Deciphering Defects in Yb2–xEuxCdSb2 and Their Impact on Thermoelectric Properties

Layered Zintl phases with A2MPn2 stoichiometry are an underexplored class of potential thermoelectric materials with complex and diverse chemistry. The solid solution Yb2–xEuxCdSb2 is an example of the promise these compounds hold, as one composition, Yb1.64Eu0.36CdSb2, has reported one of the highest zTs of any Zintl phase material at 525 K. The present study examines changes in structure and bonding of this solid solution that impacts thermoelectric performance. Pair distribution function analysis is combined with electronic structure modeling to take a chemical bonding-based approach to deconvolute the impact of defects on thermal and electronic properties in Yb2–xEuxCdSb2. Yb2–xEuxCdSb2 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples were synthesized, and thermoelectric properties and defect chemistry were investigated. Samples from the middle of the series x = 0.2 and 0.3 were found to be the most highly defected, exhibiting Yb and Sb vacancies, as well as distortions in the Yb–Sb coordination spheres that correlate with thermoelectric properties. The highest efficiency is reported for x = 0.4 (zT ≈ 0.9 at 525 K), and the thermoelectric quality factor predicts that x = 0.3 could achieve zT > 2 by synthetically tuning the defect structure and thereby carrier concentration. The strategy of investigating local structure outlined in this study can be applied to a variety of other thermoelectric materials to provide insight into the hidden role of defect chemistry in understanding the structure–property relationships in extended solids.