Study on the possibility of band gap widening of thermoelectric semiconductor α-SrSi₂ by isoelectronic elements incorporation

The material α-SrSi₂ has considerable application potential as a near room temperature thermoelectric material. However, it has a very narrow band gap which can vanish due to impurities, defects, and grain boundaries. To obtain guidelines for controlling the band gap and improving the thermoelectric properties, we have studied the substitution of isoelectronic impurities (Mg, Ca, and Ba for Sr, and C, Ge, and Sn for Si) through first-principles calculations using the Gaussian-Perdew-Burke-Ernzerhof hybrid functional. From these calculations, it was found that: (1) The band gap change due to Ca, Ba, Ge, and Sn substitutions is almost the same as that predicted from the change in the lattice constant of pure α-SrSi₂. (2) Substitution with C was energetically unfavorable and therefore high levels of substitution would be difficult to achieve. However, it was expected to increase the band gap, contrary to the prediction from the decrease in the lattice constant. (3) An increase in the Mg substitution of the Sr site from 0 at. % to 8 at. % caused a decrease in the lattice constant from 0 % to −1.189