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Mn 4+ -activated hexafluoroaluminates are promising red-emitting phosphors for white light emitting diodes (w-LEDs). Here, we report the synthesis of Na₃AlF₆:Mn 4+ , K₃AlF₆:Mn 4+ and K₂NaAlF₆:Mn 4+ phosphors through a simple two-step co-precipitation method. Highly monodisperse large (~20 μm) smoothed-octahedron shaped crystallites are obtained for K₂NaAlF₆:Mn 4+ . The large size, regular shape and small size distribution are favorable for application in w-LEDs. All Mn 4+ -doped hexafluoroaluminates show bright red Mn 4+ luminescence under blue light excitation. We compare the optical properties of Na₃AlF₆:Mn 4+ , K₃AlF₆:Mn 4+ and K₂NaAlF₆:Mn 4+ at room temperature and 4 K. The luminescence measurements reveal that multiple Mn 4+ sites exist in M₃AlF₆:Mn 4+ (M = Na, K), which is explained by the charge compensation that is required for Mn 4+ on Al 3+ 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 Mn 4+ -doped hexafluoroaluminates interesting red phosphors for w-LEDs. The new insights reported on the synthesis and optical properties of Mn 4+ in the chemically and thermally stable hexafluoroaluminates can contribute to the optimization of red-emitting Mn 4+ phosphors for w-LEDs.

Co-Precipitation Synthesis and Optical Properties of Mn4+-Doped Hexafluoroaluminate w-LED Phosphors