Mn2+ activated Ca-α-SiAlON – broadband deep-red luminescence and sensitization by Eu2+, Yb2+ and Ce3+

Abstract

The covalent character of (oxy)nitrides has enabled tuning of emission color for new Eu²⁺- and Ce³⁺-doped phosphors and widespread implementation in white light LEDs. Mn²⁺-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 Mn²⁺ 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 Mn²⁺ ions resulting in a variation in emission maximum and quenching temperature for differently coordinated Mn²⁺ ions. Interestingly, the present results deviate from previous reports on narrow band orange emission from Mn²⁺ in Ca-α-SiAlON. Careful analysis reveals that the orange emission likely originates from an AlN:Mn²⁺ impurity phase. Sensitization of the Mn²⁺ emission by strongly absorbing Eu²⁺, Yb²⁺ and Ce³⁺ ions was observed but with low efficiency as energy transfer to Mn²⁺via exchange interaction is ineffective for the relatively large (>5 Å) nearest neighbor separation in Ca-α-SiAlON. The new insights on Mn²⁺ emission show that for efficient orange/red emission in (oxy)nitrides, a tetrahedral coordination is required for Mn²⁺ while deep red/NIR emission can be expected for higher coordination numbers but is prone to thermal quenching. Sensitization of the Mn²⁺ emission is efficient only for exchange interaction and requires short (<5 Å) nearest neighbor separation.