Site occupation of Al doping in Lu₂SiO₅: The role of ionic radius versus chemical valence

Lu₂SiO₅:Ce (LSO:Ce) serving as a core material for radiation detectors, plays a crucial role in the design and development of positron emission tomography (PET) devices. Experiment has confirmed that low concentration of Al doping can significantly enhance the light yield, decay time, rise time, energy resolution, and afterglow level of the LSO:Ce crystals. The mechanisms regarding the lattice site occupancy of Al in LSO, while closely associated with the performance improvements, are not yet fully understood. Particularly, it is unclear either the ionic radius or the chemical valence plays a more critical role in determining the site occupancy. In this study, we utilized first-principles calculations based on density functional theory (DFT) to study the lattice site occupancy of Al in LSO crystals and to explore their impact on the electronic structure. Our results indicate that with changes in the growth environment, as reflected by the atomic chemical potentials, Al can occupy either the Si sites or the Lu₂ sites, and it is not inclined to occupy the Lu₁ sites. The doping of Al at the Si site introduces a shallow acceptor level, which may contribute to the suppression of trap concentration and affect the ratio of Ce³⁺ to Ce⁴⁺ within the crystal, thereby influencing its scintillation properties.