Calcium Graphite
Bulk samples of calcium graphite CaC6 are obtained by immersing highly oriented pyrolytic graphite in a well-chosen liquid Li–Ca alloy for long time (10 days at 350 °C). The crystal structure of CaC6 belongs to the R-3m space group. The graphite interlayer distance increases upon Ca intercalation from 3.35 to 4.524 Å, and the carbon-carbon distance increases from 1.42 to 1.444 Å. Among the superconducting graphite intercalation compounds, CaC6 exhibits the highest critical temperature Tc = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa). Superconductivity in these compounds is thought to be intimately related to the role of an interlayer state, a free electron like band lying roughly 2 eV (0.32 aJ) above the Fermi level; superconductivity only occurs if the interlayer state is occupied.
Read more about this topic: Graphite Intercalation Compound