Origin of large thermoelectric power in off-stoichiometric Fe₂VAl-based alloys

The valence-band and core-level soft x-ray photoelectron spectroscopy has been applied to investigate the origin of the large thermoelectric power in the off-stoichiometric and partially-substituted Fe-based Heusler-type alloys Fe_2−x−yIr_yV_1+xAl and Fe_2−xV_1+x−yTi_yAl. The non-rigid-band-like change in the valence-band electronic structure near the Fermi level E_F is found for the off-stoichiometric change x in Fe and V concentrations, whereas the rigid-bandlike shift of E_F is recognized on the Ir or Ti substitution y. The non-rigid-band-like change may be induced by the excess-V or Fe (anti-site defects)-derived states in the pseudogap of Fe₂VAl, while the rigid-band-like shift may be caused by a common band formation, where the d states of the substituted elements are incorporated into the main Fe-V 3d bands. The V2p core-level spectra consistently show the satellite structure due to the excess V as the anti-site defect. A model electronic structure is presented for explaining the observed enhancement of their thermoelectric power.