Radiative and nonradiative decay processes responsible for long-lasting phosphorescence ofEu2+-doped barium silicates

Abstract

UV or visible excitation for ${\mathrm{Eu}}^{2+}$-doped ${\mathrm{Ba}}_2\mathrm{Si}{\mathrm{O}}_4$ and ${\mathrm{Ba}}_3\mathrm{Si}{\mathrm{O}}_5$ crystals produces broadband luminescence with peak wavelengths of 510 and 590 nm, respectively. The intensities of the long-lasting phosphorescence were measured as functions of temperature and time. The radiative decay times are distributed in the wide range of $1\text{–}{10}^3\mathrm{s}$ besides the ${\mathrm{Eu}}^{2+}$ fluorescence lifetime $(\sim {10}^{-6}\mathrm{s})$. The decay curves after one second in the temperature range of 100–500 K fit $t^{-n}(0.2<n<3)$. The temperature dependence of the long-lasting phosphorescence intensities integrated in a time domain obeys the modified Arrhenius’ equation with thermal activation energies including radiative and nonradiative decay processes. These results suggest that electrons and holes produced by UV excitation move back to ${\mathrm{Eu}}^{2+}$ sites in the crystals through thermal hopping and tunneling, and recombine radiatively at ${\mathrm{Eu}}^{2+}$.