Thermoelectric Properties and Electronic Structure of the Zintl-Phase Sr_3AlSb_3

Abstract

The Zintl-phase Sr_3AlSb_3, which contains relatively earth-abundant and nontoxic elements, has many of the features that are necessary for good thermoelectric performance. The structure of Sr_3AlSb_3 is characterized by isolated anionic units formed from pairs of edge-sharing tetrahedra. Its structure is distinct from previously studied chain-forming structures, Ca_3AlSb_3 and Sr_3GaSb_3, both of which are known to be good thermoelectric materials. DFT predicts a relatively large band gap in Sr_3AlSb_3 (E_g≈1 eV) and a heavier band mass than that found in other chain-forming A_3MSb_3 phases (A=Sr, Ca; M=Al, Ga). High-temperature transport measurements reveal both high resistivity and high Seebeck coefficients in Sr_3AlSb_3, which is consistent with the large calculated band gap. The thermal conductivity of Sr_3AlSb_3 is found to be extremely low (≈ 0.55 W mK^(−1) at 1000 K) due to the large, complex unit cell (56 atoms per primitive cell). Although the figure of merit (zT) has not been optimized in the current study, a single parabolic band model suggests that, when successfully doped, zT≈ 0.3 may be obtained at 600 K; this makes Sr_3AlSb_3 promising for waste-heat recovery applications. Doping with Zn^(2+) on the Al^(3+) site has been attempted, but does not lead to the expected increase in carrier concentration.

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