Thermal and transport properties of the Heusler-type Fe2VAl1-xGex (0≤ x ≤0.20) alloys:: Effect of doping on lattice thermal conductivity, electrical resistivity, and Seebeck coefficient

We report on the thermoelectric properties of the Heusler-type Fe2VAl1-xGex alloys with compositions 0 <= x <= 0.20. While Fe2VAl (x=0) exhibits a semiconductorlike behavior in electrical resistivity, a slight substitution of Ge for Al causes a significant decrease in the low-temperature resistivity and a large enhancement in the Seebeck coefficient, reaching -130 mu V/K for x=0.05 at around room temperature. Comparison with the Fe2VAl1-xSix system demonstrates that the compositional variation of the Seebeck coefficient falls on a universal curve irrespective of the doping elements (Ge and Si), both of which are isoelectronic elements. The net effect of doping is most likely to cause a rigid-bandlike shift of the Fermi level from the central region in the pseudogap. In spite of a similar decrease in the electrical resistivity with composition of Ge and Si, the thermal conductivity decreases more rapidly for the Ge substitution. It is concluded that doping of heavier atoms such as Ge reduces more effectively the lattice thermal conductivity while retaining the low electrical resistivity as well as the large Seebeck coefficient.