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-x EuxSi5N8 (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 Eu-Sr1 or Eu-Sr2 content, the structural rigidity decreases gradually and the occupied 5d state in the excited Eu-2+(4f(6)5d(1)) ion becomes more delocalized, which may result in an enlarged Stokes shift of the 5d -> 4f emission and thus its redshift as observed experimentally. (C) 2017 Elsevier B.V. All rights reserved.