Electronic structure and thermoelectric properties of Zintl compounds A3AlSb3 (A = Ca and Sr): first-principles study

Abstract

Experimentally synthesized Zn-doped Sr₃AlSb₃ exhibited a smaller carrier concentration than Zn-doped Ca₃AlSb₃, which induces a lower thermoelectric figure of merit (ZT) than Zn-doped Ca₃AlSb₃. We used first-principles methods and the semiclassical Boltzmann theory to study the reason for this differing thermoelectric behavior and explored the optimal carrier concentration for high ZT values via p-type and n-type doping. The covalent AlSb₄ tetrahedral arrangement exhibited an important effect on the electronic structure and thermoelectric properties. p-type Ca₃AlSb₃ may exhibit good thermoelectric properties along its covalent AlSb₄ chain due to its double band degeneracy at the valence band edge and small effective mass along its one-dimensional chain direction. Zn doping the Al site exhibited higher formation energy for Sr₃AlSb₃ than Ca₃AlSb₃, which explains the lower carrier concentration for Zn-doped Sr₃AlSb₃ than Zn-doped Ca₃AlSb₃. The double band degeneracy at the valence band edge for Ca₃AlSb₃ may also help to increase the carrier concentration. Sr₃AlSb₃ containing isolated Al₂Sb₆ dimers can exhibit a high thermoelectric performance via heavy p-type doping with a carrier concentration above 1 × 10²⁰ holes per cm³. Moreover, the ZT maxima for the n-type Sr₃AlSb₃ can reach 0.76 with a carrier concentration of 4.5 × 10²⁰ electrons per cm³.