Effect of manganese on the crystal structures and luminescence properties of mn doped Sr2SiO4:Eu2+ phosphors

In Sr2SiO4:Eu2+ phosphors, the difference between the size of Sr(I) and the diameter of Eu2+ ion is larger than that between the size of Sr(II) and the diameter of Eu2+ ion. So Eu(I) is dominantly influenced by crystal field. The bond length of O-Eu bonds becomes shorter after incorporation of Mn atoms, which results in the increase of the crystal field and splitting of Eu5d states. Its band gap decreases from 4.45 eV to 3.69 eV, which results in a redshift of its absorption spectrum. Eu(II) is mainly effectively affected by Nephelanxetic effect. The average covalency of OEu(II) bonds is increased as Mn doped, so that the difference between Eu5d and 4f levels decreases. Consequently, its band gap decreases from 4.44 eV to 3.76 eV, which lead to a redshift of its absorption spectrum.