Relativistic density-functional study of the solid solubility of transition metal/γ-uranium alloys: The role of d-d orbital interactions

Research highlights: → The highlights of the present paper demonstrate that the maximum solid solubility (MSS) of transition metal (TM) atoms in solid γ-U phase, which was determined experimentally, can be explained in terms of both the TM d orbital energy (Md) and the TMd-U6d orbital overlap populations (OOP), which were obtained using first-principle calculations based on relativistic density-functional theory. Interestingly, when the correlation map between Md and the OOP is made, γ-U/TM alloys with a lower MSS appear in the lower left of the OOP-Md map, whereas those with a higher MSS appear in the upper right. Thus, the map obtained using the OOP and Md is useful for designing not only γ-U/TM alloys but also other alloy systems. - Abstract: The alloying behavior of transition metals (TMs) in solid γ-phase uranium (γ-U), which is expected to be used as fuel for next-generation nuclear reactors, was investigated using the discrete-variational Dirac-Fock-Slater molecular orbital method. Using a model cluster, U8/TM, as the minimum unit of γ-U/TM alloys, we evaluated the d-orbital energy of the TM (Md), the bond order between the TM and U atoms, and the orbital overlap population (OOP) between the atomic orbitals of the TM and U atoms. We subsequently examined the effect of these quantities on the maximum solid solubility (MSS) of the γ-U/TM alloys. The interaction between the U-6d and TM-d atomic orbitals was found to play a key role in determining the MSS of the γ-U/TM alloys. The magnitude of the MSS can be explained in terms of the stabilization energy, which is affected by the Md and the OOP, formed by d-d orbital interactions. We also mapped the MSS of γ-U/TM alloys using the Md and the OOP as the alloying parameters. These results will assist the quantum design of nuclear fuel materials.