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0980-II04-05

Thermoelectric Properties of Ba-Ge Based Type-III Clathrate Compounds Jung-Hwan Kim, Norihiko L. Okamoto, Kyosuke Kishida, Katsushi Tanaka, and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan ABSTRACT The crystal structures and thermoelectric properties of Ba-Ge based type-III clathrate compounds in Ba-Al-Ge and Ba-In-Ge systems have been investigated as a function of Al and In content. The absolute values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases with increasing Al and In content. The increase in electrical resistivity and Seebeck coefficient is discussed in terms of the number of the excess electrons deduced from the Zintl concept, on the other hand, the decrease in lattice thermal conductivity is discussed in terms of an anisotropic deformation of the open-dodecahedron cage encapsulating Ba atom. High ZT values of 0.7 and 0.87 are obtained at 780 and 580 °C for Ba24Al12Ge88 and Ba24In16Ge84, respectively. INTRODUCTION Clathrate compounds consisting of polyhedral cages and guest atoms encapsulated in the cages have attracted considerable attention in recent years as promising thermoelectric materials. The “rattling” of guest atoms in oversized cages scatters heat-carrying phonons, resulting in low thermal conductivity, while electrical conductivity remains relatively high because electronic conduction mainly takes place through the cage framework [1,2]. Clathrate compounds form in a variety of structure types, depending on the combination of different constituting cages. Among the various types, type-III clathrate compound, formulated as M24X100 (M=alkali or alkali earth metals, X=group IV and/or III elements) comprises a cage structure consisting of eight pentagonal dodecahedra (Fig. 1a) and 12 open dodecahedra (Fig. 1b). The encapsulated guest atoms (M) are located in 8c and 12d sites in pentagonal dodecahedra and open dodecahedra, and the X atoms are located in six different crystallographic sites of the space group of P4132, respectively [3,4]. The ‘rattling’ motion of guest atoms is considered to be most intensive at M(2) atom site located in open dodecahedron, especially in x-direction, since anomalously large values of atomic displacement parameters (ADPs) are obtained for M(2) atom site and ADPaniso value of M(2) is much larger in x-direction than in the other directions [5]. The binary type-III clathrate compound in Ba-Ge system, Ba24Ge100, has been reported to have a very low lattice thermal conductivity below 1W/mK [4,6]. However, since the electron

concentration of Ba24Ge100 is far larger than that of typical thermoelectric materials, this is considered to result in high electronic thermal conductivity and poor thermoelectric properties. Indeed, in our recent study, alloying Ga, which possesses 3 valence electrons, in Ba24Ge100 decreases thermal conductivity and improves thermoelectric properties significantly [7].

Figure 1. (a) Dodecahedral and (b) o