Thermoelectric properties and electronic structure of the Zintl phase Sr_5Al_2Sb_6

Abstract

The Zintl phase Sr_5Al_2Sb_6 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 Sr_5Al_2Sb_6 is characterized by infinite oscillating chains of AlSb_4 tetrahedra. It is distinct from the structure type of the previously studied Ca_5M_2Sb_6 compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr_5Al_2Sb_6 (∼0.55 W mK^(-1) at 1000 K) was found to be lower than that of the related Ca_5M_2Sb_6 compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr_5Al_2Sb_6, 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 Sr_5Al_2Sb_6, consistent with the large band gap and valence-precise structure. Doping with Zn^(2+) on the Al^(3+) site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr_5Al_2Sb_6 suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca_5Al_2Sb_6 could be achieved in this system.

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