Electronic structure tuning of α-SrSi2 by isotropic strain and isoelectronic impurity incorporation: A first-principles study for enhancement of low-temperature thermoelectric performance

Interest in thermoelectric (TE) materials has revived in recent years because TE materials realize not only the utilization of distributed unused thermal energy, such as exhaust heat from automobiles and factories and solar heat, but also cold power generators and self-power supplies for wireless sensors. However, because the bandgap of low-temperature TE materials is relatively small, the precise calculation of its physical properties is difficult to achieve by first-principles calculations based on conventional density functional theory. The present study investigates the effects of isotropic strain and incorporation of isoelectronic impurities on the TE transport properties of extremely narrow-gap semiconducting α-SrSi2. By adopting the Gaussian–Perdew–Burke–Ernzerhof hybrid functional, the analysis clarifies the relationship between the lattice distortion and the electronic structure in α-Sr4–xAxBySi8–y (A = Mg, Ca, or Ba; B = C, Ge, Sn, or Pb) and elucidates the TE transport properties. In particular, an irregular bandgap expansion was observed in α-Sr4CSi7, suggesting that the TE performance can be maximized by appropriate tuning of the carrier concentration.