We study from first principles the luminescence of Lu2βSiO5β:Ce3+
(LSO:Ce), a scintillator widely used in medical imaging applications, and
establish the crucial role of oxygen vacancies (VOβ) 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 ΞSCF 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
Ce-VOβ complexes together with the other Ce-based luminescent center.
Further, the luminescence thermal quenching behavior is analyzed. The 4fβ5d
crossover mechanism is found to be very unlikely, with a large crossing energy
barrier (Efdβ) in the one-dimensional model. The alternative mechanism
usually considered, namely the electron auto-ionization, 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 Ce3+,β and in the
ionized electronic state Ce4+. The results indicate that such electron
auto-ionization cannot explain the thermal stability difference between the
high- and low-energy emission bands. Finally, a hole auto-ionization 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.Comment: 13 pages, 8 figures, accepted by Phys. Rev. Material