Ab initiostudy of luminescence in Ce-dopedLu2SiO5: The role of oxygen vacancies on emission color and thermal quenching behavior

We study from first principles the luminescence of ${\mathrm{Lu}}_{2}{\mathrm{SiO}}_{5}$:${\mathrm{Ce}}^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the generated spectrum. The excitation energy, emission energy, and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a $\mathrm{\ensuremath{\Delta}}\mathrm{SCF}$ analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread of the low-energy emission band originates from a whole set of different $\mathrm{Ce}\ensuremath{-}{\mathrm{V}}_{\mathrm{O}}$ complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The $4f\ensuremath{-}5d$ crossover mechanism is found to be very unlikely, with a large crossing energy barrier (${E}_{fd}$) in the one-dimensional model. The alternative mechanism usually considered, namely the electron autoionization, is also shown to be unlikely. In this respect, we introduce a new methodology in which the time-consuming accurate computation of the band gap for such models is bypassed. We emphasize the usually overlooked role of the differing geometry relaxation in the excited neutral electronic state ${\mathrm{Ce}}^{3+,*}$ and in the ionized electronic state ${\mathrm{Ce}}^{4+}$. The results indicate that such electron autoionization cannot explain the thermal stability difference between the high- and low-energy emission bands. Finally, a hole autoionization process is proposed as a plausible alternative. With the already well-established excited-state characterization methodology, the approach to color center identification and thermal quenching analysis proposed here can be applied to other luminescent materials in the presence of intrinsic defects.