A key factor improving the thermoelectric properties of Zintl compounds A5M2Pn6 (A = Ca, Sr, Ba; M = Ga, Al, In; Pn = As, Sb)

The electronic structure and transport properties of A5M2Pn6 (A = Ca, Sr, Ba; M = Ga, Al, In; Pn = As, Sb) are investigated by using the first-principles calculations and Boltzmann transport theory, respectively. The results show that the order of the width of these band gaps is Ca5Ga2As6 (0.37 eV) > Ca5Al2Sb6 (0.35 eV) > Ca5In2Sb6 (0.32 eV) > Sr5In2Sb6 (0.29 eV) > Ba5In2Sb6 (0.27 eV) > Ca5Ga2Sb6 (0.088 eV). For intrinsic A5M2Pn6, increasing the band gap is better for improving their thermoelectric properties. For doped A5M2Pn6, increasing the band gap is helpful to improve the high-temperature thermoelectric properties of A5M2Pn6. However, at 300 K, when the band gap is small enough, regardless of n-type or p-type doping, A5M2Pn6 exhibits good thermoelectric properties. Our analysis shows that the band gaps can be roughly adjusted by changing the electronegativity difference between Pn and A, and subtly adjusted by changing the electronegativity difference between Pn and M atoms. So the electronegativity difference among the constituent element is a very important factor affecting the thermoelectric properties.