Nonstoichiometry in the Zintl Phase Yb_(1−δ)Zn_2Sb_2 as a Route to Thermoelectric Optimization

Abstract

Classically, Zintl phases are defined as valence-precise line compounds and are thus expected to exhibit intrinsic semiconducting behavior. Contradicting this definition are AZn_2Sb_2 Zintl compounds (A = Ca, Sr, Eu, Yb), which exhibit metallic behavior due to high concentrations of cation vacancies, according to recent density functional calculations. Here, we use synchrotron diffraction and high-temperature electronic and thermal transport properties to show that the phase width of Yb_(1−δ)Zn_2Sb_2 is wide enough to allow for significant variation and optimization of the thermoelectric properties within the single phase region. Samples with nominal compositions of Yb_xZn_2Sb_2 (0.98 < x < 1.05) were synthesized using a solid-state process. With decreasing synthetic Yb content, synchrotron X-ray diffraction reveals decreased lattice parameters, decreased occupancy of the Yb site, and a relaxation of the tetrahedral angles within the Zn_2Sb_2 sheets. In Yb-deficient samples, the carrier concentration can be controlled by varying x, whereas, in samples with excess Yb, the carrier concentration remains constant and p-type. Fully intrinsic semiconducting behavior was not obtained, suggesting that a slightly Yb-deficient composition is thermodynamically preferable to the valence-precise stoichiometry of δ = 0. Tuning the vacancy concentration provides a new route to controlling the electronic properties in Yb_(1−δ)Zn_2Sb_2 and leads to a 50% improvement in the thermoelectric figure of merit (zT = 0.85 at 773 K) compared to previously reported values for unalloyed YbZn_2Sb_2.

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