Magnetism\nand Thermoelectric Properties of the Zintl\nSemiconductor: Eu<sub>21</sub>Zn<sub>4</sub>As<sub>18</sub>

Compositional diversity\nand intriguing structural features\nhave\nmade Zintl phases excellent candidates as thermoelectric materials.\nZintl phase with 21-4-18 composition has shown high thermoelectric\nperformance in the mid- to high-temperature ranges. The complex crystal\nstructure and favorable transport properties of these compounds indicate\nthe potential for high thermoelectric efficiency. Arsenic-based Eu<sub>21</sub>Zn<sub>4</sub>As<sub>18</sub>, belonging to the Ca<sub>21</sub>Mn<sub>4</sub>Sb<sub>18</sub> structure type, exhibits a semiconductor-like\np-type transport behavior and has a calculated band gap of 0.49 eV.\nThe compound is paramagnetic at high temperatures, with an antiferromagnetic\ntransition occurring at <i>T</i><sub>N</sub> = ∼10\nK. The moment obtained from the Curie–Weiss data fit aligns\nwith Eu<sup>2+</sup> ions. At the same time, the field-dependent measurement\nat 2 K indicates complex magnetic ordering with a saturation moment\nconsistent with Eu<sup>2+</sup> ions. Pristine Eu<sub>21</sub>Zn<sub>4</sub>As<sub>18</sub> exhibits an ultralow lattice thermal conductivity\nof 0.40 W m<sup>–1</sup> K<sup>–1</sup> at 873 K. Electronic\ntransport properties measurement shows evidence of bipolar conduction\nacross much of the measured temperature range (450–780 K).\nHowever, the Seebeck coefficient remains extremely high (>440 μV\nK<sup>–1</sup>) across this range, indicating the potential\nfor high <i>zT</i> if an appropriate dopant is found. This\nwork represents the first report on the temperature-dependent thermal\nconductivity, Seebeck coefficient, and thermoelectric efficiency of\nthe arsenic-containing Zintl phase with 21-4-18 composition, showcasing\nits promise for further optimization of the thermoelectric performance.