Understanding the Intrinsic P‑Type Behavior and Phase Stability of Thermoelectric α‑Mg3Sb2

α-Mg3Sb2 is an excellent thermoelectric material through excess-Mg addition and n-type impurity doping to overcome its persistent p-type behavior. It is generally believed that the role of excess-Mg is to compensate the single Mg vacancy to realize n-type carrier conduction. In contrary to this belief, the present work indicates that the role of excess-Mg is to compensate the electronic charge of defect complex (VMg(2) + MgI)1–. The Mg solubility in α-Mg3+x Sb2 is quite small when only considering a single defect, but it enlarged up to x = 0.011 with the defect complex (VMg(2) + MgI)1–, which is more reasonable as supported by experiments. Under Mg-poor conditions, VMg(1) 2– and VMg(2) 2– are the dominant defects, and their concentrations can reach (1.05–1.18) × 1019 cm–3 at 1200 K. Under Mg-rich conditions, (VMg(2) + MgI)1– is found to be the dominant reason for strong p-type behavior, and their concentrations can reach as high as 3.5 × 1020 cm–3, which shifts the Fermi level closer to the valence band maximum. The predicted carrier concentrations in the range 1017–1020 cm–3 are in the same range found experimentally for pure p-type α-Mg3Sb2.