First-Principles Study of Bi³⁺-Related Luminescence and Electron and Hole Traps in (Y/Lu/La)PO₄

Bismuth ion-doped phosphate crystals have shown rich luminescence phenomena. However, the complexity and variety of Bi³⁺-related transitions bring great challenges to the understanding of the underlying mechanisms, rendering it hard to rationally design new phosphors and optimize their performance. In this work, we perform first-principles calculations based on the generalized gradient approximation of density functional to obtain the excited state equilibrium geometric structures and then calculate the electronic structures for various Bi³⁺-related excited states in phosphates RPO₄:Bi³⁺ (R = Y, Lu, La) by utilizing the hybrid density functional method. The experimentally measured excitation and emission features are well interpreted by our theoretical calculations. Specifically, we reveal that the emission in LaPO₄:Bi³⁺ is of charge transfer nature, whereas the dominant emission in YPO₄:Bi³⁺ or LuPO₄:Bi³⁺ is the characteristic A band emission. Trapped holes above the valence band maximum due to intrinsic defects are deemed to play a role in the charge-transfer emission of LaPO₄. Our calculations show that the excited state of the Bi³⁺ pair in YPO₄ or LuPO₄ is (Bi³⁺-Bi³⁺)*, rather than Bi²⁺-Bi⁴⁺. Such a Bi³⁺ pair contributes to the longer wavelength emission. Furthermore, our calculations on charge transition levels show that Bi³⁺ ions can act as electron and hole traps in RPO₄ (R = Y, Lu, La). Our work indicates that first-principles calculations can be useful in exploring the diverse luminescence processes in Bi³⁺-doped inorganic insulators.