Effect of Guest Substitution on Thermoelectric Properties of Clathrate Compounds

We have investigated the electronic and thermoelectric properties of polycrystalline guest-substituted clathrate compounds with the formula Ba_8-xA_xAu₆Ge₄₀ (A = Sr, Eu; nominal x = 0-4). The compounds show p-type conduction and degenerated semiconductor behavior. The Seebeck coefficient and the electrical conductivity are not largely affected by guest substitution and exhibit carrier concentration dependence that is similar to that for p-type Ba₈Au_xGe_46-x. The carrier effective mass is relatively large, and also depends on the carrier concentration, as is the case for p-type Ba₈Au_xGe _46-x. The valence band x-ray photoelectron spectroscopy study and band structure calculation show that the guest Eu 4f states with localized nature are located below about 1.5 eV from the valence band top, suggesting that the influence of Eu substitution on the p-type transport properties may be small. The lattice thermal conductivity kappa_L decreases greatly with increasing guest composition up to x = 2, and the reduction of kappa_L for Eu substitution is larger than that for Sr substitution. As a result, thermoelectric figure of merit ZT is improved as compared to that for p-type Ba₈Au_xGe_46-x