Co-precipitation Synthesis and Optical Properties of Mn⁴⁺-doped Hexafluoroaluminate w-LED Phosphors

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.