Dehydration of the Uranyl Peroxide Studtite, [UO 2 (η 2 -O 2 )(H 2 O) 2 ]·2H 2 O, Affords a Drastic Change in the Electronic Structure: A Combined X-ray Spectroscopic and Theoretical Analysis

• Published in: Inorganic chemistry Vol. 57; no. 4; pp. 1735 - 1743
• Main Authors: Vitova, Tonya, Pidchenko, Ivan, Biswas, Saptarshi, Beridze, George, Dunne, Peter W, Schild, Dieter, Wang, Zheming, Kowalski, Piotr M, Baker, Robert J
• Format: Journal Article
• Language: English
• Published: United States 19-02-2018
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.7b02326

The minerals studtite, [UO
(η
-O
)(H
O)
]·2H
O, and metastudtite, [UO
(η
-O
)(H
O)
], are uranyl peroxide minerals that are major oxidative alteration phases of UO
under conditions of geological storage. The dehydration of studtite has been studied using X-ray photoelectron spectroscopy (XPS) and X-ray 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 M
-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 5fσ* orbital is sensitive to variations in the U═O
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 σ* transition toward U═O
bond variation. Small structural changes upon dehydration have been shown to have an important electronic effect on the uranyl fragment.