Effect of carbon vacancies and oxygen impurities on the dynamical and thermal properties of uranium monocarbide

An influence of carbon vacancies and substitutional oxygen impurities on the phonon dynamics and thermal properties of uranium monocarbide is investigated within the density functional theory incorporating strong local Coulomb and spin-orbit interactions. Anharmonicity of lattice vibrations is taken into account via the quasi-harmonic theory. Results obtained for the modeled UC${}_{1-x}$ and UC${}_{1-x}$O_x compositions with $x=3$% show that the changes in atomic displacements and force constants induced by mono-vacancies and substitutional oxygen impurities in the carbon sublattice of UC remain restricted to the immediate neighborhood of the incorporated defects. The most sensitive to point defects in the carbon sublattice of UC are the optical phonon branches, while the phonon densities of states and the phonon-dependent thermodynamical properties remain affected to a lesser extent. The X-ray powder diffraction experiments have been performed to examine the temperature evolution of the lattice parameter of highly stoichiometric UC between 20 and 310 K. The present experimental studies supplement existing literature data by extending description of the uranium monocarbide structure below room temperature. They also support and validate our theoretical predictions about behavior of the investigated system at low temperatures.