Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

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Using density functional theory we investigate the occurrence of superconductivity in AC₆ with A=Mg,Ca,Sr,Ba. We predict that at zero pressure, Ba and Sr should be superconducting with critical temperatures (Tc) 0.2 K and 3.0 K, respectively. We study the pressure dependence of Tc assuming the same symmetry for the crystal structures at zero and finite pressures. We find that the SrC₆ and BaC₆ critical temperatures should be substantially enhanced by pressure. On the contrary, for CaC₆ we find that in the 0 to 5 GPa region, Tc weakly increases with pressure. The increase is much smaller than what shown in several recent experiments. Thus we suggest that in CaC₆ a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although MgC₆ is unstable, the synthesis of intercalated systems of the kind MgₓCa₁₋ₓCy could lead to higher critical temperatures.