Abstract
The lack of high efficiency thermoelectric materials hinders their deployment into wide ranging applications such as power generation from waste heat and solid state heating and cooling, which could lead to significant energy savings. Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1; and occurs at, or above, 1000 K making them unfavorable especially for intermediate temperature applications. In this report, we demonstrate that transition metal doping introduces charge distortion and lattice defects into these materials which increases the ionized impurity scattering of carriers and point defect scattering of lattice phonons, respectively; leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the band gap of these materials can be tuned between 0.1 eV and 0.5 eV by adjusting the transition metal content, making it possible to design type I clathrates with excellent thermoelectric properties between 500 K and 1000 K.
