Cation vacancy evolution and enhanced thermoelectric properties in nonstoichiometric Eu_xZn₂Sb₂ (0.95 ≤ x ≤ 1.03)

Zintl compounds AB₂Sb₂ (A and B are generally group IIA or IIB elements) are promising thermoelectric (TE) materials, and the cation vacancies are typically the dominant defects serving as a means of tuning their TE properties. Since both A and B are cations, the competition of the cation vacancies V_A and V_B in response to changes in the chemical environment directly affects the system's TE performance. Here, using EuZn₂Sb₂ as a representative system, we found that the defect formation energies of V_Eu and V_Zn vary systematically with the chemical potential. In Eu-poor and Zn-rich conditions, V_Eu dominates, while V_Zn becomes more important under Zn-poor and Eu-rich conditions. By varying the nominal Eu content, the chemical potential of nonstoichiometric Eu_xZn₂Sb₂ (0.95 ≤ x ≤ 1.03) can be effectively tuned. As a result, an anomalous evolution of carrier concentration, i.e., first decreasing and then increasing as the Eu content is reduced, is observed, leading to a pertinent change in TE properties. Consequently, a maximum TE figure of merit zT of 0.77 at 773 K is achieved for Eu_0.97Zn₂Sb₂. This work highlights the importance of understanding the evolution of intrinsic defects for enhancing TE performance.