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Hydrogen-alkali-metal-graphite ternary intercalation compounds Toshiaki Enoki Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan

Seiichi Miyajima College of Humanities and Sciences, Nihon University, Sakurajosui, Setagaya-ku, Tokyo 156, Japan

Mizuka Sano Department of Chemistry, Kumamoto University, Kumamoto 860, Japan

Hiroo Inokuchi Institute for Molecular Science, Okazaki 444, Japan (Received 26 May 1989; accepted 1 September 1989) Alkali-metal-graphite intercalation compounds (alkali-metal-GIC's) absorb hydrogen in two ways: physisorption and chemisorption. Hydrogen uptake through the physisorption process occurs at low temperatures below about 200 K in higher stage alkali-mctal-GIC's, where hydrogen molecules are stabilized to form a two-dimensional condensed phase in the galleries of the graphite sheets. The concentration of absorbed hydrogen molecules is saturated at a rate of H2/alkali metal atom ~2. The hydrogen physisorption shows a strong isotope effect and a swelling effect on c-axis lattice expansion. In the case of hydrogen uptake through the chemisorption process, dissociated hydrogen species arc stabilized in the intercalate spaces. The activity of the chemisorption increases in the order Cs < Rb < K. The introduction of hydrogen generates a charge transfer from the host alkali metal-GIC's to the hydrogen since hydrogen has strong electron affinity. The hydrogenated potassium-GIC's have intercalates consisting of K + -H"-K + triple atomic layer sandwiches which are inserted between metallic graphite sheets. The inserted two-dimensional hydrogen layer is suggested to consist of H " ions with a weakly metallic nature. The superconductivity of the hydrogenated potassium-GIC is also discussed in terms of the change in the electronic and lattice dynamical properties by hydrogen uptake. The hydrogen-absorption in alkali-mctal-GIC's is an interesting phenomenon in comparison with that in transition metal hydrides from the point of hydrogen storage. The hydrogen-alkali-metal-ternary GlC's obtained from hydrogen absorption have novel electronic properties and lattice structures which provide attractive problems for GIC research. The studies of hydrogen-alkali-metal ternary GIC's are reviewed in this article. I. INTRODUCTION

Polycyclic condensed aromatic hydrocarbons shown in Fig. 1 form charge transfer complexes with alkali metals. The charge transfer complexes between graphite and alkali metals are called "alkali-metal-graphite intercalation compounds" (alkali-metal-GIC's), where graphite is an extreme of the polycyclic condensed aromatic hydrocarbons, consisting of an infinite network of benzene rings. In these GIC's, alkali-metal atoms are intercalated in the galleries between graphite sheets. The compound with alkali-metal atoms in every gallery is called a stage-1 GIC, while ones with alkali-metal atoms in every n gallery are named stage-ra GIC's. The approximate compositions of stage-1 and stage-n (n > 2) alkali-metal-GIC's are described as C8M and Ci2nM, w