Electronic and thermoelectric properties of YbyFe4-xNixSb12 filled skutterudites

Polycrystalline p-type Yb_yFe_4-xNi_xSb₁₂ (nominal x=0-0.8, y=1) filled skutterudite compounds have been grown by a spark plasma sintering technique and their electronic and thermoelectric properties have been investigated. The electronic structure of p-type Yb_yFe_4-xNi_xSb₁₂ compounds has been studied by x-ray photoelectron spectroscopy. In these materials, valence-band and core-level photoelectron spectra clearly indicate the intermediate valence (mixed valence) states of Yb between divalent and trivalent. Further the valence-band spectra show that the Yb 4f states are hybridized with the Fe (Ni) 3d and Sb 5p states. The lattice thermal conductivity for these compounds is extremely low (∼0.01 W/cmK). The Seebeck coefficient increases with increasing carrier concentration in contrast to a behavior of ordinary semiconductors. This behavior can be attributed to an enhancement of density of states effective mass due to the narrow 4f band effects. The dimensionless thermoelectric figure of merit ZT∼1 (750-850 K) was estimated for p-type Yb_yFe_4-xNi_xSb₁₂ samples with x=0.6 and 0.8.