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.