Fabrication of new superconducting materials, CaxK1−xCy (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 CaC 6 . 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 x K 1−x C y . Their structure resembles that of KC 8 . T c increased continuously with increasing x. Furthermore, the pressure dependence of T c in Ca 0.6 K 0.4 C 8 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 CaC 6 , 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. [ABSTRACT FROM AUTHOR]