Stabilization of Eu²⁺ in Li₂B₄O₇ with the BO₃ network through U⁶⁺ co-doping and defect engineering

Owing to the unique 4f-5d transitions and the involvement of 5d electrons, the divalent europium (Eu²⁺) ion is extensively used as a dopant ion in luminescent materials for phosphor-converted light emitting diodes (pc-LEDs) and other technological applications. Earlier reports in most of the cases have shown that the reduction of Eu³⁺ to Eu²⁺ requires very high temperatures and large hydrogen flux. In this study, a co-doping strategy with higher valent U⁶⁺ ions was utilized to successfully stabilize Eu²⁺ ions in the Li₂B₄O₇ (LTB) host with both the BO₃ and BO₄ network in low H₂ flux of only 8%. It is postulated that charge transfer occurs from U to Eu, resulting in the reduction of the charged state of Eu and the reaction probably proceeds via the formation of paramagnetic transient [U⁵⁺-Eu³⁺] species in the co-doped LTB. The same is also believed to be facilitated by the enhanced formation of Li-O type vacancy clusters in co-doped samples and enhanced oxygen vacancies in a reducing atmosphere. We believe this work will pave a new pathway for stabilizing the unusual oxidation state of lanthanides and transition metal ions through co-doping with hexavalent uranium ions.