Enhancing thermoelectric properties in TiNiSi structure-type semimetal ZrNiSi by doping

Abstract

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 ZrNiSi of this family. We show that by doping with Sb for Si in ZrNiSi, S and σ can both be increased simultaneously for initial Sb doping defying the oppositely directed trend commonly observed in most materials. In the doped samples, σ at 300K increases from 1000Scm−1 to as high as 2500Scm−1; at the same time, the peak value of S, which is −20μVK−1 in ZrNiSi, increases by more than a factor of two. The simultaneous enhancement of σ and S has been explained using the first-principles density functional theory based band structure calculations. The as-cast (i.e., unannealed) ZrNiSi1−xSbx 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 (κ) in the doped samples drops significantly from 12Wm−1K−1 (x=0) to nearly 2Wm−1K−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 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.