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 Sm(3+)-, Dy(3+)- and TM(3+)-activated Ca-alpha-SiAlON phosphors are interpreted as the N(3-)-to-rare earth charge transfer transition (CTT). From the energies of the charge transfer transitions and from the optical data presented for the Eu(2+) ion. the location of the divalent rare earth ion energy levels relative to the valence and the conduction band of Ca-a-SiAlON is derived. The salient features of the energy-level diagram are shown to be practical in explaining the temperature-dependent variations of the Eu(2+) and Yb(2+) luminescence efficiency in Ca-a-SiAlON. A comparative study pertaining to the nature of the Yb(2+) and Eu(2+) ion luminescence in Ca-alpha-SiAlON and in SrSi(2)O(2)N(2) is presented. A tentative energy-level diagram of the trivalent rare earth ions in Ca-alpha-SiAlON is also constructed. (c) 2009 Published by Elsevier B.V.