First Principle Study of Uranium Nitrides UN and UN2 Using DFT and DFT + U

Abstract

First principle calculation based on density functional theory (DFT) was used to evaluate some physical properties of Uranium Nitrides. Adsorption of oxygen O atom and O2 molecule on/in (001) surfaces of both Uranium monoNitride (UN) and diNitride (UN2) was then studied and compared mutually. To treat the strong correlation effects caused by 5f Uranium valence electrons, Hubbard-U advanced (DFT + U) approach was employed to correct the exchange correlation functional GGA and PBE which are based on generalized gradient approximation. The functional are developed for the Vienna Abinitio Simulation Package (VASP) and were used with the projector-augmented wave (PAW) pseudo potentials. The structural and elastic-mechanical UN and UN2 properties were calculated within DFT and DFT + U methods. Then, Potential Energy Surfaces (PES) concepts which correspond to the interaction between O atom (respectively O2 molecule) and (001) on-surfaces / sub-surfaces uranium nitrides for several positions were determined to identify favorable adsorption sites. Physical properties calculation results of UN or UN2 are in order of magnitude of other theoretical values and show an acceptable precision compared to experiments. Hubbard U value of the DFT + U formalism was optimized to achieve Antiferromagnetic (AFM) UN configuration and was effective at U  1.625 eV. Optimization of UN2 was accomplished to attain experimental cell parameter of 5.31 A° and was reached for U  2.6 eV. According to our calculation results, O2 diffusion through UN(001) and UN2(001) clean surfaces have demonstrated dissociation of the molecule from a distance of approximately d  1.5 Å. Favored on surface modes for O atom adsorption were found to be near the bridge site for UN(001) and UN2(001). The O incorporation through UN(001) surface was at the bridge site, nevertheless, for UN2, merging of O atom in the (001) surface bridge site was not allowed.