Elemental tellurium as a chiralp-type thermoelectric material

The thermoelectric transport properties of elemental tellurium are investigated by density functional theory combined with the Boltzmann transport equation in the rigid band approximation. We find that the thermoelectric transport properties parallel and perpendicular to the helical chains are highly asymmetric (almost symmetric) for $p$- ($n$-) type doped tellurium due to the anisotropic (isotropic) hole (electron) pockets of the Fermi surface. The electronic band structure shows that the lone-pair derived uppermost heavy-hole and extremely light-hole lower valence bands offer the opportunity to obtain both a high Seebeck coefficient and electrical conductivity along the chains through Sb or Bi doping. Furthermore, the stairlike density of states yields a large asymmetry for the transport distribution function relative to the Fermi energy which leads to large thermopower. The calculations reveal that tellurium has the potential to be a good $p$-type thermoelectric material with an optimum figure of merit $zT$ of 0.31 (0.56) at room temperature (500 K) at a hole concentration around $1\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$. Exploiting the rich chemistry of lone pairs in chiral solids may have important implications for the discovery of high-$zT$ polychalcogenide-based thermoelectric materials.