Nature of empty states in superconducting CaC6and related Li-Ca ternary graphite intercalation compounds using polarized x-ray absorption near-edge structure at the CaKedge

Abstract

The angular dependence of the x-ray absorption near-edge structure (XANES) was recorded at the calcium $K$ edge in three graphite intercalated compounds: CaC${}_{6}$, Li${}_{3.1}$Ca${}_{2.1}$C${}_{6}$ (phase $\ensuremath{\beta}$), and Li${}_{0.4}$Ca${}_{2.7}$C${}_{6}$ (phase $\ensuremath{\alpha}$). The polarized XANES spectra provide experimental pictures of the Ca ${p}_{x,y}$ and ${p}_{z}$ empty states of these three materials. In the case of CaC${}_{6}$, first-principles calculations based on density functional theory were performed, including XANES modeling and projected density-of-state simulations. A good agreement is obtained between theoretical and experimental polarized spectra. This enables us to assign the main XANES peaks in terms of orbital hybridization. The thorough analysis of the CaC${}_{6}$ polarized XANES spectra is used to interpret the spectral differences observed between the three compounds. The absence of pre-edge for all these phases indicates that Ca atoms are in locally centrosymmetric sites. The presence of Li in the graphitic interlayer drastically reduces the anisotropy of the Ca $p$ empty states, as compared to CaC${}_{6}$. Structural and electronic information is extracted from the spectra of the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases, leading to substantial results, in absence of fully determined crystallographic structures.