Structure and Thermoelectric Properties of Mg_1–xTi_xNiSb Quaternary Half-Heusler Alloys

In comparison with intensively investigated traditional ternary half-Heusler compounds, quaternary half-Heusler alloys exhibit inherently lower lattice thermal conductivities due to their complex crystal structures. In this work, based on the Slater–Pauling rule, we successfully design and synthesize single-phase Mg 1– x Ti x NiSb ( x = 0.3–0.7) alloys by adjusting the ratio of nominal MgNiSb and TiNiSb components with, respectively, 17 and 19 valence electrons, which avoids the use of the costly ScNiSb half-Heusler compound and provides an economically efficient approach to improve thermoelectric performance. Due to the strong disorder effect at the Mg/Ti site in the Mg 0.5 Ti 0.5 NiSb sample, the lattice thermal conductivity of Mg 0.5 Ti 0.5 NiSb is significantly reduced, reaching ∼2.6 W m –1 K –1 at 300 K. By tuning the Mg/Ti ratio, both p-type and n-type Mg 1– x Ti x NiSb alloys are obtained. The p-type Mg 0.55 Ti 0.45 NiSb and n-type Mg 0.35 Ti 0.65 NiSb achieve dimensionless thermoelectric figure of merits zT s of 0.36 at 973 K and 0.40 at 873 K, respectively. Moreover, the electrical and thermal transport properties of these samples can be further optimized by doping Cu or Co to reduce the carrier concentration. As a result, cost-effective and Hf-free p-type Mg 0.55 Ti 0.45 Ni 0.95 Cu 0.05 Sb and n-type Mg 0.35 Ti 0.65 Ni 0.95 Co 0.05 Sb with, respectively, zT values of 0.48 at 873 K and 0.52 at 773 K are achieved, demonstrating the promising potential of developed half-Heusler alloys with low cost.