Role of Volume Change on the Physics of Thermoelectric Half-Heusler Compounds

Doping at particular sites is a common method for increasing the thermoelectric efficiency of materials, by tuning carrier concentration and electronic structure. A secondary effect of doping, as well as defects, is to induce a volume change, usually referred to as chemical pressure, that may affect the thermoelectric efficiency. Theoretical investigations usually ignore the role of volume change in thermoelectric improvement, mostly for computational limitations. In this work, we address the role of chemical pressure on the thermoelectric properties of TaFeSb, MOsSb (M = Ta, Nb), and NRuAs (N = Ta, Nb, V) by using ab initio electronic structure calculations. We calculate the effect of both negative and positive pressure on the electronic structure, the Seebeck coefficient, electrical and thermal conductivity, as well as the power factor and thermoelectric performance. We argue that volume change, occurring because of defects or doping, should be regarded as an essential parameter to determine the thermoelectric efficiency accurately, as exemplified by TaFeSb. Among the investigated compounds, TaRuAs stands out for the peculiar behavior of electronic and thermoelectric properties with respect to volume change. NbOsSb also stands out, as the sole compounds whose thermoelectric efficiency is maximal in the ground state and cannot be increased via a moderate volume change. Overall, we predict that TaRuAs can be an excellent candidate for thermoelectric applications, due to its large thermoelectric efficiency at zero pressure and the possibility of increasing it by a small volume change. Direct calculations of TaRuAs0.875Bi0.125 demonstrate the improved thermoelectric properties while also providing an estimate of the accuracy of our chemical-pressure-based modeling of the doping process.