The orthorhombic TiNiSi structure-type compounds show interesting electronic structures comprising in most cases a pseudogap in the density of states and several small electron and hole pockets at the Fermi energy. These features are promising and can be exploited to test their potential as thermoelectric materials for waste heat conversion. Here, we investigate the effect of electron doping in the semimetallic member $\mathrm{ZrNiSi}$ of this family. We show that by doping with Sb for Si in $\mathrm{ZrNiSi}, S$ and $\sigma$ can both be increased simultaneously for initial Sb doping defying the oppositely directed trend commonly observed in most materials. In the doped samples, $\sigma$ at $300\mathrm{K}$ increases from $1000\mathrm{S}{\mathrm{cm}}^{-1}$ to as high as $2500\mathrm{S}{\mathrm{cm}}^{-1}$; at the same time, the peak value of $S$, which is $-20\mu \mathrm{V}{\mathrm{K}}^{-1}$ in $\mathrm{ZrNiSi}$, increases by more than a factor of two. The simultaneous enhancement of $\sigma$ and $S$ has been explained using the first-principles density functional theory based band structure calculations. The as-cast (i.e., unannealed) ${\mathrm{ZrNiSi}}_{1-x}{\mathrm{Sb}}_x$ samples show phase segregation due to a spinodal-type decomposition with two coexisting TiNiSi structure-type phases with different Sb/Si ratios. The thermal conductivity ($\kappa$) in the doped samples drops significantly from $12\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ ($x=0$) to nearly $2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ ($x=0.2$) at 300 K. As a result, the peak thermoelectric figure of merit ($zT$) increases from 0.005 $(x=0)$ to 0.023 $(x=0.2)$. Further enhancement in $zT$ is obtained by codoping of Hf (Zr site) and Sb (Si site), which improves the phase stability and chemical homogeneity while keeping the thermal conductivity still very low due to Zr-Hf point mass fluctuation, resulting in a peak $zT$ value of 0.055, i.e., almost an order of magnitude higher value than for the pristine $\mathrm{ZrNiSi}$. We show that the thermoelectric properties of TiNiSi structure-type semimetals can be enhanced by aliovalent doping. This principle can be employed on other members of the TiNiSi to improve the $zT$ in this family of compounds further.

• Received 18 November 2021
• Revised 13 April 2022
• Accepted 9 May 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.065401