Defect Chemistry for N-Type Doping of Mg₃Sb₂-Based Thermoelectric Materials

Abstract

N-Type Mg3Sb2-based Zintl compounds have recently been discovered to be a promising class of thermoelectric materials. Effective n-type dopants are crucial for realizing high thermoelectric performance. Here, using first-principles defect calculations, we investigate that Tm and Ce are effective n-type dopants in Mg3Sb2 and explain why n-type conduction can be successfully achieved by a simple doping without extra Mg. Under Mg-rich conditions, the maximal achievable free carrier concentrations for Tm and Ce substitution on Mg sites at 750 K exceed 1020 cm–3, approaching the optimal carriers of ∼1020 cm–3. Under Mg-poor conditions, the miraculous n-type conduction achieved by a simple extrinsic n-type doping can be explained by the sufficiently low defect formation energy for the substitutional donor defect.

Probing Efficient N‐Type Lanthanide Dopants for Mg3Sb2 Thermoelectrics

“…It is evident that Sm, Dy, Er, Pm, and Tb are a bit less efficient and show lower free electron concentrations than Te for n-type doping since they show a bit deeper donor levels even though they have lower formation energies. Besides La, Ce, Pr, and Tm that have been confirmed by previous calculations, [22,24,25] Nd, Gd, Lu, and Ho, as expected, are more effective as n-type dopants than Te and doping with these lanthanide elements results in high maximum achievable electron concentrations ≳10 20 cm −3 at 900 K. Among these efficient n-type dopants, Nd and Tm show the lowest formation energies of the donor defects Nd Mg1 (+1) and Tm Mg1 (+1) throughout the bulk gap. The predicted maximum achievable free electron concentrations at the growth temperature of 900 K for the potential n-type dopants Nd, Gd, Lu, and Ho are respectively 9.90 × 10 19 , 1.30 × 10 20 , 1.27 × 10 20 , and 1.17 × 10 20 cm −3 , which are slightly lower than those (>2 × 10 20 cm −3 ) of the n-type dopants La, Ce, and Pr.…”

“…For the anion site doping, Se and S also have been reported as effective n-type dopants for Mg 3 Sb 2 , [14,21] whereas they are less efficient than Te due to the higher substitution defect formation energies. Recent defect calculations [22][23][24][25] predicted that the group-3 elements (Sc and Y) as well as several lanthanides including La, Pr, Ce, and Tm are efficient n-type cation-site dopants for Mg 3 Sb 2 and doping with these elements on the Mg sites is able to achieve higher electron concentration than doping with chalcogens on the anion sites. The subsequent experiments [24,[26][27][28][29][30][31][32] confirmed Sc, Y, La, Pr, and Ce as efficient n-type dopants on the Mg sites for Mg 3+ Sb 2−x Bi x .…”

High-Performance Mg 3 Sb 2- x Bi x Thermoelectrics: Progress and Perspective

“…Both the defect calculations and single-crystal studies give a simple answer [ 48 , 97 ]. That is, only slightly excess Mg (less than 0.1%) is necessary to guarantee the system to be n -type if efficient dopants are chosen [ 171 , 195 , 196 ]. However, for the experimental preparation of polycrystalline samples, the real conditions are more complicated.…”

“…After the significant role of Mg-excess for realizing n -type doping was found [ 97 ], defect calculations predicted that Mg substitution with trivalent (or higher) cations can be even more effective than Se and Te doping to achieve high electron density above 1 × 10 20 cm −3 [ 171 , 195 , 196 ]. In experiments, group 3 elements (Sc and Y) were found to be the most effective cation dopants which not only enable the realization of high carrier concentration ( Figure 7(d) ) but also have a weak effect on carrier transport [ 48 , 73 , 92 , 173 , 192 ].…”

Enhanced Thermoelectric Performance in N‐Type Mg3.2Sb1.5Bi0.5 by La or Ce Doping into Mg

“…Based on the satisfactory doping effect of La at the Mg site, another lanthanide element (i.e., Ce) was chosen for further studies. First‐principles calculations demonstrated that Ce could be a more effective dopant as compared with La or Y 24,25,35. The electrical properties of Ce‐doped Mg 3 Sb 2 are shown in Figure .…”