Synthesis and superconducting properties of CaC6

Among the superconducting graphite intercalation compounds, CaC6 exhibits the highest critical temperature T-c = 11.5 K. Bulk samples of CaC6 are obtained by immersing highly oriented pyrographite pieces in a well-chosen liquid Li-Ca alloy for 10 days at 350 degrees C. The crystal structure of CaC6 belongs to the R (3) over barm space group. In order to study the superconducting properties of CaC6, magnetisation was measured as a function of temperature and direction of magnetic field applied parallel or perpendicular to the c-axis. Meissner effect was evidenced, as well as a type II superconducting behaviour and a small anisotropy. In agreement with calculations, experimental results obtained from various techniques suggest that a classical electron-phonon mechanism is responsible for the superconductivity of CaC6. Application of high pressure increases the T-c up to 15.1 K at 8 GPa.