Electronic structure and transport of and

We have carried out detailed ab initio electronic structure calculations of a novel thermoelectric compound (Formula presented) and its parent (Formula presented) the best room-temperature thermoelectric known to date. The calculations were carried out using the self-consistent full-potential linearized augmented plane-wave method within density functional theory. The generalized gradient approximation of Perdew, Burke, and Ernzerhof was used to treat the exchange and correlation potential. For both systems, inclusion of spin-orbit interaction is crucial in understanding the gap structure near the Fermi energy. The calculated theoretical values of the gaps agree surprisingly well with experiment. A detailed comparison of the band structures of these two compounds indicates that the effective masses (expressed in units of free-electron mass) for both of them are highly anisotropic and comparable in magnitude. They lie in the range 0.01-0.4. The major difference between the two compounds is the large degeneracy of the band extrema in (Formula presented) which is most likely the origin of its large thermoelectric figure of merit.