Thermal stability of p-type Ag-doped Mg₃Sb₂ thermoelectric materials investigated by powder X-ray diffraction

The thermal stability of the p-type Mg2.985Ag0.015Sb2 material with reported good thermoelectric performance is systematically investigated using powder X-ray diffraction (PXRD). Rietveld refinements were performed to extract the quantitative compositional and structural information from PXRD data. For both densified bulk samples and powdered samples of Mg2.985Ag0.015Sb2 studied under vacuum, a secondary phase, Sb, appears upon the first heating process at temperatures above 500 K, but the amount of the Sb phase stabilizes in subsequent thermal cycles. This is consistent with the good repeatability of the electrical resistivity data after the first cycle measurement, and it indicates that the appearance of the Sb phase does not result in structural decomposition or degradation of the thermoelectric properties. To investigate the effect of the Mg vacancy concentration on the formation of the secondary Sb phase, Mg3.5-xAgxSb2 (x = 0 and 0.015) samples with nominal excess Mg were synthesized and characterized. Compared with Mg2.985Ag0.015Sb2, it is found that Mg3.5-xAgxSb2 (x = 0.015) shows no Sb phase appearing for the bulk sample after heat treatment, and only a small amount of Sb emerges in the powdered sample. This reveals that decreasing the Mg deficiency is beneficial for reducing the amount of emerging Sb secondary phase. In addition, there is less Sb appearing in the Ag-doped powder sample of Mg3.5-xAgxSb2 (x = 0.015) than the undoped Mg3.5Sb2 powder sample, indicating that Ag doping at the Mg sites can inhibit the appearance of the Sb phase to some extent. Our work implies that the emergence of the Sb phase is unlikely due to the decomposition of the Mg3Sb2 structure and can be hindered by reducing the Mg deficiency or Ag doping on the Mg sites.