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–xTi_xNiSb (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_0.5Ti_0.5NiSb sample, the lattice thermal conductivity of Mg_0.5Ti_0.5NiSb 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–xTi_xNiSb alloys are obtained. The p-type Mg_0.55Ti_0.45NiSb and n-type Mg_0.35Ti_0.65NiSb achieve dimensionless thermoelectric figure of merits zTs 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_0.55Ti_0.45Ni_0.95Cu_0.05Sb and n-type Mg_0.35Ti_0.65Ni_0.95Co_0.05Sb 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.