Metal intercalation to graphite produces various types of superconductors. The highest superconducting transition temperature Tc (onset temperature, Tc onset, of 11.5 K) was found in Ca intercalated graphite, denoted CaC6. Tc onset increased up to 15.1 K at 7.5 GPa, implying a positive pressure dependence. However, no new metal-intercalated graphite superconductors with Tc 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, CaxK1-xCy. Their structure resembles that of KC8. Tc increased continuously with increasing x. Furthermore, the pressure dependence of Tc in Ca0.6K0.4C8 was investigated over a wide pressure range from 0-43 GPa. Tc (= 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 Tc. 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.
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