H. Anno, M. Hokazono, M. Kawamura, K. Matsubara

Semiconductor clathrate compounds are energetically being studied as a new class of thermoelectric material based on the concept of Phonon Glass and Electron Crystal. The glasslike low thermal conductivity in clathrate compounds is due to the strong interaction of localized guest vibrations with the host acoustic modes. On the other hand, these materials have relatively good electronic conduction through the covalent sp³ hybridized framework, which dominates the band structure and electronic transport properties of the compounds. Thorough investigation of the guest-host interaction and its relation to the physical properties is of great importance to gain some insight into the design and development of novel thermoelectric clathrates. We have systematically prepared polycrystalline Si- and Ge-based clathrate compounds with elemental substitution of the framework and investigated their electronic and thermoelectric properties. In Ge clathrates the Seebeck coefficient is significantly enhanced at high carrier concentration by transition element Pt substitution of the framework, while the carrier mobility is hardly affected. As a result, the dimensionless thermoelectric figure of merit was improved by Pt substitution. The effects of transition element substitution on the band structure and the electronic properties are also discussed.