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 $5f-5f$ Coulomb interaction $U$ and spin-orbit coupling (DFT+$U$+SOC) were also performed for UC and UC$_2$. While the U $L_3$ HERFD-XAS spectra of the studied samples reveal little difference, the U $M_4$ HERFD-XAS spectra show certain sensitivity to the varying carbon content in uranium carbides. The observed gradual changes in the U $M_4$ 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$_2$ 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 indicate the importance of taking into account $U$ and SOC for the description of the electronic structure of actinide carbides.