Enhanced thermoelectric performance in n-type Mg3.2Sb1.5Bi0.5 doping with lanthanides at the Mg site

•Gd and Ho have increased the DOS of the valence band near the Fermi level, and the d electrons of Gd and Ho have made extraordinary contributions.•Gd and Ho are effective n-type cation site dopants with higher doping efficiency and better carrier concentration than the anion site dopants chalcogenide elements.•For Mg3.2Gd0.02Sb1.5Bi0.5 and Mg3.2Ho0.02Sb1.5Bi0.5 samples, we obtain high zT values of 1.61 and 1.55 at 725 K, respectively.•Gd as well as Ho are an alternative effective n-type doping element for higher thermoelectric performance.