Nanophase Separation in K_1–xCa_xC₈ Revealed by X-ray Fluorescence Holography and Extended X-ray Absorption Fine Structure

Even at low Ca concentrations, the binary-element intercalated graphite K 1– x Ca x C 8 exhibits high superconducting transition temperatures T c, closer to that of CaC 6 (11.5 K) than KC 8 (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– K α and Ca– K β holograms showed that K 0.7 Ca 0.3 C 8 did not take the solid-solution type random distribution of Ca and K atoms; instead, a nanoscale phase separation of CaC 6 and KC 8 was observed, which was also supported by the EXAFS results. While the lattice constant of K 0.7 Ca 0.3 C 8 was close to that of KC 8, 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 K 1– x Ca x C 8 was discussed with two scenarios of percolation of CaC 6 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.