We report the effects of partial substitution of Ge onto the Ga sites of Fe2VGa by measuring electrical resistivity, Seebeck coefficient and thermal conductivity as a function of temperature. It is found that Ge substitution effectively dopes electrons to the system and thus causes a dramatic decrease in the electrical resistivity. The Seebeck coefficient changes sign from positive to negative upon replacing Ga by Ge, which is in good agreement with a simple band filling picture. In addition, the magnitude of the Seebeck coefficient gradually increases and attains a maximum value of 85 νV K-1 at around 120 K for Fe2VGa0.9Ge 0.1. Such a variation of Seebeck coefficient can be understood by means of rigid band-like shifting of the Fermi level across the pseudogap. The thermal conductivity is also reduced and a detailed analysis based on the Debye approximation indicates that the extrinsic disorder introduced by Ge substitution in Fe2VGa has a minor contribution to the point defect scattering. Other lattice imperfections, such as antisite disorder, may be the main source for the point defect scattering which shows no systematic variation with Ge concentration. While the thermoelectric performance improves with the partial substitution of Ge, the largest figure-of-merit (ZT) value among these presently investigated alloys is still an order of magnitude lower than the conventional thermoelectric materials.
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