Structure, luminescence and energy transfer in Ce3+ and Mn2+ codoped γ-AlON phosphors

Mn2+ doped gamma-AlON is an interesting narrow-band green emitting phosphor which can be used in wide color gamut white LED backlights for liquid crystal displays ( LCDs). However, the spin-forbidden transition of 3d(5) in Mn2+ results in a quite low absorption efficiency, gamma-AlON: Mn2+ thus has a small external quantum efficiency. Besides Eu2+, Ce3+ is another promising sensitizer and is expected to improve the emission of Mn2+ through a distinctive energy transfer process. In this work, a series of Ce3+-Mn2+ codoped gamma-AlON phosphors were synthesized by gas pressure sintering at 1800 degrees C for 2 h under 0.5MPa nitrogen gas pressure. The Rietveld refinement analysis shows that Al atoms occupy both 8a and 16d sites, while codoped Ce3+ and Mn2+ ions substitute the 8a Al atoms only in the tetrahedral sites. The Al-27 solid state NMR spectrum further confirms the chemical shifts of AlO4 and AlO6 sites at 71.4 and 0 ppm, respectively. The measured PL, CL and decay times have evidenced that a brisk energy transfer occurs between the Ce3+ and Mn2+ ions. The energy transfer mechanism from Ce3+ to Mn2+ in gamma-AlON is a quadrupole-quadrupole interaction and the critical distance is calculated to be 15.97 A. Upon UV or blue light excitation, the codoped gamma-AlON phosphor has higher luminescence intensity, quantum efficiency and thermal stability than the Mn2+-doped gamma-AlON due to the energy transfer and low total concentration of Ce3+ and Mn2+ ions. Meanwhile, the absorption, internal quantum efficiency and external quantum efficiency are raised to 60.2%, 61.3% and 36.9% upon 310 nm excitation.