Luminescence properties of red-emitting M2Si5N8:Eu2+ (M = Ca, Sr, Ba) LED conversion phosphors

The influence of the type of the alkaline-earth ion and the Eu2+ concentration on the luminescence properties of Eu2+-doped M2Si5N8 (M Ca, Sr, Ba) has been investigated. XRD analysis shows that Eu2+-doped Ca2Si5N8 forms a limited solid solution with a maximum solubility of about 7 mol% having a monoclinic lattice. The Eu2+ ion can be totally incorporated into Sr2Si5N8 and Ba2Si5N8 forming complete solid solutions with orthorhombic lattices. M2Si5N8:Eu2+ (M = Ca, Sr) shows typical broad band emission in orange to red spectral range (600-680 nm) depending on the type of M and the europium concentration. Ba2Si5N8:Eu2+ shows yellow to red emission with maxima from 580 to 680 nm with increasing Eu2+ content. The long-wavelength excitation and emission is attributed to the effect of a high covalency and a large crystal-field splitting on the 5d band of Eu2+ in the nitrogen environment. With increasing the europium concentration, the emission band shows a red-shift for all M2Si5N8:Eu2+ compounds due to changing Stokes shift and the reabsorption by Eu2+. The conversion (i.e. quantum) efficiency increases going from Ca to Ba and Sr under excitation at 465 nm. In particular, Sr2Si5N8:Eu2+ with a quantum efficiency of 75-80% and a thermal quenching of only a few percent at 150 deg. C, demonstrates to be a highly promising red-emitting conversion phosphor for white-LED applications