Reddish-orange long-lasting phosphorescence of Ca₂ Si ₅ N₈: Eu²⁺, Tm³⁺ phosphor

Reddish-orange long-lasting phosphorescence materials, various amounts of Eu²⁺ - and Tm³⁺ -co-doped phosphors (Ca₂ Si₅ N₈: Eu²⁺, Tm³⁺), were prepared by the conventional high temperature solid-state reaction method, and their luminescence properties were systematically investigated by photoluminescence spectra, afterglow spectra, afterglow decay curves, and thermoluminescence spectra. The Ca₂ Si₅ N₈: Eu²⁺, Tm³⁺ materials exhibited strong reddish-orange emission bands in a wavelength range of 500-750 nm with peak positions at about 600 nm due to the 5d→4f transition of the Eu²⁺ ion. Furthermore, these phosphors emitted strong reddish-orange long-lasting phosphorescence with an afterglow time of more than 1 h after turning off the activating lamp in the light perception of the dark-adapted human eye (0.32 mcd/ m²). Such afterglow of Ca₂ Si₅ N₈: Eu²⁺, Tm²⁺ phosphors was attributed to the recombination of holes and electrons that were trapped within the lattice defect centers. The results of the thermoluminescence spectra indicated that the increase in the predominating band at 350 K, which was associated with the charge-trapping centers, was responsible for the enhancement of the afterglow properties of Ca₂ Si₅ N₈: Eu²⁺, Tm³⁺.