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

Abstract

Nitride red phosphors of Sr 2 Si 5 N 8 :Eu 2+ 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 Sr 2 Si 5 N 8 :Eu 2+ 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 N 2 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 Sr 2 Si 5 N 8 :Eu 2+ 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 N 2 , 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. SrSiO 3 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 Sr 2 Si 5 N 8 :Eu 2+ phosphor in the process of industrialization. • The bonds between Si atom and O impurity detached in the crystal lattice. • A shrinkage of the unit cell occurs after thermal treatment in N 2 atmosphere. • Structural defects favor non-radiative transitions and decreased PL intensity.