NbCoSn-based half-Heusler alloys are promising n-type high-temperature thermoelectric materials, but their relatively high thermal conductivities prevent the further improvement in thermoelectric properties. In this work, an effective way to reduce the lattice thermal conductivity without degradation of electrical transport properties via the isoelectronic substitution of Ta for Nb in NbCoSn-based alloys has been demonstrated. The synthesis and thermoelectric properties of Nb 1−x Ta x CoSn 0.9 Sb 0.1 (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25) solid solutions are reported. The large mass fluctuation and chemical bond softening caused by Ta alloying significantly suppress the lattice thermal conductivity. Meanwhile, the Ta content has little effect on the carrier mobility due to the close atomic size and similar chemical nature between Ta and Nb. Benefiting from the significant reduction of lattice thermal conductivity and preservation of electrical transport performance after Ta alloying, a ZT value of ∼0.7 is achieved in Nb 0.75 Ta 0.25 CoSn 0.9 Sb 0.1 at 973 K, which is about 32% higher than that of a NbCoSn 0.9 Sb 0.1 matrix. This work demonstrates a valid approach to enhance the thermoelectric performances of half-Heusler alloys.

Half-Heusler materials are promising candidates for high-temperature power generation and have relatively high lattice thermal conductivity compared to other thermoelectric material systems. In this work, we report novel p-type YbNiSbbased half-Heusler alloys with a low lattice thermal conductivity (∼3.6 W m −1 K −1 at 340 K) that resulted from their large Gruneisen parameter, low sound speed, and low Debye temperature. All YbNiSb-based alloys exhibit a high carrier mobility of 30−50 cm 2 V −1 s −1 at room temperature because of their relatively small effective mass. Importantly, the structural analysis reveals that Yb-rich Yb 1.3 Ni 0.9 Sb 0.8 exhibits Yb/Ni and Yb/Sb substitution, indicating a wide homogeneity region of the YbNiSb phase experimentally. The adjustable Yb and Ni contents in YbNiSb-based alloys can modify the band structure around the Fermi level and significantly affect electrical transport properties. Additionally, by doping Ta at Yb sites, the carrier concentration and lattice thermal conductivity of these alloys can be manipulated. Consequently, a peak zT value of 0.45 at 823 K was achieved for Yb 0.95 Ta 0.05 NiSb. Our work demonstrates that YbNiSb-based alloys are promising p-type thermoelectric materials and suggests the possibility of exploring novel thermoelectric alloys in rare-earth nickel pnictides via tuning their composition and crystal structure.

The nominal composition TiNiSb with 19 valence electrons is demonstrated to be composed of off-stoichiometric half-Heusler phase and impurities. In this work, the Ti_1-<i>x</i>NiSb (<i>x</i>=0, 0.10, 0.15, 0.20, 0.25) samples were first prepared by ball milling and spark plasma sintering. The single-phase Ti_0.9NiSb sample, deviating from the theoretical composition Ti_0.75NiSb base on 18-electron rule, is obtained, which might be ascribed to the small defect formation energy of Ti filling the vacancy as well as our ball-milling preparation method. With the single-phase Ti_0.9NiSb sample as the base material, a small amount of Sc is used to partially replace Ti in order to further reduce the carrier concentration. Thus, the Ti_{1-<i>x</i>-<i>y</i>}Sc<i>_y</i>NiSb (<i>x</i>=0.10,0.15;<i>y</i>=0.03,0.05) samples are designed to investigate the effect of Sc doping on the thermoelectric properties. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analysis confirms the single-phase nature of the Ti_{1-<i>x</i>-<i>y</i>}Sc<i>_y</i>NiSb samples. Energy-dispersive X-ray spectroscopy (EDS) results indicate that the actual compositions of the Ti_{1-<i>x</i>-<i>y</i>}Sc<i>_y</i>NiSb samples agree well with their nominal compositions, and all the elements distribute uniformly in the sample. Moreover, the doping of Sc can increase the content of Ti vacancy while maintaining the single-phase structure, which could be attributed to the higher binding energy between Sc and Sb because the electronegativity of Sc is less than that of Ti. Both the substitution of Sc for Ti and the increase of the Ti vacancies significantly reduce the carrier concentration, which decreases from~13.6 × 10²¹ cm^-3 for Ti_0.9NiSb to~3.4 × 10²¹ cm^-3 for Ti_0.8Sc_0.05NiSb. The decreased carrier concentration results in greatly increased Seebeck coefficient, and thus the Ti_0.8Sc_0.05NiSb sample achieve a power factor as high as 17.7 μW/(cm·K²) at 973 K. Although the lattice thermal conductivity of Sc-doped samples increases slightly owing to the reduction of electron-phonon scattering and the enhancement of chemical bonds, the total thermal conductivity decreases dramatically due to the greatly reduced electronic thermal conductivity. Finally, the Ti_0.8Sc_0.05NiSb sample reaches a <i>zT</i> value of~0.42 at 973K, which is 180% higher than that of Ti_0.9NiSb sample. Despite that the thermoelectric performance of our sample is still inferior to those of the state-of-the-art off-stoichiometric 19-electron half-Heusler alloys, this work demonstrates that the thermoelectric performance of Ti_1-<i>x</i>NiSb can be further improved by non-isoelectronic doping.