Structural, electronic, mechanical, and thermodynamic properties of UN2: Systematic density functional calculations

We have performed a comparative study of UN2 and β-U2N3using the generalized gradient approximation (GGA) and the GGA + U approaches based on the density functional theory (DFT). The lattice parameters obtained from the GGA + U calculations can be ameliorated appreciably, however, the density of states (DOS) is insensitive to Hubbard U for both compounds. Our Bader analysis shows that the effective charges increase with decreasing U:N ratio (from UN to UN2). The f electrons in UN2 and β-U2N3 seem less localized than that in UN phase. The effects of Hubbard U on mechanical properties is evident, while the phonon dispersion depends weakly on the changes of Hubbard U. Based on our phonon dispersion data, the lattice vibration energy, thermal expansion, and specific heat are obtained by utilizing the quasiharmonic approximation (QHA).