Effect of carbon content on electronic structure of uranium carbides

The electronic structure of UC$_x$ (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$_4$ and L$_3$ edges. The full-relativistic density functional theory calculations taking into account the Coulomb interaction U and spin-orbit coupling (DFT+U+SOC) were also performed for UC and UC$_2$. While the U M$_4$HERFD-XAS spectra of the studied samples reveal little difference, the U HERFD-XAS spectra show certain sensitivity to the varying carbon content in uranium carbides. The observed gradual changes in the U 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 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$_2$, 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.