Silicon and Germanium Network Polyhedra viewed from Soft X-ray Spectroscopy
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1044-U05-02
Silicon and Germanium Network Polyhedra viewed from Soft X-ray Spectroscopy Jun Tang1, Takeshi Rachi1, Marcos A Avila2, Toshiro Takabatake2, FagZhum Guo3, Keisuke Kobayashi3, Koji Akai4, and Katsumi Tanigaki1 1 Graduate of Science, Department of Physics, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, 980-8578, Japan 2 Dep. Quantum Matter, Hiroshina University, Kagamiyama 1-3-1, Higashi-Hiroshima, Japan 3 SPring-8, 1-1-1 Mikazuki-machi, Sayo-gun, Hyogo, Japan 4 Computer Media Center, Yamaguchi University, 2-16-1 Tokiwadai, Ube, 755-8611, Japan ABSTRACT The electronic states of Ba8Ga16Ge30 and Sr8Ga16Ge30 are studied by soft x-ray photoelectron spectroscopy (XPS) at a high-energy facility. In Ba8Ga16Ge30, three bands are resolved in the valence band region. Resonance experiments together with theoretical calculations show that the three band structures in the valence band are mainly constructed by the Ge/Ga 4s and 4p wave functions with little contributions of Ba 5s, 5p and 5d. The valence band of Sr8Ga16Ge30 shows a similar feature to that of Ba8Ga16Ge30. It is clearly shown that the atomic orbitals of Ba and Sr make little hybridization with the orbitals made by the framework polyhedra. This provides the understanding of the thermoelectricity in clathrates, and assists the design in high performance thermoelectric materials in this family. INTRODUCTION Recently, thermoelectric materials have attracted intense research interests because the conversion from thermal to electric energy is very promising to solve the energy resource problems in this century. For this purpose, lots of materials have been searched to dates, such as degenerate semiconductors, half heusler alloys, heavy fermion compounds, cobalt oxides and so forth [1–7]. A new concept of ”phonon glass, electron crystal” (PGEC) was introduced by Slack in 1995 [8], and this is expected to become one of the most useful design methods in power electric materials. In the PGEC concept, heat transfer is thought to be suppressed by rattling phonons, even though electric transport holds to be high due to the periodic regulations in the Bloch lattice, which results to give rise to high figure of merits ZT=TS2σ/κ·, where S denotes the Seebeck coefficient, σ is the electric conductivity, and κ is the thermal conductivity. Host-guest type materials such as, clathrates, skutterdites and pyrochlores have been studied as the very perspective candidates along this concept. Such demonstrations have recently been reported for type I clathrates M8Ga16Ge30 (M=Sr,Ba,Eu) and promising applications are proposed due to the their observed high conversion coefficiency. Although their structural and thermoelectric properties have widely been studied [1-8], energetics of the endohedral atomic elements as well as the guest/host interactions between the endohedral atoms and the outer surrounding polyhedral frameworks have not yet understood sufficiently. For having the real scientific insights to the PGEC concept, the electronic states of the atomic elements confined in
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