Pseudogap Formation and Thermoelectric Properties of Heusler-type Compounds

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The thermoelectric power of Fe_2VA1 can be expected to be well enhanced by doping or off-stoichiometry, because the density of states rises sharply in both sides of the pseudogap. We have investigated the temperature dependence of the Seebeck coefficient and the electrical resistivity in the Fe_2VAI-based alloys, in addition to the measurement of the thermal conductivity at room temperature, to clarify the doping effect on the thermal and transport properties. The compositional variation of the Seebeck coefficient falls on a universal curve, irrespective of the doping elements, when plotted against the valence electron concentration (VEC), instead of the composition of the doping elements. The net effect of doping is most likely to cause an appreciable shift of the Fermi level from the central region in the pseudogap without modifying the band structure in any essential manner. The substitution of heavy atoms certainly acts in favor of the development of thermoelectric materials because of a substantial reduction of the lattice thermal conductivity. Co-doping of heavier atoms is also effective to reduce the lattice thermal conductivity while retaining the low electrical resistivity as well as the large Seebeck coefficient. For the Heusler compound Fe_<2-x>Co_xTiAl with VEC=6, the compositional variation of the Seebeck coefficient is in line with a pseudogap scenario where the off-stoichiometry causes a rigid-band-like shift of the Fermi level from the central region in the pseudogap. The low Seebeck coefficient may be attributed to an atomic disorder between Fe and Co