Enhanced thermoelectric properties of Zintl compound KCaBi by tensile strain: A first-principles study

Recently Zintl phase compound KCaBi has emerged as a potential candidate for thermoelectric materials because of its extremely low lattice thermal conductivity. Applying strain is an effective approach for modulating the properties of thermoelectric materials. In this study, first-principles calculations and Boltzmann transport theory were used to explore the optimization of thermoelectric properties of the Zintl compound KCaBi under tensile strains ranging from 2% to 4%. Because of the low lattice thermal conductivity, the Zintl compound KCaBi exhibits excellent thermoelectric properties, with a maximum $zT$ value of 1.39 for n-type KCaBi and 2.09 for p-type KCaBi at 800 K. Furthermore, applying tensile strain can moreover reduce the lattice thermal conductivity, thereby improving the thermoelectric performance of KCaBi. Under tensile strain of 4%, the optimal $zT$ of n-type KCaBi at 300 K along (a(b), c) direction increases from (0.25, 0.40) to (0.65, 1.05), while that of p-type KCaBi along (a(b), c) direction increases from (0.75, 0.78) to (1.31, 1.71). However, the tensile strain causes substantial reduction in the band gap of KCaBi, which leads to the bipolar effect at higher temperatures. This largely weakens the enhancement of thermoelectric performance by tensile strain, especially in n-type KCaBi. Despite of the bipolar effect at high temperature, there is still a considerable improvement of the average $zT$ by tensile strain. Our results demonstrate that both n-type and p-type KCaBi exhibit great potential for thermoelectric applications, and applying tensile strain can effectively improve the thermoelectric properties.