Dehydration of the Uranyl Peroxide Studtite, [UO2(eta(2)-O-2)(H2O)(2)]center dot 2H(2)O, Affords a Drastic Change in the Electronic Structure: A Combined X-ray Spectroscopic and Theoretical Analysis

The minerals studtite, [UO2(eta(2)-O-2)(H2O)2]center dot 2H(2)O, and metastudtite, [UO2(eta(2)-O-2)(H2O)(2)], are uranyl peroxide minerals that are major oxidative alteration phases of UO2 under conditions of geological storage. The dehydration of studtite has been studied using X-ray photoelectron spectroscopy (XPS) and Xray absorption spectroscopy. XPS of the U 4f region shows small but significant differences between studtite and metastudtite, with the 4f binding energy of studtite being the highest reported for a uranyl mineral studied by this technique. Further information about the changes in the electronic structure was elucidated using U M4- edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy, which directly probes f orbital states. The transition from the 3d to 5f sigma* orbital is sensitive to variations in the U=O-axial bond length and to changes in the bond covalency. We report evidence that the covalence in the uranyl fragment decreases upon dehydration. Photoluminescence spectroscopy at near-liquid helium temperatures reveals significant spectral differences between the two materials, correlating with the X-ray spectroscopy results. A theoretical investigation has been conducted on the structures of both studtite and metastudtite and benchmarked to the HR-XANES spectra. These illustrate the sensitivity of the 3d to 5f sigma* transition toward U=O-axial bond variation. Small structural changes upon dehydration have been shown to have an important electronic effect on the uranyl fragment.