Understanding the Local and Electronic Structures toward Enhanced Thermal Stable Luminescence of CaAlSiN3:Eu2+

It is a great challenge to maintain thermally stable luminescence of red phosphors in white light-emitting diodes (LEDs), because of the large Stokes shift. For the purpose of overcoming this challenge, this work elucidates the intrinsic mechanism of the thermal quenching luminescence of CaAlSiN3:Eu2+. The empty 5d orbital of Eu2+ is partly filled with electrons upon Eu2+ increasing, as observed using XANES; and the exceptional expansion of the local Eu-N bond length, the ratio of which is far larger than the volume expansion of crystal lattice brought by doping Eu2+, is measured using EXAFS. The shift of Fermi level predicted with the first-principles calculations is confirmed by the valence band spectra. Therefore, the changeable distribution of electrons on the excited substates and then thermal delocalization to the conduction band are the intrinsic mechanisms of thermal quenching luminescence of CaAlSiN3:Eu2+. The results provide a solid basis for exploring the methods to enhance the thermal stable luminescence of CaAlSiN3:Eu2+.