Effect of carbon content on electronic structure of uranium carbides

Abstract The electronic structure of UC $$_x$$ x (x = 0.9, 1.0, 1.1, 2.0) was studied by means of x-ray absorption spectroscopy (XAS) at the C K edge and measurements in the high energy resolution fluorescence detection (HERFD) mode at the U $$M_4$$ M 4 and $$L_3$$ L 3 edges. The full-relativistic density functional theory calculations taking into account the $$5f-5f$$ 5 f - 5 f Coulomb interaction U and spin-orbit coupling (DFT+U+SOC) were also performed for UC and UC $$_2$$ 2 . While the U $$L_3$$ L 3 HERFD-XAS spectra of the studied samples reveal little difference, the U $$M_4$$ M 4 HERFD-XAS spectra show certain sensitivity to the varying carbon content in uranium carbides. The observed gradual changes in the U $$M_4$$ M 4 HERFD spectra suggest an increase in the C 2p-U 5f charge transfer, which is supported by the orbital population analysis in the DFT+U+SOC calculations, indicating an increase in the U 5f occupancy in UC $$_2$$ 2 as compared to that in UC. On the other hand, the density of states at the Fermi level were found to be significantly lower in UC $$_2$$ 2 , thus affecting the thermodynamic properties. Both the x-ray spectroscopic data (in particular, the C K XAS measurements) and results of the DFT+U+SOC calculations indicate the importance of taking into account U and SOC for the description of the electronic structure of actinide carbides.