Effect of hydrogenation on structure and superconducting properties of CaC6

Abstract

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.