Formation of the amorphous phase in the carbothermal reduction and nitridation route to SrSi2O2N2:Eu2+: a new understanding of the catalytic effect of carbon in the synthesis of Sr2Si5N8:Eu2+ for white LEDs

This study provides an explanation for the mechanism of carbothermal reduction contributing to the synthesis of a Sr2Si5N8 : Eu2+ nitride phosphor, in terms of the heat released and propagated within the local region, which makes the actual temperature far higher than that determined in the reaction. Two pathways of carbon mixing were designed for synthesizing SrSi2O2N2 : Eu2+ at relatively lower temperatures than that required for synthesis of Sr2Si5N8 : Eu2+. However, formation of the nitride, Sr2Si5N8 : Eu2+, was observed by mixing carbon with the raw materials, rather than placing a layer of carbon underneath for heat conduction, indicating that close contact of carbon with the raw materials was a precondition of nitride formation. In addition, the presence of an amorphous layer on the surface of the phosphor particles, identified using a high-resolution transmission electron microscope, provided a clue to understanding the mechanism of the nitridation reaction at relatively low temperatures.