Structural, electronic, and thermodynamic properties of UN: Systematic density functional calculations

Abstract

A systematic first-principle study is performed to calculate the lattice parameters, electronic structure, and thermodynamic properties of UN using the local-density approximation (LDA)+U and the generalized gradient approximation (GGA)+U formalisms. To properly describe the strong correlation in the U 5f electrons, we optimized the U parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) configurations. Our results show that by choosing the Hubbard U around 2 eV within the GGA+U approach, it is promising to correctly and consistently describe the above mentioned properties of UN. The localization behavior of 5f electrons is found to be stronger than that of UC and our electronic analysis indicates that the effective charge of UN can be represented as U 1.71+ N 1.71− . As for the thermodynamic study, the phonon dispersion illustrates the stability of UN and we further predict the lattice vibration energy, thermal expansion, and specific heat by utilizing the quasiharmonic approximation. Our calculated specific heat is well consistent with experiments.

Quantum oscillations of nitrogen atoms in uranium nitride

“…2a are obtained from a fit of the data to a simplified model 16 that includes only nearest-neighbour U-N (65 ± 2 N m − 1 ) and U-U (50 ± 4 N m − 1 ) force constants. This model suffices for illustrative purposes, although more detailed models 2,[8][9][10]21,22 give a closer match to the observed dispersion. Some of those models also include N-N force constants, although they are found to be negligibly small compared with their U-N and U-U counterparts.…”

“…They have been extensively studied owing to their vast array of puzzling physical and magnetic properties 7 , including unusually high electronic-specific heats and drastic suppression of ordered magnetic moments. Among these systems, UN has received significant attention [8][9][10][11][12] recently owing to its potential use as a high-temperature nuclear fuel [13][14][15] .…”

Adsorption and diffusion of oxygen atom on UN2(100) surface and subsurface: a density functional theory study (DFT and DFT + U)

“…The calculated lattice constant of UN 2 is 5.276 Å (experimental value 5.31 [16]) and the bulk modulus calculated by using the third-order Birch-Murnaghan equation of states (EOS) [17] is 250 GPA in the order of other theoretical values (252 [15], from 235.8 to 264.6 Gpa [18]). Nevertheless, the lattice constant was somewhat smaller than the experimental value.…”

“…The bulk UN 2 , UN 2 (100) surface and oxygen Bulk calculation was performed without spin polarization consideration, since free UN 2 is a nonmagnetic (NM) phase through PBE52 calculation [15]. The calculated lattice constant of UN 2 is 5.276 Å (experimental value 5.31 [16]) and the bulk modulus calculated by using the third-order Birch-Murnaghan equation of states (EOS) [17] is 250 GPA in the order of other theoretical values (252 [15], from 235.8 to 264.6 Gpa [18]).…”

First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels

“…The lattice parameters, electronic structure, as well as the thermodynamic properties of UN using LDA+U and GGA+U semi-empirical schemes and plane wave (PW) approach were presented in [32]. The total energy dependences on Hubbard U-parameter for UN bulk in FM and AFM states obtained in those calculations show that the FM state is preferable for the range of U-parameter between 0 and 2 eV while the AFM state could be favorable for Uparameter larger than 2 eV.…”