First-principles study of the magnetic properties of NpN, PuN and AmN

The electronic structure, magnetic, dynamical and thermodynamic properties of the actinide nitrides, NpN, PuN, and AmN are explored using ab initio pseudopotential and density functional perturbation theories. The results show that PuN exhibits half-metallic properties, while AmN and NpN are found to be metallic; the calculated total magnetic moments are 3.11μB for NpN, 4.85μB for PuN and 6.00μB for AmN. The electronic structure calculation indicates, for the minority-spin, that the states close to Fermi-level are mainly due to the N-p states with a small admixture of Np, Pu and Am d and f states, while for the majority spin, the states close to Fermi-level exhibit mainly N-p character with a strong admixture of huge Np, Pu and Am f states, and a small admixture of Np, Pu and Am d orbitals. Moreover, the phonon band structure of NpN, PuN and AmN shows that the longitudinal and transverse acoustic phonon modes have flatness along a large part of the Brillouin zone, which gives–consequently–two sharp peaks for the acoustic modes. Finally we used quasiharmonic approximation to determine the temperature dependence of mean-squared displacements, entropy, and heat capacity. We found that for high temperatures, N atoms show higher vibrational amplitudes as compared to Am, Pu and Np atoms. In addition, we found that the entropy and heat capacity originate principally from actinides (Am, Pu and Np)-acoustic modes and from N-optical modes.