Abstract
The influence of the type of the alkaline-earth ion and the Eu 2+ concentration on the luminescence properties of Eu 2+-doped M 2Si 5N 8 (M = Ca, Sr, Ba) has been investigated. XRD analysis shows that Eu 2+-doped Ca 2Si 5N 8 forms a limited solid solution with a maximum solubility of about 7 mol% having a monoclinic lattice. The Eu 2+ ion can be totally incorporated into Sr 2Si 5N 8 and Ba 2Si 5N 8 forming complete solid solutions with orthorhombic lattices. M 2Si 5N 8:Eu 2+ (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. Ba 2Si 5N 8:Eu 2+ shows yellow to red emission with maxima from 580 to 680 nm with increasing Eu 2+ 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 Eu 2+ in the nitrogen environment. With increasing the europium concentration, the emission band shows a red-shift for all M 2Si 5N 8:Eu 2+ compounds due to changing Stokes shift and the reabsorption by Eu 2+. The conversion (i.e. quantum) efficiency increases going from Ca to Ba and Sr under excitation at 465 nm. In particular, Sr 2Si 5N 8:Eu 2+ with a quantum efficiency of 75–80% and a thermal quenching of only a few percent at 150 °C, demonstrates to be a highly promising red-emitting conversion phosphor for white-LED applications.
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