Fabrication of new superconducting materials, Ca_xK_1-xC_y (0 < x < 1)

Metal intercalation to graphite produces various types of superconductors. The highest superconducting transition temperature T_c (onset temperature, T_c^onset, of 11.5 K) was found in Ca intercalated graphite, denoted CaC₆. T_c^onset increased up to 15.1 K at 7.5 GPa, implying a positive pressure dependence. However, no new metal-intercalated graphite superconductors with T_c^onset higher than 11.5 K at ambient pressure have so far been reported. To search for new graphite superconductors, we successfully synthesized binary-element-intercalated graphite, Ca_xK_1-xC_y. Their structure resembles that of KC₈. T_c increased continuously with increasing x. Furthermore, the pressure dependence of T_c in Ca_0.6K_0.4C₈ was investigated over a wide pressure range from 0-43 GPa. T_c (= 9.6 K at 0 GPa) increased to 11.6 K at 3.3 GPa, and decreased to 2.0 K at 41 GPa. This behavior is similar to that of CaC₆, albeit with a lower maximum T_c. X-ray diffraction patterns were measured under high pressures of 0-24 GPa, and suggest a structural transition at 15 GPa. Evidence is given for superconducting graphite involving binary metal intercalation.