Computer simulation of defect clusters in UO2 and their dependence on composition

Density functional theory investigation of the layered uranium oxides U3O8 and U2O5

“…The bending is slightly different for each of the four distinct uranium sites and the direction alternates with each layer, maintaining a stacking direction (on average) orthogonal to the layers. However the bonds deviate far more from straight than any of the U 3 On relaxation, the coordination at uranium sites remains the same and the symmetry is retained. The bent axial bond chains have straightened out slightly, such that the range of angles is from 179.7 to 175.0°.…”

“…The oxides under study are not as well characterised as the Mott-Hubbard insulating UO 2 or the 5f 0 charge-transfer insulating UO 3 . No electronic data could be found for δ-U 2 O 5 , which is lower than both UO 2 (2.14 eV 54 ) and α-UO 3 (2.63 eV 31 ).…”

“…7 Hexagonal and monoclinic symmetry have been debated for γ-U 3 O 8 with the most recent reports suggesting a hexagonal structure. 8 Uranium charge configuration in U 3 O 8 has also been the subject of much investigation with older reports suggesting a U 4+ /U 6+ arrangement 29,35 and more recent experiments finding U 5+ /U 6+ . 28,36 Computer simulation studies using force fields with an averaged uranium charge of 5 1/3+ gave the best structural fit to experiment; 11 a result corroborated by pure DFT calculations exemplified by Geng et al 32 However more recently DFT + U calculations suggest a preferred configuration of U 5+ /U 6+ .…”

“…1 The oxidation of fluorite UO 2 occurs by incorporating oxygen interstitials, giving rise to the fluorite based U 4 O 9 (UO 2.25 ) and U 3 O 7 (UO 2.33 ) phases. 2,3 Following this there is a transition to layered type structures, where the fluorite type lattice has reached oxygen saturation point, marked by meta-stable U 2 O 5 (UO 2.5 ) which can exist in both fluorite based form (α-, β-and γ-polymorphs) and layered type (δ-U 2 O 5 ). 4,5 The layered oxides are defined by their uranium-oxygen planes linked by short, collinear UO bonds resembling those of the UO 2 2+ (uranyl) ion, although the bonds are typically longer (2.10 Å).…”

Effects of Zr doping on the surface energy and surface structure of UO2: Atomistic simulations

“…This method has been http://dx.doi.org/10.1016/j.apsusc.2015.05.168 0169-4332/© 2015 Elsevier B.V. All rights reserved. widely used to study the point defects formation energies and the stability of defect clusters in UO 2 fuel [26][27][28][29]. Sometimes scholars also applied this method to investigate the surface energy of cubic UO 2 and the absorption of molecules on the surface of UO 2 and other oxide nuclear fuels [30][31][32][33][34].…”