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

TL;DR

This study systematically investigates the structural, electronic, and thermodynamic properties of uranium nitride (UN) using advanced density functional theory methods, highlighting the importance of the Hubbard U parameter for accurate modeling.

Contribution

It introduces a systematic approach to optimize the U parameter in DFT+U calculations for UN, improving the accuracy of predicted properties and providing detailed thermodynamic insights.

Findings

GGA+U with U around 2 eV accurately describes UN properties.

UN exhibits stronger 5f electron localization than UC.

Calculated specific heat matches experimental data.

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)+\emph{U} and the generalized gradient approximation (GGA)+\emph{U} formalisms. To properly describe the strong correlation in the U $5f$ electrons, we optimized the \emph{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 \emph{U} around 2 eV within the GGA+\emph{U} approach, it is promising to correctly and consistently describe the above mentioned properties of UN. The localization behavior of 5$f$ 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.