Giant Thermoelectric Effect in Rare Earth Sulfoiodides

Using first-principles calculations and the electron–phonon Wannier and Boltzmann transport equation, the electronic and lattice transport properties along with thermoelectric performance of FeOCl-type compounds ScXI (X = S, Se, Te) are investigated. It is found that the transport coefficients that determine the thermoelectric properties are significantly affected by electron–phonon coupling, with optical phonon scattering contributing importantly to the transport properties that produce low lattice thermal conductivity at room temperature. Near the Fermi level, the conductivity and relaxation time of holes are 1 order of magnitude larger than that of electrons, while both of them contribute negligibly to the thermal conductivity. The significantly enhanced power factor of holes along with low lattice thermal conductivity give rise to remarkable thermoelectric performance, with ZT of p-type ScSI reaching 1.02. More importantly, we find that both compressive and tensile strain engineering significantly suppress the lattice thermal conductivity by enhancing the phonon scattering, while retaining the electronic band structures almost unchanged near the Fermi level. As a result, a giant thermoelectric figure of merit is predicted in ScSI with a ZT value as high as 4.56 under isotropic strain ratio (−0.6%), corresponding with a hydrostatic pressure of 1.08 GPa. The present work not only uncovers a new family of high-performance thermoelectric materials but also casts physical insights on optimizing the figure of merits for potential applications.