Correlation between pseudo-gap electronic structures and thermoelectric properties in Fe_2 VAl intermetallic compounds

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The Heusler-type Fe_2VAl compound has been theoretically predicted to have a pronounced pseudo-gap across the Fermi level and experimentally proved to show the drastic change in the electronic transport properties and the large enhancement of the thermoelectric power on the deviation of the composition from its stoichiometry. The purpose of this research is to experimentally clarify the relation between their electronic structures and thermoelectric properties of the Heusler-type Fe_2VAl-related alloys with use of the normal and inverse photoelectron spectroscopy, and thus to obtain a new guideline for developing thermoelectric materials.In this study, the electronic structures have been investigated for the stoichiometric Fe_2VAl, off-stoichiometric (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_<2-x>V_<1+x>Al, and substituted Fe_2VAl_<1-z>Si_z. It is experimentally shown for the first time that the existence of the pseudo-gap and the relative position of the Fermi level in the pseudo-gap are strongly related to their thermoelectric properties. The change in the electronic structure near the Fermi level and thermoelectric properties with the composition is explained qualitatively by a rigid band model for (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_2VAl_<1-z>Si_z but not for Fe_<2-x>V_<1+x>Al. This arises from the large modification of the electronic structure near the Fermi level due to the transition metal d states in contrast to the sp valence states of Al and Si. These mid-gap d states are responsible for the unusual transport properties at low temperatures. Thus it is important to control the d states near the Fermi level for the further development of these Heusler-type alloys as thermoelectric materials. We suggest that the substitution of the forth element for the Fermi level positioning and the microscopic structural control for the low thermal conductivity are effective for the development.