Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu2ZnSb2 Alloys

AMX compounds with the ZrBeSi structure type tolerate a vacancy concentration of up to 50% on the M-site in their planar honeycomb MX-layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu1−xZn0.5xSb solid solution. The transition from a fully-occupied honeycomb (EuCuSb) to one with a quarter of the atoms missing (EuZn0.5Sb) has wide-ranging structural consequences; we observe a significant non-linear expansion of the average bond lengths in the honeycomb, consistent with anion-anion repulsion, and increasing atomic displacement parameters on the M and Sb-sites. Increasing the vacancy concentration causes lattice softening and a corresponding decrease in sound velocity, as well as a rapid increase in point defect scattering, leading to a drop in room temperature lattice thermal conductivity from 3 W/mK to 0.5 W/mK. The effect of increasing Zn and vacancy concentration on the electronic properties is more nuanced, leading to a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as the composition varies from EuCuSb to EuZn0.5Sb.