Outstanding thermoelectric performance of n-type half-Heusler V(Fe_1−xCo_x)Sb compounds at room-temperature

The effects of cobalt substitution at the Fe-site on the crystal structure, electronic structure and thermoelectric (TE) properties of half-Heusler (HH) V(Fe_1−xCo_x)Sb compounds are investigated. Crystal structural analysis using powder X-ray diffraction clarified that all compounds have deficient HH structures (Huang et al., Chem. Mater., 32, 5173-5181, 2020). With Co-substitution at the Fe 4c sites, defects are observed in the V 4a and Fe 4c sites, and a small amount of Fe atoms enter into 4d sites, which is normally vacant in an ideal HH structure. The deficient HH structures of the V(Fe_1−xCo_x)Sb compounds are supported by electronic structural analysis using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). All compounds exhibit n-type behaviour in both calculations and experiments with deficient HH structures. The absolute value of the Seebeck coefficient |S|, decreases with an increase in the cobalt content x. The tendency of the experimental |S| value to decrease corresponds well with the |S| determined by KKR-CPA calculations. Cobalt substitution acts as an electron dopant that effectively tunes the carrier concentration. The optimised carrier concentration of 2.1×10²⁰ cm⁻³ leads to the highest power factor of 5.7×10⁻³ W/K²m at room temperature for the V(Fe_0.99Co_0.01)Sb compound. The V(Fe_1−xCo_x)Sb compounds are thus potential candidates as n-type TE materials for power generation near room temperature.