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

Abstract

α-Mg 3 Sb 2 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 (V Mg(2) + Mg I ) 1− . The Mg solubility in α-Mg 3+x Sb 2 is quite small when only considering a single defect, but it enlarged up to x = 0.011 with the defect complex (V Mg(2) + Mg I ) 1− , which is more reasonable as supported by experiments. Under Mg-poor conditions, V Mg(1) 2− and V Mg(2) 2− are the dominant defects, and their concentrations can reach (1.05−1.18) × 10 19 cm −3 at 1200 K. Under Mg-rich conditions, (V Mg(2) + Mg I ) 1− is found to be the dominant reason for strong p-type behavior, and their concentrations can reach as high as 3.5 × 10 20 cm −3 , which shifts the Fermi level closer to the valence band maximum. The predicted carrier concentrations in the range 10 17 −10 20 cm −3 are in the same range found experimentally for pure p-type α-Mg 3 Sb 2 .

Enhanced Thermoelectric Properties of n‐type Mg3Sb2 by Excess Magnesium and Tellurium Doping

“…Previously, excess Mg is beneficial to the n‐type carrier conduction in Mg 3 Sb 2 has been reported . The negatively charged Mg vacancies V Mg are the most stable defects in Mg 3 Sb 2 compounds .…”

Insights into the design of thermoelectric Mg3Sb2 and its analogs by combining theory and experiment

“…Several experimental and theoretical efforts 54,60,61,63,65,121,122 reveal that a large amount of excess Mg is not needed for achieving n-type properties as claimed by Tamaki et al 55 . A tiny amount of excess Mg or even no excess Mg is required to realize n-type properties as long as great care is taken to avoid the Mg loss during the synthesis, consistent with the earlier report without excess Mg by Zhang et al 54 The samples with smaller amounts of excess or no excess Mg even show better performance and stability than those with large amounts of excess Mg. 63,122 Various reports with different amounts of excess Mg may arise from the difficulty in preparing these materials without the Mg loss due to the high reactivity, easy oxidation, and high vapor pressure of Mg. Fig.…”

“…55,61 The n-type doping in Mg 3 Sb 2 -based materials is challenging and only recently discovered to be successful. 51,[53][54][55] Several defect calculations 55,61,121 were conducted to explain the n-type behavior in Mg 3 Sb 2 under the Mg-excess condition. The results vary from different methods applied in defect energy calculations.…”

“…Ohno et al 61 concluded from calculations using the PBE functional 108 with finite-size and band-gap corrections that under the Mg-excess condition the Mg vacancy suppression rather than the Mg interstitial 55 is responsible for n-type properties (Fig. 8), while Chong et al 121 based on more accurate calculations with the HSE06 functional 131 revealed that the role of the excess Mg for achieving n-type behavior is to compensate the electronic charge of the defect complex (V Mg2 + Mg i ) 1− . It is clear that defect calculations are usually sensitive to the applied theoretical methods; however, the robust relative trend of defect formation energies is supposed to render insightful guidance for the carrier transport.…”

Let’s Uncomplicate: Mg Complexes in Mg3Sb2

“…In a recent article, Chong et al challenged the role of Mg-rich growth conditions in suppressing Mg vacancy formation. 5 The authors claim that a Mg defect complex (V Mg + Mg i ) 1 is the dominant defect under Mg-rich conditions; the excess Mg compensates the defect complexes rather than the Mg vacancies. We regret to inform that there are serious shortcomings in the methodology and results of this paper.…”