New insights into the mechanical and thermal properties of UN_1-xC_x from first-principles calculations

Uranium nitride (UN) is considered to be a very promising candidate of accident-tolerant fuel. Since UN is commercially synthesized by carbothermic reduction-nitridation, the carbon in synthesized UN is unavoidable. Interestingly, UN_1-xC_x solid solutions obtained by isothermal conversion exhibit some more excellent properties, such as corrosion resistance. But the effect of composition on the mechanical and thermal properties is not clear yet. In this work, first-principles calculations were carried out to investigate the stabilities, mechanical and thermal properties of UN_1-xC_x (x = 0.00, 0.25, 0.50, 0.75 and 1.00). The impact of the on-site Coulomb repulsion term and spin-orbit coupling (SOC) on the properties of UN_1-xC_x were also discussed. The on-site Coulomb repulsion term is indispensable for the magnetic ground state, mechanical and thermal transport properties. Moreover, SOC has a negligible influence on the mechanical properties and can overestimate the thermal conductivities of UN_1-xC_x. Formation enthalpy, phonon dispersion and elastic constants calculations were preformed to prove the thermodynamic, dynamic and mechanical stability of UN_1-xC_x, respectively. The absence of a band gap in the density of states confirms the metallic nature of studied UN_1-xC_x. Theoretical analyses show that large-size carbon doping can induce lattice expansion and softening, resulting in the reduction of elastic moduli. The more localized density of states of UN_1-xC_x with carbon concentration demonstrate the weaker bond strength, which further verifies the variation of mechanical properties. The thermal conductivities of UN_1-xC_x calculated using PBE+U decrease firstly, then increase with the carbon concentration and increase with the temperature.