Preparation and characterization of a new graphite superconductor: Ca 0.5 Sr 0.5 C 6

We have produced a superconducting binary-elements intercalated graphite, Ca x Sr 1−x C 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 x Sr 1−x C y sample was fabricated with the nominal x value of 0.8, i.e ., Ca 0.8 Sr 0.2 C 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 SrC 6 -type hexagonal-structure rather than CaC 6 -type rhombohedral-structure. Consequently, the chemical formula of Ca x Sr 1−x C y sample could be expressed as ‘Ca 0.5(2) Sr 0.5(2) C 6 ’. The XRD pattern of Ca 0.5(2) Sr 0.5(2) C 6 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 YbC 6 , SrC 6 , KC 8 , CaC 6 and Ca 0.6 K 0.4 C 8 . 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 x Sr 1−x C y , are fully investigated, and suitable combinations of elements are suggested for binary-elements intercalated graphite.