Study of persistent luminescence mechanisms of TB³⁺ and PR³⁺ doped materials with X-ray absorption spectroscopy

Full text: Research on persistent luminescence materials has expanded since mid 1990s due to their versatile applications. Despite the experimental and theoretical advance made for Eu²⁺ doped materials, the similar phenomena arising from trivalent rare earths (R³⁺) have received much less attention. Among such materials, CdSiO₃:R³⁺ show persistent emission from the R³⁺ ions or defects or both when doped with Tb³⁺ and Pr³⁺ or La³⁺, Gd³⁺, and Lu³⁺ or Dy³⁺ and Sm³⁺, respectively. Understanding the mechanism of persistent luminescence of these materials requires the knowledge of dopant's valence and environment and if the photoionization process produces R²⁺ or R^IV. The synchrotron radiation (SR) XANES measurements on CdSiO₃:R³⁺ show only the trivalent Pr³⁺, Eu³⁺ and Tb³⁺, even if the Tb persistent luminescence was induced by X-rays. The photoionization of Pr³⁺ and Tb³⁺ during charging persistent luminescence thus creates Pr³⁺-h⁺ and Tb³⁺-h⁺ pairs instead of Pr^IV and Tb^IV. The EXAFS data confirmed that R³⁺ replace Cd²⁺ in CdSiO₃, requiring charge compensation and formation of defects responsible for storing energy for persistent luminescence. The XANES and EXAFS results aided to design the mechanism of persistent luminescence from Pr³⁺ and Tb³⁺ in CdSiO₃. The band gap energy and the 4fⁿ ground energy level positions vis-à-vis the host's band system were determined with the SR VUV-UV excitation spectroscopy. Finally, the trap energies were determined with thermoluminescence measurements. (author)