Thermoelectric properties and electronic structure of the Zintl phase Sr₅Al₂Sb₆

The Zintl phase Sr₅Al₂Sb₆ has a large, complex unit cell and is composed of relatively earth-abundant and non-toxic elements, making it an attractive candidate for thermoelectric applications. The structure of Sr₅Al₂Sb₆ is characterized by infinite oscillating chains of AlSb₄ tetrahedra. It is distinct from the structure type of the previously studied Ca₅M₂Sb ₆ compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr₅Al₂Sb₆ (∼0.55 W mK^-1 at 1000 K) was found to be lower than that of the related Ca₅M ₂Sb₆ compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr₅Al₂Sb₆, in agreement with the experimentally determined band gap of E_g ∼ 0.5 eV. High temperature electronic transport measurements reveal high resistivity and high Seebeck coefficients in Sr₅Al₂Sb₆, consistent with the large band gap and valence-precise structure. Doping with Zn ²⁺ on the Al³⁺ site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr₅Al₂Sb₆ suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca₅Al₂Sb₆ could be achieved in this system.