4f and 5d energy levels of the divalent and trivalent lanthanide ions in M _2Si _5N _8 (M=Ca, Sr, Ba)

摘要

Optical data of Sm, Tb and Yb doped Ca _2Si _5N _8 and Sr 2Si 5N 8 phosphors that have been prepared by solid-state synthesis, are presented. Together with luminescence data from literature on Ce ~3 and Eu ~2 doping in the M _2Si _5N _8 (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 Eu ~2 d-f emission. Tb ~3 substitutes on both a high energy and a low energy site in Ca _2Si _5N _8, due to which excitation at 4.77 eV led to emission from both the ~5D _3 and ~5D _4 levels, while excitation at 4.34 eV gave rise to mainly ~5D _4 emission. Doping with Sm resulted in typical Sm ~3 f-f line absorption, as well as an absorption band around 4.1 eV in Ca _2Si _5N _8 and 3.6 eV in Sr _2Si _5N _8 that could be identified as the Sm ~3 charge transfer band. Yb on the other hand was incorporated in both the divalent and the trivalent state in Ca _2Si _5N _8.