Co-Precipitation Synthesis and Optical Properties of Mn(4+)-Doped Hexafluoroaluminate w-LED Phosphors

Mn(4+)-activated hexafluoroaluminates are promising red-emitting phosphors for white light emitting diodes (w-LEDs). Here, we report the synthesis of Na(3)AlF(6):Mn(4+), K(3)AlF(6):Mn(4+) and K(2)NaAlF(6):Mn(4+) phosphors through a simple two-step co-precipitation method. Highly monodisperse large (~20 μm) smoothed-octahedron shaped crystallites are obtained for K(2)NaAlF(6):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 Na(3)AlF(6):Mn(4+), K(3)AlF(6):Mn(4+) and K(2)NaAlF(6):Mn(4+) at room temperature and 4 K. The luminescence measurements reveal that multiple Mn(4+) sites exist in M(3)AlF(6):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.