Experimental data from literature on the thermal quenching of the Eu²⁺ 5d-4f emission in the M_xSi_yN_z (M = alkali, alkaline earth or rare earth) nitridosilicates have been collected and evaluated. No clear correlation was observed between the activation energy for thermal quenching and the Stokes shift, suggesting that non-radiative relaxation via a thermally excited cross-over from the 5d excited state to the 4f ground state is not the main reason for thermal quenching in the nitridosilicates. Based on literature data on rare-earth charge transfer transitions, host-lattice bandgap and Eu²⁺ 5d-4f emission energy, the energy difference between the Eu²⁺ 5d level and the bottom of the host-lattice conduction band has been determined. This energy difference correlates fairly well with the quenching temperature, suggesting that thermal ionization of the 5d electron to the conduction band is responsible for the thermal quenching of the Eu²⁺ 5d-4f emission in the nitridosilicates. The energy difference between the lowest 5d level and the bottom of the conduction band, and consequently the quenching temperature, increases with increasing Si/N ratio of the nitridosilicates. From this, it is concluded that the combined effect of the larger Stokes shift and the raise in energy of the bottom of the host lattice conduction band with increasing Si/N ratio is stronger than the decrease of the centroid shift and crystal field splitting of the Eu²⁺ 5d level.

The red-emitting Sr ₂ Si ₅ N ₈ :Eu ²⁺ phosphor with a superior quantum efficiency and suitable emission spectrum has been widely used as a promising down-conversion material in white light-emitting diodes. However, its thermal degradation under high temperature handicaps its large scale application, and therefore must be reduced. Here, we proposed to increase the thermal stability of Sr ₂ Si ₅ N ₈ :Eu ²⁺ by coating a nanometer-order Al ₂ O ₃ film on each phosphor particle using an atomic layer deposition approach in a fluidized bed reactor. The deposited Al ₂ O ₃ layer was quite uniform and conformal when using O ₃ as the oxidizer, and its thickness could be controlled by the dosage type, deposition temperature and cycle numbers, which largely affects the photoluminescence properties and thermal degradation of the title phosphor. Thermal gravimetric analysis results showed that the oxidation temperature of the coated phosphor increased from 700 to 850 °C, suggesting that the coating layer has the function of anti-oxidation. Meanwhile, the coated phosphor particle surface became hydrophobic. Consequently, the thermal degradation of phosphor powders in air at 200 °C was greatly reduced and the stability of the fabricated LEDs with coated powders was also improved. Prospectively, the proposed approach provides a new strategy to improve the thermal stability of other phosphors.