Origin of thermal degradation of Sr _2-xSi ₅N ₈:Eu _x phosphors in air for light-emitting diodes

The orange-red emitting phosphors based on M 2Si 5N 8:Eu (M = Sr, Ba) are widely utilized in white light-emitting diodes (WLEDs) because of their improvement of the color rendering index (CRI), which is brilliant for warm white light emission. Nitride-based phosphors are adopted in high-performance applications because of their excellent thermal and chemical stabilities. A series of nitridosilicate phosphor compounds, M 2-xSi 5N 8:Eu x (M = Sr, Ba), were prepared by solid-state reaction. The thermal degradation in air was only observed in Sr 2-xSi 5N 8:Eu x with x = 0.10, but it did not appear in Sr 2-xSi 5N 8:Eu x with x = 0.02 and Ba analogue with x = 0.10. This is an unprecedented investigation to study this phenomenon in the stable nitrides. The crystal structural variation upon heating treatment of these compounds was carried out using the in situ XRD measurements. The valence of Eu ions in these compounds was determined by electron spectroscopy for chemical analysis (ESCA) and X-ray absorption near-edge structure (XANES) spectroscopy. The morphology of these materials was examined by transmission electron microscopy (TEM). Combining all results, it is concluded that the origin of the thermal degradation in Sr 2-xSi 5N 8:Eu x with x = 0.10 is due to the formation of an amorphous layer on the surface of the nitride phosphor grain during oxidative heating treatment, which results in the oxidation of Eu ions from divalent to trivalent. This study provides a new perspective for the impact of the degradation problem as a consequence of heating processes in luminescent materials.