Nanophase Separation in K1–x Ca x C8 Revealed by X‑ray Fluorescence Holography and Extended X‑ray Absorption Fine Structure

Even at low Ca concentrations, the binary-element intercalated graphite K1–x Ca x C8 exhibits high superconducting transition temperatures T c, closer to that of CaC6 (11.5 K) than KC8 (0.169 K). To investigate the behaviors of K and Ca within the graphite matrix, their local structures have been investigated by using X-ray fluorescence holography and extended X-ray absorption fine structure (EXAFS) spectroscopy. The atomic images reconstructed from the K–Kα and Ca–Kβ holograms showed that K0.7Ca0.3C8 did not take the solid-solution type random distribution of Ca and K atoms; instead, a nanoscale phase separation of CaC6 and KC8 was observed, which was also supported by the EXAFS results. While the lattice constant of K0.7Ca0.3C8 was close to that of KC8, we found a nanoscale Ca layer dispersed within the sample. The Ca nanolayer was offset from the center between the C sublayers. The superconducting behavior found in K1–x Ca x C8 was discussed with two scenarios of percolation of CaC6 and deformation of graphene based on such a specific inhomogeneous binary element monatomic layers. This study represents an important step for understanding the superconducting properties in a nanoscale phase-separated system.