Mechanism of long-lasting phosphorescence process of Ce3+-doped Ca2Al2SiO7 melilite crystals -: art. no. 235108

UV excitation of Ce3+-doped Ca2Al2SiO7 melilite crystals produces long-lasting phosphorescence (1-10(3) s) from Ce3+ ions in addition to the intrinsic Ce3+ 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 Ce3+ in the crystals and that electrons are trapped at O2- vacancies as F+ centers, the wave functions extending toward Al3+ ions, while holes are self-trapped at Al3+ ions accompanied by Si4+ vacancies in the nearest neighbors along the <110> or <(1) over bar 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(-n)(n<1). 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 Al4+ produced in the crystal by the UV excitation of Ce3+ move back to Ce3+ sites through tunneling and thermal hopping, and the retrapped holes in the form of Ce4+ recombine radiatively the trapped electrons through tunneling.