Abstract: Heavy rare-earth element doping can effectively strengthen phonon scattering, suppress the lattice thermal conductivity, and enhance the overall thermoelectric performance of GeTe. However, the large electronegativity difference between rare-earth elements (such as La, Eu, and Gd) and Ge refrains the doping limit of rare-earth elements below 1 mol.% in GeTe. Here, compared with other rare earth elements, Lu was found to have a relatively small radius and electronegativity difference with Ge, which can induce a high doping level in GeTe. The result shows that Lu doping effectively reduces the lattice thermal conductivity from 0.77 W m^-1 K^-1 of GeTe to 0.35 W m^-1 K^-1 of Ge_0.98Lu_0.02Te at 673 K, and further induces a high zT value of 1.5 in Ge_0.98Lu_0.02Te at 673 K. Extra Sb alloying optimizes the carrier concentration from 1.02 × 10²¹ cm^-3 of Ge_0.98Lu_0.02Te to 1.77 × 10²⁰ cm^-3 of Ge_0.90Lu_0.02Sb_0.08Te, which results in a reasonable power factor of 33.82 µW cm^-1 K^-2 and a low electrical thermal conductivity of 0.75 W m^-1 K^-1 at 673 K in Ge_0.90Lu_0.02Sb_0.08Te. Correspondingly, a peak zT of 1.75 at 673 K and an average zT of 0.92 within the temperature range of 303-723 K are obtained in Ge_0.9Lu_0.02Sb_0.08Te. This study indicates that Lu and Sb co-doping can effectively boost the thermoelectric performance of GeTe-based thermoelectric materials.

Key words: Thermoelectric, Rare-earth, Thermal conductivity, Carrier concentration