Effect of hydrogenation on structure and superconducting properties of CaC6

We report the effects of hydrogenation on the structure and superconducting properties of good quality bulk superconducting CaC6 samples prepared from Madagascar graphite single crystals using a Ca-metal vapour transport method. The hydrogenation of a CaC6 sample takes several hours during which, at different hydrogen exposure times, the structural and superconducting studies were carried out using X-ray diffraction, Raman scattering, and temperature and field dependence of magnetization measurements. These studies reveal that only a mixture of CaH2 and graphite is obtained upon hydrogenation of CaC6. The successive suppression of superconducting volume fraction and critical temperature is observed when increasing the hydrogenation time. The results reveal that there is a definite relation between structure and superconductivity in the hydrogenated CaC6 samples. The changes in stoichiometry and inevitable Ca-sub-lattice disorder during the deintercalation process are considered to be the main sources for the change in the superconducting transition temperature.