GGA + U studies of the early actinide mononitrides and dinitrides

Abstract

We present a detailed comparative study of the electronic and mechanical properties of the early actinide mononitrides and dinitrides within the framework of the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA [PBE]) and GGA+U implementations of density functional theory with the inclusion of spin–orbit coupling. The dependence of selected observables of these materials on the effective U-parameter is investigated in detail. The properties include the lattice constant, bulk modulus, charge density distribution, hybridization of the atomic orbitals, energy of formation and the lattice dynamics. The inclusion of the Hubbard U parameter results in a proper description of the 5f electrons, and is subsequently used in the determination of the structural and electronic properties of these compounds. The mononitrides and dinitrides of the early actinides are metallic except for UN2, which is a semiconductor. These actinide nitrides are non-magnetic with the exception of UN, NpN, PuN, NpN2 and PuN2 that are magnetic systems with orbital-dependent magnetic moments oriented in the z-axis. We observed that ThN2 is elastically unstable to isotropic pressure. We discovered that UN2 is thermodynamically unstable, but may be stabilized by N vacancy formation.