Computational Investigation of the Formation of Hyperstoichiometric Uranium Dioxide (UO 2+x )
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Computational Investigation of the Formation of Hyperstoichiometric Uranium Dioxide (UO2+x) Frances Skomurski, Udo Becker, and Rodney Ewing Geological Sciences, University of Michigan, 2534 C.C. Little Building, 1100 North University Ave., Ann Arbor, MI, 48109
ABSTRACT Understanding the mechanisms behind the formation of hyperstoichiometric UO2 phases is important because oxidation of uranium atoms upon the addition of excess oxygen to the UO2 structure leads to volume changes that increase the susceptibility of spent fuel to corrosion. While a variety of diffraction and spectroscopic studies have been used to investigate structural changes as UO2 oxidizes to U4O9, the effect of interstitial oxygen on the charge distribution of uranium in hyperstoichiometric UO2 remains inconclusive. In this study, quantum mechanical techniques were used to model the effects of interstitial oxygen on the structure and charge distribution of atoms in a simplified U4O9 unit cell. A density functional theory-based approach was used to optimize the geometry and charge distribution of a variety of U4O9 starting models with different U4+, U5+ and U6+ charge configurations. Results from our calculations suggest that the formation of one U5+ per addition of interstitial oxygen at a perpendicular bisector site is favorable; this oxidation event is accompanied by partial reduction of the interstitial oxygen atom. Deflection of two lattice oxygen atoms along the body diagonal of the cubic site surrounding the U5+ is also observed upon the addition of one interstitial oxygen atom. Structural and bond length data are compared with experimental data whenever possible. INTRODUCTION The oxidation of uranium dioxide (UO2) is accompanied by an increase in the ratio of U5+ or U6+ relative to U4+, which leads to volume changes of the unit cell. The cubic fluorite structure of UO2 can accommodate excess oxygen atoms to form hyperstoichiometric phases such as U4O9. Beyond U4O9, however, the fluorite structure is no longer stable, and tetragonal and orthorhombic phases are observed for U3O7 and U3O8, respectively [1]. While neutron, x-ray, and electron diffraction techniques have been used to determine the location of interstitial atoms and refine the structures of uranium oxides [2], uncertainty still surrounds the charge distribution of U atoms in higher oxide phases. Recently, a synchrotron-based study using EXAFS was used to determine the local bonding environment for U in U4O9, and the shortest O-U bonds measured were 1.7 Å, indicative of U6+ [3]. This study was followed by a neutron diffraction study on U4O9 where no O-U bond-lengths shorter than 2.2 Å were observed [4], indicating that uranyl molecules are not present in the U4O9 structure. While U5+ and U6+ have distinct bonding environments in naturally occurring minerals such as wyartite [5], experimental studies on synthetic uranium oxides show that U6+ phases such as CaUO4 can exhibit U-O bond-lengths up to 2.21 Å, making bond-length alone difficult for determining the oxidation
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