Electronic structure of Yb-filled CoSb3 skutterudites studied by X-ray photoelectron spectroscopy

The valence-band structure of Yb-filled CoSb₃ skutterudites has been investigated by X-ray photoelectron spectroscopy for an understanding of the effect of Yb filling on the electronic and thermoelectric properties of CoSb₃. The valence-band and core-level spectra clearly indicated the intermediate valence states of Yb between divalent and trivalent in Yb-filled CoSb₃, in good agreement with the results of band calculations made by the full-potential linearized augmented-plane-wave method. The effect of Yb filling on the electronic properties (carrier mobility, effective mass, lattice thermal conductivity, etc.) is discussed in terms of an electronic model obtained from band calculations. The electronic properties of CoSb₃ are substantially affected by Yb filling, resulting in the large thermoelectric figure of merit for Yb filling as compared to other rare-earth filling. It is suggested that the mixed valence states of Yb play an important role in the improvement of thermoelectric properties