Solid state speciation of uranium and its local structure in Sr₂CeO₄ using photoluminescence spectroscopy

An effort was taken to carry our speciation study of uranium ion in technologically important cerate host Sr₂CeO₄ using time resolved photoluminescence spectroscopy. Such studies are not relevant only to nuclear industry but can give rich insight into fundamentals of 5f electron chemistry in solid state systems. In this work both undoped and varied amount of uranium doped Sr₂CeO₄ compound is synthesized using complex polymerization method and is characterized systematically using X-ray diffraction (XRD), Raman spectroscopy, impedance spectroscopy and scanning electron microscopy (SEM). Both XRD and Raman spectroscopy confirmed the formation of pure Sr₂CeO₄ which has tendency to decompose peritectically to SrCeO₃ and SrO at higher temperature. Uranium doping is confirmed by XRD. Uranium exhibits a rich chemistry owing to its variable oxidation state from +3 to +6. Each of them exhibits distinct luminescence properties either due to f-f transitions or ligand to metal charge transfer (LMCT). We have taken Sr₂CeO₄ as a model host lattice to understand the photophysical characteristics of uranium ion in it. Emission spectroscopy revealed the stabilization of uranium as U (VI) in the form of UO₆^6- (octahedral uranate) in Sr₂CeO₄. Emission kinetics study reflects that uranate ions are not homogeneously distributed in Sr₂CeO₄ and it has two different environments due to its stabilization at both Sr²⁺ as well as Ce⁴⁺ site. The lifetime population analysis interestingly pinpointed that majority of uranate ion resided at Ce⁴⁺ site. The critical energy-transfer distance between the uranate ion was determined based on which the concentration quenching mechanism was attributed to electric multipolar interaction. These studies are very important in designing Sr₂CeO₄ based optoelectronic material as well exploring it for actinides studies.