Phase equilibria in the U-Si system from first-principles calculations

Abstract

Density functional theory calculations have been used with spin-orbit coupling and onsite Coulomb correction (GGA+U) methods to investigate the U-Si system. Structural prediction methods were employed to identify alternate stable structures. Convex hulls of the U-Si system were constructed for each of the methods to highlight the competing energetics of various phases. For GGA calculations, new structures are predicted to be dynamically stable, but these have not been experimentally observed. When the GGA+U (U eff > 1.3 eV) method is considered, the experimentally observed structures are predicted to be energetically preferred. Phonon calculations were used to investigate the energy predictions and showed that the use of the GGA+U method removes the significant imaginary frequencies observed for U 3 Si 2 when the correction is not considered. Total and partial electron density of states calculations were also

Thermal and transport properties of U3Si2

“…The negative Hall coefficient together with positive Seebeck coefficient, indicate that a simple free electron model cannot be used to describe the electronic properties of this system. DFT calculations using spin-orbit coupling and an on-site coulomb correction [31] [32] indicate that besides uranium 5f-electrons there are also several overlapping states, in particular 6d and 3p at the Fermi level. Interplay between conventional and extraordinary components with differing electron-like or hole-like character could lead to anomalous values for RH [33].…”

Non-stoichiometry in U3Si2

“…practically, this small energy could be negated by entropic effects or small variations in the 164 energy associated with the methods [19]. Despite the small energy, as the reaction is predicted 165 to be favourable this phase will be considered to judge the potential deviation in stoichiometry 166 for U 3 Si 2 in the presence of U 5 Si 4 .…”

“…Recent theoretical work by Noordhoek et al [18] has been carried out to identify a reliable 68 method to simulate U-Si compounds. It was shown that the use of an on-site Coulombic 69 correction (GGA+U) method [19] was vital to predict the experimentally observed phases as 70 structurally and thermodynamically stable. A +U value of 1.5eV was suggested as appropriate 71…”

Transport properties in dilute UN(X) solid solutions (X=Xe,Kr)

“…The work on silicide [19][20][21][22] or carbide 23 is only starting but the work on oxide fuels has already received many contributions. It is outside the scope here to mention all of them, but Xe has been studied in U O 2 24 , other actinide oxide fuels 25 , and its diffusion following complex paths has also been investigated 26 , as well as the effect of nonstoichiometry and irradiation conditions on its diffusion properties 27 .However, krypton has never been considered in such works either for UN, except when swelling was the sole phenomenon being investigated 17 .…”