The high thermopower of the Zintl compound Sr5Sn2As6 over a wide temperature range: first-principles calculations

Abstract

The thermoelectric properties and the electronic structure of Sr₅Sn₂As₆ were studied according to the first principles and semi-classical Boltzmann theory. To elucidate the thermoelectric performance of Sr₅Sn₂As₆, we simulated its carrier concentration, Seebeck coefficient, and electrical conductivity, and provided an estimated value for the thermoelectric figure of merit ZT. For pure Sr₅Sn₂As₆, the largest Seebeck coefficient (S) is 248 (μV K⁻¹) at 500 K, and the minimum S is 202 (μV K⁻¹) at 1200 K. The large Seebeck coefficient over a wide temperature range most likely results from the appropriate band gap (0.55 eV) of Sr₅Sn₂As₆. By studying the carrier concentration dependence of the transport properties, the ZT value for p-type doping was found to be ∼1.4 times that of n-type doping, which is mainly due to the larger effective mass of the valence band. Moreover, for n-type doping, both the Seebeck coefficient and the electrical conductivity along the z-direction are much larger than those along the other directions, due to the large band degeneracy and the light band, which results in a highest ZT value of 3.0 along the z-direction, with a carrier concentration of 9.4 × 10¹⁹ electrons per cm³ at 950 K. The highest ZT value for p-type along the z-direction is 1.7, with a carrier concentration of 1.2 × 10²⁰ holes per cm³ at 950 K. Meanwhile, the minimum lattice thermal conductivity of Sr₅Sn₂As₆ is small (0.47 W m⁻¹ K⁻¹), and is comparable to those of Ca₅M₂Sb₆ (M = Al, Ga, In). Hence, good thermoelectric performance along the z-direction for n-type Sr₅Sn₂As₆ was predicted.