Enhanced Thermoelectric Performance in N‐Type Mg 3.2 Sb 1.5 Bi 0.5 by La or Ce Doping into Mg

N‐type Mg 3.2 Sb 1.5 Bi 0.5 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 Mg 3.2 Sb 1.5 Bi 0.5 samples. High electron carrier concentration ≈9 × 10 19 cm −3 is obtained in Mg 3.18 La 0.02 Sb 1.5 Bi 0.5 and Mg 3.185 Ce 0.015 Sb 1.5 Bi 0.5 , 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 Mg 3.2 Sb 1.5 Bi 0.5 . The highest ZT value ≈1.6 is achieved in Mg 3.19 La 0.01 Sb 1.5 Bi 0.5 at 693 K, along with a ZT ≈1.50 in Mg 3.19 Ce 0.01 Sb 1.5 Bi 0.5 at 693 K, indicating that lanthanides provide a promising doping strategy for Mg 3.2 Sb 1.5 Bi 0.5 ‐based materials.