission and can be used to realize superior narrow-band red Mn⁴⁺ phosphors for w-LEDs.

Mn⁴⁺-activated hexafluoroaluminates are promising red-emitting phosphors for white light emitting diodes (w-LEDs). Here, we report the synthesis of Na₃AlF₆:Mn⁴⁺, K₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ phosphors through a simple two-step co-precipitation method. Highly monodisperse large (~20 μm) smoothed-octahedron shaped crystallites are obtained for K₂NaAlF₆:Mn⁴⁺. The large size, regular shape and small size distribution are favorable for application in w-LEDs. All Mn⁴⁺-doped hexafluoroaluminates show bright red Mn⁴⁺ luminescence under blue light excitation. We compare the optical properties of Na₃AlF₆:Mn⁴⁺, K₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ at room temperature and 4 K. The luminescence measurements reveal that multiple Mn⁴⁺ sites exist in M₃AlF₆:Mn⁴⁺ (M = Na, K), which is explained by the charge compensation that is required for Mn⁴⁺ on Al³⁺ sites. Thermal cycling experiments show that the site distribution changes after annealing. Finally, we investigate thermal quenching and show that the luminescence quenching temperature is high, around 460–490 K, which makes these Mn⁴⁺-doped hexafluoroaluminates interesting red phosphors for w-LEDs. The new insights reported on the synthesis and optical properties of Mn⁴⁺ in the chemically and thermally stable hexafluoroaluminates can contribute to the optimization of red-emitting Mn⁴⁺ phosphors for w-LEDs.