4f and 5d energy levels of the divalent and trivalent lanthanide ions in M2Si5N8 (M=Ca, Sr, Ba)

Optical data of Sm, Tb and Yb doped Ca2Si5N8 and Sr2Si5N8 phosphors that have been prepared by solid-state synthesis, are presented. Together with luminescence data from literature on Ce3+ and Eu2+ doping in the M2Si5N8 (M=Ca, Sr, Ba) hosts, energy level schemes were constructed showing the energy of the 4f and 5d levels of all divalent and trivalent lanthanide ions relative to the valence and conduction band. The schemes were of great help in interpreting the optical data of the lanthanide doped phosphors and allow commenting on the valence stability of the ions, as well as the stability against thermal quenching of the Eu2+d–f emission. Tb3+ substitutes on both a high energy and a low energy site in Ca2Si5N8, due to which excitation at 4.77 eV led to emission from both the 5D3 and 5D4 levels, while excitation at 4.34 eV gave rise to mainly 5D4 emission. Doping with Sm resulted in typical Sm3+f–f line absorption, as well as an absorption band around 4.1 eV in Ca2Si5N8 and 3.6 eV in Sr2Si5N8 that could be identified as the Sm3+ charge transfer band. Yb on the other hand was incorporated in both the divalent and the trivalent state in Ca2Si5N8.