Nonstoichiometry in the Zintl Phase Yb_1−δZn₂Sb₂ as a Route to Thermoelectric Optimization

Abstract

Classically, Zintl phases are defined\nas valence-precise line compounds\nand are thus expected to exhibit intrinsic semiconducting behavior.\nContradicting this definition are <i>A</i>Zn₂Sb₂ Zintl compounds (<i>A</i> = Ca, Sr, Eu,\nYb), which exhibit metallic behavior due to high concentrations of\ncation vacancies, according to recent density functional calculations.\nHere, we use synchrotron diffraction and high-temperature electronic\nand thermal transport properties to show that the phase width of Yb_1−δZn₂Sb₂ is wide enough\nto allow for significant variation and optimization of the thermoelectric\nproperties within the single phase region. Samples with nominal compositions\nof Yb_<i>x</i>Zn₂Sb₂ (0.98\n< <i>x</i> < 1.05) were synthesized using a solid-state\nprocess. With decreasing synthetic Yb content, synchrotron X-ray diffraction\nreveals decreased lattice parameters, decreased occupancy of the Yb\nsite, and a relaxation of the tetrahedral angles within the Zn₂Sb₂ sheets. In Yb-deficient samples, the carrier\nconcentration can be controlled by varying <i>x</i>, whereas,\nin samples with excess Yb, the carrier concentration remains constant\nand <i>p</i>-type. Fully intrinsic semiconducting behavior\nwas not obtained, suggesting that a slightly Yb-deficient composition\nis thermodynamically preferable to the valence-precise stoichiometry\nof δ = 0. Tuning the vacancy concentration provides a new route\nto controlling the electronic properties in Yb_1−δZn₂Sb₂ and leads to a 50% improvement in the\nthermoelectric figure of merit (<i>zT</i> = 0.85 at 773\nK) compared to previously reported values for unalloyed YbZn₂Sb₂.