On the photophysics and speciation of actinide ion in MgAl2O4 spinel using photoluminescence spectroscopy and first principle calculation: A case study with uranium

Actinide chemistry is very interesting not from scientific perspective but also from technological importance. Elucidating the valence state and coordinating environment of actinide ion like uranium in technologically important magnesium aluminate spinel (MAS) is important to fully understand its hazardous and other harmful effect in human as well as environment. Magnesium aluminate spinel doped with 1.0 mol % of Uranium ion has been synthesized using citric acid assisted gel-combustion route at 800 °C. The as prepared powder is characterised using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS) and density functional theory (DFT) calculations. Uranium is an interesting element because it exhibits multiple oxidation state and each one of them is having characteristics fluorescence behavior. TRPLS is used to investigate the oxidation state and coordination behavior of uranium in MgAl2O4. Indeed in our earlier work on undoped and lanthanide ion doped MAS; it was oberved that in undoped sample itself defect induced emission could be seen in visible region which was probed using DFT. Here on doping uranium in MAS; complete energy of host is transferred to uranium ion which is explained using DFT. From excitation and emission spectroscopy it was observed that uranium stabilizes in +6 oxidation state in the form of UO22+ ion. Based on luminescence lifetime and its comparison with the emission profile of uranyl fluoride crystal it was inferred that majority of uranium is occupying relatively asymmetric MgO4 polyhedra and minority substitutes AlO6. The site stability of the uranyl ion in MAS was also validated using DFT based first principle calculations. Time resolved emission shows the uranyl at Mg2+ site differs from the one at Al3+ site in terms of peak position and intensity.