Mechanism of long-lasting phosphorescence process ofCe3+-dopedCa2Al2SiO7melilite crystals

Abstract

UV excitation of ${\mathrm{Ce}}^{3+}$-doped ${\mathrm{Ca}}_{2}{\mathrm{Al}}_{2}{\mathrm{SiO}}_{7}$ melilite crystals produces long-lasting phosphorescence $(1\ensuremath{-}{10}^{3} \mathrm{s})$ from ${\mathrm{Ce}}^{3+}$ ions in addition to the intrinsic ${\mathrm{Ce}}^{3+}$ luminescence with a lifetime of 40 ns. The distribution of the radiative decay rates is due to recombination of distant pairs of electron and hole in the crystals. The electron spin-resonance study gives evidence that pairs of electron and hole are produced through UV excitation of ${\mathrm{Ce}}^{3+}$ in the crystals and that electrons are trapped at ${\mathrm{O}}^{2\ensuremath{-}}$ vacancies as ${F}^{+}$ centers, the wave functions extending toward ${\mathrm{Al}}^{3+}$ ions, while holes are self-trapped at ${\mathrm{Al}}^{3+}$ ions accompanied by ${\mathrm{Si}}^{4+}$ vacancies in the nearest neighbors along the $〈110〉$ or $〈1\ifmmode\bar\else\textasciimacron\fi{}10〉$ direction. The intensities of the phosphorescence at the peak wavelength of 410 nm were measured as functions of temperature and time. The decay curves of the phosphorescence at various temperatures fit ${t}^{\ensuremath{-}n}(nl1).$ The temperature dependence of the intensities integrated in a time domain obeys the Arrhenius' equation with a thermal activation energy of 243 meV. These results support that the self-trapped holes ${\mathrm{Al}}^{4+}$ produced in the crystal by the UV excitation of ${\mathrm{Ce}}^{3+}$ move back to ${\mathrm{Ce}}^{3+}$ sites through tunneling and thermal hopping, and the retrapped holes in the form of ${\mathrm{Ce}}^{4+}$ recombine radiatively the trapped electrons through tunneling.