Abstract
We report on an advanced density-functional theory (DFT) approach for investigating the ground-state and thermodynamical properties of uranium mononitride (UN). The electronic structure for UN at zero temperature is obtained from DFT that utilizes the generalized gradient approximation (GGA) for the electron exchange and correlation functional and includes spin-orbit interaction and an extension with orbital polarization. Thermodynamical properties are computed within the quasi-harmonic approximation in the Debye–Grüneisen model while anharmonicity is captured in the self-consistent ab initio lattice dynamics (SCAILD) scheme. Anharmonic phonons have heretofore never been modeled from first-principles for UN but they turn out to be important. The computed free energy compares well with that of a CALPHAD (CALculation of PHAse Diagrams) assessment of available experimental data.
Highlights
Uranium mononitride (UN) is an actinide compound that forms in the cubic sodium-chloride structure (B1), similar to other actinide nitrides and carbides
Density-functional theory is in principle correct, but it depends on practical simplifications
The formation enthalpy can be modeled within the density-functional theory (DFT) approach because the energy of the constituents of UN as well as the compound itself can be calculated
Summary
Uranium mononitride (UN) is an actinide compound that forms in the cubic sodium-chloride structure (B1), similar to other actinide nitrides and carbides. The location of the uranium and nitrogen atoms are interchangeable in this phase that is a face-centered cubic, with one atom in the origin and the other in the cube center. The B1 actinide-nitride and actinide-carbon systems are characterized by high melting temperatures and substantial electrical and thermal conductivities. In the case of UN, the U-U distance is greater (~3.5 Å) than that of α-uranium (~2.8 Å) and a magnetic moment forms on the uranium atom in UN but not in α-uranium. Uranium mononitride has received a lot of attention from both experimental [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] and computational angles [30,31,32,33,34,35,36,37,38,39,40,41,42,43] due to its potential as a nuclear fuel for fast-breeder reactors [44]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call