The efficiency of energy conversion in thermoelectric generators (TEGs) is directly proportional to electrical conductivity and Seebeck coefficient while inversely to thermal conductivity. The challenge is to optimize these interdependent parameters simultaneously. In this work, the problem is addressed with a novel approach of nanostructuring and constructive electronic structure modification to achieve a very high value of dimensionless figure of merit ZT greater than 3.6 at 1000 K with negative Seebeck coefficient. Supersaturated solid-solutions of Si-Ge containing 1 atomic percent Fe and 10 atomic percent P are prepared by high-energy ball milling. The bulk samples consisting of ultra-fine nano-crystallites 9.7 nm are obtained by the sophisticated low-temperature & high-pressure sintering process. Despite that the electrical resistivity is slightly high due to the localization of electrons is associated with the highly disordered structure and low electrical density of states near the chemical potential, a very low thermal conductivity k{appa} less than 1 W m-1K-1 and very large magnitude of Seebeck coefficient exceeding 470 uV K-1 are achieved in association with the nanostructuring and the Fe 3d impurity states, respectively, to realize a very large magnitude of ZT.
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