Enhancement of superconductivity and evidence of structural instability in intercalated graphite CaC6 under high pressure

We measured the temperature dependent resistivity, rho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, T-c, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximate to 8 GPa, a jump of rho and a sudden drop of T-c down to approximate to 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the T-c increase and that higher pressures greater than or similar to 8 GPa lead to a structural transition into a new phase with a low T-c less than or similar to 3 K.