Hf/Sb co-doping induced a high thermoelectric performance of ZrNiSn: First-principles calculation

Previous experiments showed that Hf/Sb co-doping in ZrNiSn impressively improved the electrical conductivity (σ). To explore the physical reasons for this improvement, the electronic structures of Hf_xZr_1-xNiSn_1-ySb_y (x = 0, 0.25, 0.5; y = 0, 0.02) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. 50% Hf doping at Zr site in ZrNiSn simultaneously increases the degeneracy and dispersion of energy bands near the conduction band edge, which are helpful to optimizing Seebeck coefficient and slightly improving σ. Furthermore, 2% Sb co-doping at Sn site in Hf_0.5Zr_0.5NiSn not only increases total density of states near the Fermi energy but also retains high mobility, and N _v reaches eleven at the conduction band minimum, thereby inducing a large improvement in σ. Additionally, the Bader charge analysis shows the reason why Sb co-doping supplies more electrons. It is most likely derived from that Sb loses more electrons and Sb-Ni has a stronger hybridization than Sn-Ni. Moreover, we predict that the ZT of Hf_0.5Zr_0.5NiSn_0.98Sb_0.02 at 1000 K can reach 1.37 with the carrier concentration of 7.56 × 10¹⁸ cm^-3, indicating that Hf/Sb co-doping may be an effective approach in optimizing thermoelectric properties of ZrNiSn alloy compounds.