Enhanced Thermoelectric Performance in N‐Type Mg_3.2Sb_1.5Bi_0.5 by La or Ce Doping into Mg

Abstract

N‐type Mg3.2Sb1.5Bi0.5 materials are prepared by cation‐site doping with lanthanides (La, Ce). Both La‐ and Ce‐doped samples exhibit a higher doping limit and greater efficiency than those of chalcogen (Te, Se, S)‐doped n‐type Mg3.2Sb1.5Bi0.5 samples. High electron carrier concentration ≈9 × 1019 cm−3 is obtained in Mg3.18La0.02Sb1.5Bi0.5 and Mg3.185Ce0.015Sb1.5Bi0.5, which is close to the theoretical doping‐concentration limit and induces contributions from more electron bands. A higher electrical conductivity was thus obtained and is beneficial to the enhanced ZT values for lanthanide‐doped Mg3.2Sb1.5Bi0.5. The highest ZT value ≈1.6 is achieved in Mg3.19La0.01Sb1.5Bi0.5 at 693 K, along with a ZT ≈1.50 in Mg3.19Ce0.01Sb1.5Bi0.5 at 693 K, indicating that lanthanides provide a promising doping strategy for Mg3.2Sb1.5Bi0.5‐based materials.

Probing Efficient N‐Type Lanthanide Dopants for Mg3Sb2 Thermoelectrics

“…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 . Despite these significant theoretical and experimental efforts on exploring efficient n-type dopants, the n-type doping behavior of many other lanthanide dopants in Mg 3 Sb 2 remains largely unexplored so far.…”

“…The appearance of the f states below the CBM as well as their hybridization with the conduction band in Nd-doped samples can be attributed to the relatively small energy separation between the atomic orbital levels of the Nd f states and Mg 3s states. Similarly, the above mechanism may be used to explain the low electron mobilities in previously reported La-, Pr-, and Ce-doped Mg 3 Sb 2−x Bi x samples [24,26,31] because of the small energy separation between the atomic orbital energies of the f states of the dopants (La, Pr, and Ce) and 3s states of Mg (see Table S5 in the Supporting Information).…”

“…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. Recent experiments [24,26,31] have already confirmed La, Ce, and Pr as effective n-type dopants for Mg 3 Sb 2−x Bi x , while other predicted efficient n-type dopants such as Nd, Gd, Lu, Ho, and Tm still require the experimental confirmation.…”

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

“…This configuration not only provides a rough diagram to understand the structure-property relationship of Mg 3 X 2 but also gives instruction about the site preference when alloying with other elements [ 113 ]. The Mg1 sites are favorably occupied when alloying with more electropositive alkaline earth metals and lanthanides, like Ca, Sr, Ba, La, and Yb [ 114 – 118 ], whereas Mg2 sites with more electronegative Zn, Mn, and Cd metals [ 119 , 120 ].…”

“…The latter might originate from the smaller differences in the electronegativity and ionic radius between Mg and the dopants [ 86 , 89 , 159 , 197 ]. The lanthanide dopants with a larger ionic radius, such as La, Pr, and Ce [ 115 , 118 , 193 ], were also found to be effective in achieving higher electron density than Te doping ( Figure 7(d) ) buthad an adverse effect on the carrier mobility. Moreover, the cation dopants on the Mg sites seem to have one more advantage than the anion dopants.…”