Mn activated Ca-α-SiAlON - broadband deep-red luminescence and sensitization by Eu, Yb and Ce

The covalent character of (oxy)nitrides has enabled tuning of emission color for new Eu 2+ - and Ce 3+ -doped phosphors and widespread implementation in white light LEDs. Mn 2+ -doped (oxy)nitrides form a relatively unexplored class of luminescent materials with great potential as orange to near infrared (NIR) phosphor in white light and NIR LEDs. Here we report deep red broad band emission around 700 nm for Mn 2+ in Ca-α-SiAlON. The emission shows a complex and gradual temperature quenching behavior and an unexpectedly large blue shift of the band maximum from 740 nm at 75 K to 670 nm at 475 K. This peculiar luminescence behavior is explained by disorder in the local (N,O) 7 polyhedral coordination of the Mn 2+ ions resulting in a variation in emission maximum and quenching temperature for differently coordinated Mn 2+ ions. Interestingly, the present results deviate from previous reports on narrow band orange emission from Mn 2+ in Ca-α-SiAlON. Careful analysis reveals that the orange emission likely originates from an AlN:Mn 2+ impurity phase. Sensitization of the Mn 2+ emission by strongly absorbing Eu 2+ , Yb 2+ and Ce 3+ ions was observed but with low efficiency as energy transfer to Mn 2+ via exchange interaction is ineffective for the relatively large (>5 Å) nearest neighbor separation in Ca-α-SiAlON. The new insights on Mn 2+ emission show that for efficient orange/red emission in (oxy)nitrides, a tetrahedral coordination is required for Mn 2+ while deep red/NIR emission can be expected for higher coordination numbers but is prone to thermal quenching. Sensitization of the Mn 2+ emission is efficient only for exchange interaction and requires short (