Nature of empty states in superconducting CaC6 and related Li-Ca ternary graphite intercalation compounds using polarized x-ray absorption near-edge structure at the Ca K edge

The angular dependence of the x-ray absorption near-edge structure (XANES) was recorded at the calcium K edge in three graphite intercalated compounds: CaC6, Li3.1Ca2.1C6 (phase beta), and Li0.4Ca2.7C6 (phase 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 CaC6, 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 CaC6 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 CaC6. Structural and electronic information is extracted from the spectra of the alpha and beta phases, leading to substantial results, in absence of fully determined crystallographic structures. DOI: 10.1103/PhysRevB.87.075108