A new thermal degradation mechanism of red Sr2Si5N8:Eu phosphor: From the view of microstructural evolution

Nitride red phosphors of Sr2Si5N8:Eu2+ are synthesized in mild conditions and exhibit excellent luminescence properties; yet, the occurrence of thermal degradation results in a decrease of their luminous intensity and quantum efficiency at the high working temperature of wLED. This work shows that Sr2Si5N8:Eu2+ phosphors, prepared by a high-temperature solid-state reaction, undergo severe thermal degradation at high temperatures, not only in air atmosphere, but even in highly pure N2 gas. In order to shed light on the degradation mechanism, the materials obtained after heat-treatment at various temperatures up to 600 °C were thoroughly characterized at the investigated temperatures with the aid of several experimental techniques, such as in-situ HRTEM, along with XPS, luminescence spectroscopy, SEM/EDS, XRD with Rietvelt refinement. The analyses of the results proposed a novel mechanism for the thermal degradation of Sr2Si5N8:Eu2+ phosphors. More specifically, there was no evidence of alterations in the crystalline regime or in the Eu-oxidation state after the heat-treatment. Nevertheless, the results revealed that after heat-treatment in N2, the bonds between Si atom and O impurity inside the crystal lattice become unstable, resulting in their detachment and in formation of local nano-defects. SrSiO3 precipitates on the surface of the phosphor particles, which leads to the destruction of the original lattice structure and to the formation of deep defect energy levels, which causes a decrease in the luminous intensity of the phosphor. Therefore, present study provides a new insight into the thermal degradation mechanism and favors solving the shortcomings of Sr2Si5N8:Eu2+ phosphor in the process of industrialization.