Speciation and site occupancy of uranium in strontium orthosilicate by photoluminescence and X-ray absorption spectroscopy: A combined experimental and theoretical approach

• Uranium stabilizes as UO22+.
• PL is characteristic of distorted environment for UO22+.
• Majority of UO22+ occupy 9-coordinated Sr sites.
• DFT calculations also supported the experimental findings.

Identifying the oxidation state and coordination geometry of radioactive element like uranium is important to fully understand its toxicology and other harmful effect in environment. Strontium orthosilicate is taken as a model compound for that. Strontium silicate doped with 1.0 mol% of U has been synthesized using sol–gel method and characterized using X-ray diffraction (XRD), time resolved fluorescence spectroscopy (TRFS) and extended X-ray absorption fine structure (EXAFS). Uranium exhibits multiple oxidation state and each one of them is having characteristics luminescence is an interesting dopant from structural point of view. TRFS is used to investigate the oxidation state and coordination behavior of uranium in Sr2SiO4. From TRFS measurement it was observed that uranium stabilizes in +6 oxidation state in the form of uranyl ion. Based on luminescence lifetime and EXAFS studies it was inferred that uranyl is stabilized on both 9- and 10-coordinated strontium polyhedra but majority occupies relatively asymmetric 9-coordinated Sr sites. This is further confirmed using theoretical measurement.

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