Charge transfer transitions and location of the rare earth ion energy levels in Ca-α-SiAlON

The broad bands in the room-temperature excitation spectra of Sm3+-, Dy3+- and Tm3+-activated Ca-α-SiAlON phosphors are interpreted as the N3−-to-rare earth charge transfer transition (CTT). From the energies of the charge transfer transitions and from the optical data presented for the Eu2+ ion, the location of the divalent rare earth ion energy levels relative to the valence and the conduction band of Ca-α-SiAlON is derived. The salient features of the energy-level diagram are shown to be practical in explaining the temperature-dependent variations of the Eu2+ and Yb2+ luminescence efficiency in Ca-α-SiAlON. A comparative study pertaining to the nature of the Yb2+ and Eu2+ ion luminescence in Ca-α-SiAlON and in SrSi2O2N2 is presented. A tentative energy-level diagram of the trivalent rare earth ions in Ca-α-SiAlON is also constructed.