Unraveling the Critical Role of Site Occupancy of Lithium Codopants in Lu ₂ SiO ₅ :Ce ³⁺ Single-Crystalline Scintillators

Lithium codoping has emerged as an effective strategy to enhance the light yield of oxide scintillators for radiation detection applications, but the understanding of the actual role played by Li ⁺ remains unclear. In this work, we comprehensively study the effects of Li codoping on optical and scintillation properties of Lu ₂ SiO ₅ :Ce (LSO:Ce) single crystals and reveal the critical role of site occupancy of Li. High-quality LSO:Ce single crystals codoped with 0.05, 0.1, and 0.3 at. % Li ions were grown by the Czochralski method. The optical absorption spectra confirm nonconversion of stable Ce ³⁺ to Ce ⁴⁺ in Li-codoped LSO:Ce regardless of the Li codoping concentration. The photoluminescence decay kinetics suggest an enhanced ionization of the excited 5d ₁ state of Ce ³⁺ centers in highly codoped samples. A simultaneous improvement of scintillation light yield, decay time, and afterglow is achieved in LSO:Ce codoped with low concentrations of Li. The preferential occupation of Li at interstitial spaces and lutetium sites is proven to rely on its codoping concentration by using the ⁷ Li nuclear magnetic resonance technique. The concentration-dependent site occupancy of Li alters the defect structures of LSO:Ce, in particular resulting in a distinct change in the number of cerium spatially correlated oxygen vacancies confirmed by thermoluminescence and afterglow measurements.