We have obtained bulk samples of the graphite intercalation compound, ${\mathrm{CaC}}_6$, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the ${\mathrm{MC}}_6$, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.

• Received 3 June 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.087003