Low Temperature Precursor Route for Highly Efficient Spherically Shaped LED-Phosphors M2Si5N8:Eu2+ (M = Eu, Sr, Ba)

The highly efficient nitridosilicate phosphors M2Si5N8 (M = Sr, Ba, Eu) for phosphor-converted pc-LEDs were synthesized at low temperatures using a novel precursor route involving metal amides M(NH2)(2). These precursors have been synthesized by dissolution of the respective metals in supercritical ammonia at 150 degrees C and 300 bar. The thermal behavior and decomposition process of the amides were investigated with temperature programmed powder X-ray diffractometry and thermoanalytical measurements (DTA/TG). These investigations rendered the amides as suitable inter-mediates for reaction with silicon diimide (Si(NH)(2)). Thus, the desired nitridosilicate phosphors were obtained at relatively low temperatures around 1150-1400 degrees C which is approximately 300 degrees C lower compared to common synthetic approaches starting from metals or oxides. The influence of the thermal treatment on the phosphor morphology has been studied extensively. The accessibility of spherical phosphor particles represents another striking feature of this route since it improves light extraction from the crystallites due to decreasing light guiding and decreasing re-absorption inside the phosphor particle. The synthesized luminescent materials M2Si5N8: Eu2+ (M = Sr, Ba) exhibit quantum efficiencies and emission band widths (FWHM 70-90 nm) comparable to standard phosphor powders. Employment of Eu(NH2)(2) as dopant reagent for synthesis of Ba2Si5N8: Eu2+ proved favorable for the formation of spherical crystallites compared to doping with Eu metal, halides, or oxide.