First-principles study of UC₂ and U₂C₃

The electronic structure and mechanical properties of UC₂ and U₂C₃ have been systematically investigated using first-principles calculations by the projector-augmented-wave (PAW) method. Furthermore, in order to describe precisely the strong on-site Coulomb repulsion among the localized U 5f electrons, we adopt the generalized gradient approximation +U formalisms for the exchange-correlation term. We show that our calculated structural parameters and electronic properties for UC₂ and U₂C₃ are in good agreement with the experimental data by choosing an appropriate Hubbard U = 3 eV. As for the chemical bonding nature, the contour plot of charge density and total density of states suggest that UC₂ and U₂C₃ are metallic mainly contributed by the 5f electrons, mixed with significant covalent component resulted from the strong C{single bond}C bonds. The present results also illustrate that the metal-carbon (U{single bond}C) bonding and the carbon-carbon covalent bonding in U₂C₃ are somewhat weaker than those in UC₂, leading to the weaker thermodynamic stability at high temperature as observed by experiments.