Persistent luminescence mechanism of Gd³⁺ in ScPO₄

Persistent luminescence (PersL) is an important and interesting optical phenomenon. However, the popular valence-alternation theory struggles to elucidate the PersL mechanism of Gd³⁺ due to its highly stable +3 oxidation state. Here, by using the X-ray Absorption Near Edge Structure (XANES), we confirm the stable valence state of Gd³⁺ in ScPO₄, thus providing evidence to exclude the valence-alternation model for explaining the PersL of Gd³⁺ in ScPO₄. Comparative experiments on the photoluminescence and PersL spectra between ScPO₄ and Gd³⁺-doped ScPO₄ confirm the energy transfer from the self-trapped excitons of ScPO₄ to Gd³⁺. Further density functional theory (DFT) calculations reveal that Gd³⁺ doping enhances the orbital hybridization between Gd³⁺ and O^2-, thereby modifying the electronic structure, which likely facilitates the confinement of self-trapped excitons.