Thermoelectric and optical properties of the filled skutterudite YbFe4Sb12

Transport measurements, including magnetoresistivity, Hall resistivity, thermal conductivity, and thermopower, and an assessment of the dimensionless thermoelectric figure of merit ZT below room temperature are reported on a polycrystalline sample of the strongly correlated electron material YbFe4Sb12. Neutron diffraction measurements are presented, which reveal large amplitude thermal vibrations of the Yb ion, consistent with speculations that the ion is weakly bound in an oversized atomic "cage" formed in the skutterudite crystal structure. Infrared spectroscopy measurements reveal a resonance in the dissipative part of the optical conductivity sigma(1)(omega), which develops below T*approximate to 70 K, the characteristic temperature for the Yb ion valence fluctuations. The frequency dependence of the optical constants is suggestive of a hybridization band gap in the quasiparticle density of states at low temperatures.