Spectral redshift mechanism of N doped Sr2SiO4:Eu2+ phosphors

By using the first principle methods, the electronic structure and spectral characteristics of N-doped and un-doped Sr2SiO4:Eu2+ phosphors is studied. It is found that after nitrogen doping the shorter bond lengths of EuN leads to the splitting of Eu5d states due to the crystal field effect, and the stronger covalencies of EuN bonds results in the down shift of Eu5d states because of the nephelauxetic effect. These two factors come together to bring about the red-shift of the of the PL properties. Lower Eu5d energy level and wider energy band of Eu5d states for Eu(II) site occurs due to shorter bond length and greater covalency of Eu(II)N bond. Consequently, the Eu(II) site was effectively substituted with nitrogen. By increase of nitrogen content, the number of EuN bonds increases and the luminescence intensity for the phosphors increase correspondingly.