First-Principles Study of Bi-Related Luminescence and Electron and Hole Traps in (Y/Lu/La)PO.

Bismuth ion-doped phosphate crystals have shown rich luminescence phenomena. However, the complexity and variety of Bi-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 Bi-related excited states in phosphates RPO:Bi (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 LaPO:Bi is of charge transfer nature, whereas the dominant emission in YPO:Bi or LuPO:Bi 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 LaPO. Our calculations show that the excited state of the Bi pair in YPO or LuPO is (Bi-Bi)*, rather than Bi-Bi. Such a Bi pair contributes to the longer wavelength emission. Furthermore, our calculations on charge transition levels show that Bi ions can act as electron and hole traps in RPO (R = Y, Lu, La). Our work indicates that first-principles calculations can be useful in exploring the diverse luminescence processes in Bi-doped inorganic insulators.