X-site aliovalent substitution decoupled charge and phonon transports in XYZ half-Heusler thermoelectrics

In this work, the effects of niobium (Nb) doping on thermoelectric properties of n-type half-Heusler (Hf_0.25Zr_0.25Ti_0.5)_1-xNb_xNiSn (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) compounds were investigated. The samples were synthesized by arc-melting method and followed by annealing, mechanical grinding and spark plasma sintering (SPS) process. The electrical conductivity, the Seebeck coefficient, the thermal conductivity, and the Hall coefficient measurements were performed in the range of temperature from room temperature to 923 K. By changing Nb-doping amount, the electrical conductivity increased with increasing the doping level, while the Seebeck coefficient relatively less decreased at high temperatures, resulting in the highest power factor of 36 × 10⁻⁴ (W/K²m) at 773 K with x = 0.02 sample. Furthermore, a significant reduction in the lattice thermal conductivity is attributed to anharmonic phonon-phonon interactions by aliovalent substitution into multi-element site and higher degree of phase separation, which was achieved through the synthesis process of these compounds. The enhancement of the power factor combined to low thermal conductivity leads to ∼20% increase of the dimensionless figure of merit (ZT, ∼1.0 at 773 K) in comparison to the parent Hf_0.25Zr_0.25Ti_0.5NiSn compound. It is believed that this is attributed to a decoupling between the electrical conductivity, thermopower, and phonon transport by Nb-doping. The Nb-doped compounds also showed high stability of the power factor up to 873 K.