Double half-Heusler alloys X2Ni2InSb (X = Zr, Hf) with promising thermoelectric performance: The role of varying structural phases

Double half-Heusler (HH) alloys are a new class of compounds that can be seen as transmuted versions of two single half-Heusler alloys with higher flexibility of tunability of their properties. Here, we report a detailed study of the thermoelectric (TE) properties of two double HH alloys $X_2{\mathrm{Ni}}_2\mathrm{InSb}$ ($X$ = $\mathrm{Hf}$, $\mathrm{Zr}$), using first-principles calculation. These alloys exhibit a rich phase diagram with the possibility of tetragonal, cubic, and solid-solution phases. As such, a comparative study of the TE properties of all these phases is highly desired. The simulated band gap, obtained using a hybrid functional, of the ordered phase of ${\mathrm{Hf}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ and ${\mathrm{Zr}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ lies in the range 0.24–0.4 and 0.17–0.59 eV, respectively, while, for the disordered phase, it lies in between 0.05 and 0.06 eV. A simulated TE figure of merit ($ZT$) as high as 2.02 and 2.45 is obtained for ${\mathrm{Hf}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ and ${\mathrm{Zr}}_2{\mathrm{Ni}}_2\mathrm{InSb}$, respectively. In both compounds, electronic transport plays the dominant role in achieving the promising $ZT$ values. We believe this study will attract the attention not only of experimentalists but also of theoreticians from the thermoelectric community to further investigate similar double HH alloys.