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0988-QQ07-05

Structure and Electronic Properties of Silicon and Germanium Network Polyhedra Katsumi Tanigaki, Takeshi Rachi, Ryotaro Kumashiro, Takuya Nishino, Kiyotsugu Narita, and Naoya Komatsu Department of Physics, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, 980-8578, Japan

ABSTRACT Clathrates of type I to type III are presented from the two points of views. The first one is intriguing electronic states appearing from the variation in phonons. The various phonons ranging from lattice phonons, intra-cluster phonons and atomic phonons are described in connection to the electronic states created via electron-phonon interactions. The other issue is the application to thermoelectric power materials on a basis of the concept of phonon-glass-electroncrystal (PGEC). INTRODUCTION When the special conditions are constrained to the network formation in IVth group of elements like silicon and germanium, nano materials having network polyhedra are produced. Although the sp3-hybridized bonding is favored in silicon and germanium, being different from carbon where both sp2- and sp3- hybridizations are realized like graphite and diamond, a new series of materials featured by the polyhedral cage frameworks with including sp2-characters have been searched in these days and lots of such materials, so called clathrates, have been synthesized to date [1]. In these nano materials, one of the important issues is phonons other than unique electronic states constructing from the polyhedral building blocks in these crystals. Because of a variety of frameworks formulating from polyhedra as well as atoms or molecules endohedrally accommodated in the polyhedral cages, phonons different from the conventional lattice phonons are produced and these can importantly be taken into consideration in order to understand the physical properties of these materials. For instance, intra-cluster phonons are believed to play an important role for giving rise to unique electronic states and endohedral atomic phonons with showing time- and space-dependent anharmonic oscillations are thought to give rise to unconventional interactions with conduction electrons near the Fermi surfaces. The latter atomic phonons have recently been drawing much attention in materials science and are called as rattling phonons. A few topics related to these issues will be presented in this proceeding. The first topic will be electronic properties realized in silicon and germanium clathrates. The second one will be thermoelectric power influenced by the atomic phonons inside the polyhedral frameworks. EXPERIMENT Ba8Ge43 and (Ba, Sr)8Ga16Ge30 were prepared in a home-made RF furnace where the materials in the feed can be handled under an anaerobic condition without any exposure to air. Ba8Si46 was synthesized from pure Ba and Si at 800 K under 3 GPa. Na16Ba8Si136 was prepared by the distillation of Na from a solid solution Zintle phase of (Na2Ba)Si4. The distillation of the sodium in a cylindrical Ta cell with one end open in a quartz tube with controlling temperature gav