Understanding the emission redshift in Sr2Si5N8:Eu2+ with increasing Eu doping concentration from density functional calculations

The red phosphor Sr2Si5N8:Eu2+ exhibits a significant emission redshift with increasing Eu concentration, but the reason remains controversial. Here, we investigate energetic, mechanical, and electronic properties of Sr2Si5N8:Eu2+ by using density-functional theory (DFT) approaches with the periodic supercell model. Total-energy calculations for Sr2−xEuxSi5N8 (x=0.028, 0.125, 0.5) supercells reveal that Eu2+ ions occupy the two distinct Sr1 and Sr2 sites with almost equal preference, with a nearly even distribution of Eu2+ on the two Sr sites, irrespective of the doping concentration. Calculations for the Debye temperature and electronic properties show that, with increasing EuSr1 or EuSr2 content, the structural rigidity decreases gradually and the occupied 5d state in the excited Eu2+(4f65d1) ion becomes more delocalized, which may result in an enlarged Stokes shift of the 5d→4f emission and thus its redshift as observed experimentally.