Low thermal conductivity and semimetallic behavior in some TiNiSi structure-type compounds

Abstract

Motivated by recent advances in half-Heusler based thermoelectric materials, we investigated the phase stability and thermoelectric properties of compounds ZrNiSi, ZrNiGe, HfNiSi, NbCoSi, and ZrNiSb, some of which were recently reported in literature as promising half-Heuslers for thermoelectric applications using the first-principles density functional theory based calculations. Here, we show that all the named compounds actually crystallize with the orthorhombic TiNiSi structure type, which remains stable above room temperature up to at least 1100 K. In ZrNiSb, $5%$ excess Zr is required to obtain the pure orthorhombic phase. Our first-principles electronic band structure calculations reveal that they are semimetals. In ZrNiSi, ZrNiGe, and HfNiSi, the Fermi surface consists of small electron and hole pockets with electrons as the majority charge carriers. In NbCoSi and ZrNiSb, the majority carriers are holes. A pseudogaplike feature is observed in the electronic density of states with Fermi energy (${E}_{F}$) located either slightly below (ZrNiSi, ZrNiGe, and HfNiSi) or above the pseudogap (NbCoSi). In ZrNiSb no pseudogap is observed; however, the density of states at ${E}_{F}$ is still small. The electrical conductivity ($\ensuremath{\sigma}$) near room temperature is of the order of ${10}^{3}\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which is intermediate between that of the degenerate semiconductors and metallic alloys. Near room temperature the thermopower is negative for $\mathrm{ZrNi}X$ ($X$ = Si, Ge) and HfNiSi, and positive for NbCoSi and ZrNiSb as predicted theoretically. The average value of Seebeck coefficient is small, of the order of $10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Despite reasonably high electrical conductivity, the thermal conductivity ($\ensuremath{\kappa}$) of these compounds is found to be generally low ($<15\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ near $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$). In ${\mathrm{Zr}}_{1.05}\mathrm{Ni}\mathrm{Sb}$, which has the highest electrical conductivity ($\ensuremath{\approx}4000\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), $\ensuremath{\kappa}$ is as low as $\ensuremath{\approx}4\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$, of which almost 70% is estimated to be due to the electronic contribution resulting in a lattice contribution which is $<1\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. This uncommon combination of high electrical conductivity and low thermal conductivity is interesting and invites further attention.