Local Structure and Speciation-Driven UO₂²⁺ → Sm³⁺ Energy Transfer for Enhanced Luminescence in Li₂B₄O₇

Herein, we report the uranyl sensitization of Sm³⁺ emissions in uranium-codoped Li₂B₄O₇:Sm³⁺ phosphor. The uranyl speciation in codoped [Sm, U] LTB samples was determined by synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy that revealed two coordination shells for U(VI) ions with bond distances of U-O_ax (∼1.81 Å) and U-O_eq (∼2.30 Å). EXAFS fitting suggested that the uranyl moiety is present as pentagonal bipyramids (UO₇) and hexagonal bipyramids (UO₈) with five and six equatorial oxygen ligands, respectively. The alteration of the local structure of Sm³⁺ from [SmO₄] to [SmO₇] polyhedra and the changes in the coordination number of equatorial oxygen for uranyl were observed with different codoping concentrations of Sm³⁺ and uranium. Density functional theory (DFT) calculations suggested the lowering of defect formation energy for Li vacancies on codoping of Sm and U. Hence, we proposed the increase of the equatorial coordination number of UO₂²⁺ on the increase in the lithium vacancies in LTB. In addition, DFT supported the feasibility of efficient energy transfer (ET) due to the overlap of uranium and Sm³⁺ excited state levels. The influence of the same on the spectral features and UO₂²⁺ → Sm³⁺ energy transfer was investigated by time-resolved photoluminescence (PL) studies. The ET efficiency from the UO₂²⁺ to Sm³⁺ was 70.5% in 0.5 mol % codoped [Sm, U] LTB samples. The correlation of EXAFS and luminescence properties indicated a red shift in vibronic features of uranyl emission with increase in the equatorial coordination of the uranyl moiety from five to six. Additionally, a higher probability of ET was observed for uranyl speciation as UO₈ hexagonal bipyramids. Temperature-dependent emissions and decay profiles were collected under uranyl excitation to investigate the thermal dependence of ET. A high energy barrier (E_a ∼ 4027 cm^-1) was evaluated for the thermal quenching of Sm³⁺ emissions. This work provides insights into the modulation of luminescence and ET efficiency via structural changes in uranyl and Sm local environment in LTB phosphor.