Hydrogen Absorption in Graphite-Alkali-Metal Intercalation Compounds

碱金属石墨层间化合物C_nM(M=K，Rb，Cs；n=8，24)是一种在石墨层间具有碱金属插层结构的金属化合物。众所周知，这些插层化合物具有化学吸附和物理吸附氢气的活性，导致氢气堵塞在石墨层之间。对于氢的化学吸附，解离的氢原子稳定在石墨层的廊道中，吸氢体系类似于PdH_x等过渡金属氢化物，而对于物理吸附，氢分子在石墨层之间形成二维凝聚相，在那里体系表现为分子筛，如沸石。在本研究中，我们主要针对氢-化学吸附体系，通过固态性质的研究，研究了吸附的氢物种的电子结构、晶体结构以及动态和静态行为。从我们的研究结果中，我们阐明了石墨-碱金属-氢三元插层化合物的新颖的电子结构和晶格结构。也就是说，过渡金属氢化物如PdHx被认为是金属和氢原子之间的合金体系，其中氢具有离域电子结构，而碱金属氢化物如K^+H^-是具有局域电子结构的离子绝缘体。金属性质的碱金属石墨层间化合物中稳定的氢物种在较弱的传导电子作用下，在上述两种氢化物的电子结构之间具有新的电子结构。在本研究中，我们给出了钾-氢-石墨三元插层化合物的电导率、热电动势、质子核磁共振、ESR和c轴热膨胀系数的结果。

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.