An EXAFS and HR-XANES study of the uranyl peroxides [UO2(η2-O2)(H2O)2]·nH2O (n = 0, 2) and uranyl (oxy)hydroxide [(UO2)4O(OH)6]·6H2O

Abstract

The solid-sate structures of the two uranyl peroxides studtite, [UO₂(η²-O₂)(H₂O)₂]·2H₂O, and metastudtite [UO₂(η²-O₂)(H₂O)₂] have been determined by U–L₃ edge extended X-ray absorption fine structure (EXAFS) spectroscopy and show that upon removal of the interstitial water in studtite there are structural changes with a small shortening of the U–O_peroxo and small lengthening of the U–O_yl bonds. High-energy resolution X-Ray absorption near edge structure (HR-XANES) spectroscopy has been used to probe the differences in the local electronic structure and, supported by ab initio FEFF9.5.1 calculations, dehydration causes a shift to higher energies of the occupied O p-DOS and U d- and f-DOS of metastudtite. The HR-XANES spectrum of schoepite, [(UO₂)₄O(OH)₆]·6H₂O, has been measured as the White Line intensity can give information on the mixing of metal and ligand atomic orbitals. There is an indication for higher degree of ionicity for the U–OH bond in schoepite compared to the U–O₂ bond in studtite.