Preparation and characterization of a new graphite superconductor: Ca_0.5Sr_0.5C₆

We have produced a superconducting binary-elements intercalated graphite, Ca_xSr_1-xC_y, with the intercalation of Sr and Ca in highly-oriented pyrolytic graphite; the superconducting transition temperature, T _c, was ~3 K. The superconducting Ca_xSr_1-xC_y sample was fabricated with the nominal x value of 0.8, i.e., Ca_0.8Sr_0.2C_y. Energy dispersive X-ray (EDX) spectroscopy provided the stoichiometry of Ca_0.5(2)Sr_0.5(2)C_y for this sample, and the X-ray powder diffraction (XRD) pattern showed that Ca_0.5(2)Sr_0.5(2)C_y took the SrC₆-type hexagonal-structure rather than CaC₆-type rhombohedral-structure. Consequently, the chemical formula of Ca_xSr_1-xC_y sample could be expressed as 'Ca_0.5(2)Sr_0.5(2)C₆'. The XRD pattern of Ca_0.5(2)Sr_0.5(2)C₆ was measured at 0-31 GPa, showing that the lattice shrank monotonically with increasing pressure up to 8.6 GPa, with the structural phase transition occurring above 8.6 GPa. The pressure dependence of T _c was determined from the DC magnetic susceptibility and resistance up to 15 GPa, which exhibited a positive pressure dependence of T _c up to 8.3 GPa, as in YbC₆, SrC₆, KC₈, CaC₆ and Ca_0.6K_0.4C₈. The further application of pressure caused the rapid decrease of T _c. In this study, the fabrication and superconducting properties of new binary-elements intercalated graphite, Ca_xSr_1-xC_y, are fully investigated, and suitable combinations of elements are suggested for binary-elements intercalated graphite.