Hydrogen Absorption in Graphite-Alkali-Metal Intercalation Compounds

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Alkali metal graphite intercalation compounds C_nM (M=K,Rb,Cs; n=8,24) are metallic compounds, which have a structure with an intercalated alkali metal layer between graphite layers. It has been known that these intercalation compounds have activities for hydrogen chemisorption and physisorption, leading to hydrogen occlusion between graphite layers. In the case of hydrogen chemisorption, dissociated hydrogen atoms are stabilized in the gallery of graphite layers, and the hydrogen-absorbed system resembles transition metal hydries such as PdH_x, while, for the physisorption, hydrogen molecules form two-dimensional condensed phase between graphite layers, where the system behaves as molecular sieves such as zeolites. In this study, mainly for the hydrogen-chemisorbed system, we have investigated the electronic structure, crystal structure and dynamical and static behaviors of absorbed hydrogen species, by means of the investigation of solid state properties. From the results of our study, we have clarified novel elevtronic and lattice structures of the graphite-alkali metal-hydrogen ternary intercalation compounds. Namely, transition metal hydrides such as PdH_x are regarded to be alloy systems between metal and hydrogen atoms, where hydrogen possesses delocalized electronic structure, while, alkali metal hydrides such as K^+H^- are ionic insulators with localized electronic structures. The hydrogen species stabilized in the alkali metal graphite intercalation compounds with metallic nature have novel electronic structure between the electronic structures of the above two hydrides, under relatively weak effects of conduction electrons. In this study, we present the results of electrical conductivity, thermoelectric power, proton-NMR, ESR and c-axis thermal expansion coefficients of the potassium-hydrogen-graphite ternary intercalation compounds.