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0886-F10-05.1

Effect of Ga-doping on the Thermoelectric Properties of Ba-Ge Type-III Clathrate Compounds Jung-Hwan Kim, Norihiko L. Okamoto, Katsushi Tanaka and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan ABSTRACT Effect of Ga addition on the thermoelectric properties of Ba-Ge type-III clathrate has been investigated as a function of Ga content and temperature. The substitution of Ga atom for Ge atom leads to the decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with the increase in the Ga content and in temperature. Both electronic and lattice thermal conductivity decrease with the increase in the Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open-dodecahedron, respectively. A very high ZT value of 1.25 is obtained at 670 °C for Ba24Ga15Ge85. 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, because they are considered to behave as “phonon glass-electron crystal (PGEC)” [1,2]. 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. Type-III clathrate compounds, formulated as M24X100 (M=alkali or alkali earth metals, X=Si, Ge), are composed of three kinds of cage structures designated X20-pentagonal dodecahedron, open-dodecahedron and distorted cube. M (Guest) atoms are located in 8c, 12d and 4a sites of the space group of P4132 encapsulated with these three different cages, respectively [3,4]. In spite of the possibility of thermoelectric materials of high performance in view of the structural characteristic for type-III clathrate compounds, only a small amount of efforts seems to have been made to investigate type-III clathrate compounds as a thermoelectric material. Indeed, Paschen et. al. [5] and Kim et. al. [6] have reported low thermal conductivity and relatively high electrical conductivity for the type-III clathrate compound Ba24Ge100 below room temperature. Paschen et. al. [5] have noticed that the carrier (electron) concentration of Ba24Ge100 is far larger

0886-F10-05.2

than that of typical thermoelectric materials. However, no systematic study has been carried out so far on the thermoelectric properties of Ba24Ge100, especially at high temperatures. In the present study, we investigate systematically the thermoelectric properties of type-III clathrate compounds Ba24GaXGe100-X as a function of Ga content and temperature. We selected Ga as a ternary element which possesses 3 valence electrons to control the carrier concentration of the compound. We first determine the solid-solu