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We study from first principles the luminescence of Lu$_2$SiO$_5$:Ce$^{3+}$(LSO:Ce), a scintillator widely used in medical imaging applications, andestablish the crucial role of oxygen vacancies (V$_O$) in the generatedspectrum. The excitation energy, emission energy and Stokes shift of itsluminescent centers are simulated through a constrained density-functionaltheory method coupled with a ${\Delta}$SCF analysis of total energies, andcompared with experimental spectra. We show that the high-energy emission bandcomes from a single Ce-based luminescent center, while the large experimentalspread of the low-energy emission band originates from a whole set of differentCe-V$_O$ complexes together with the other Ce-based luminescent center.Further, the luminescence thermal quenching behavior is analyzed. The $4f-5d$crossover mechanism is found to be very unlikely, with a large crossing energybarrier (E$_{fd}$) in the one-dimensional model. The alternative mechanismusually considered, namely the electron auto-ionization, is also shown to beunlikely. In this respect, we introduce a new methodology in which thetime-consuming accurate computation of the band gap for such models isbypassed. We emphasize the usually overlooked role of the differing geometryrelaxation in the excited neutral electronic state Ce$^{3+,*}$ and in theionized electronic state Ce$^{4+}$. The results indicate that such electronauto-ionization cannot explain the thermal stability difference between thehigh- and low-energy emission bands. Finally, a hole auto-ionization process isproposed as a plausible alternative. With the already well-established excitedstate characterization methodology, the approach to color center identificationand thermal quenching analysis proposed here can be applied to otherluminescent materials in the presence of intrinsic defects.

Ab initio Study of Luminescence in Ce-doped Lu$_2$SiO$_5$: The Role of Oxygen Vacancies on Emission Color and Thermal Quenching Behavior