Skutterudite antimonides: Quasilinear bands and unusual transport

Abstract

Electronic-structure calculations are reported for ${\mathrm{IrSb}}_{3}$, ${\mathrm{CoSb}}_{3}$, and ${\mathrm{CoAs}}_{3}$ in the skutterudite structure. The band structures show a pseudogap around the Fermi level. The single band, which crosses the pseudogap, touches the conduction-band minimum at the \ensuremath{\Gamma} point in ${\mathrm{CoAs}}_{3}$ and ${\mathrm{IrSb}}_{3}$ and almost touches it in ${\mathrm{CoSb}}_{3}$ due to a different ordering of the conduction bands. The dispersion of this gap-crossing band is quadratic at the \ensuremath{\Gamma} point as expected, but for the antimonides it remarkably crosses over to linear behavior extremely close to the band edge, so that for doping levels as low as 3\ifmmode\times\else\texttimes\fi{}${10}^{16}$ holes/${\mathrm{cm}}^{3}$ the properties are determined by the linear dispersion. This yields interesting transport effects, such as a completely off-diagonal inverse mass tensor and unusual doping dependencies of the hole mobility and Seebeck coefficient. The Seebeck coefficients calculated from the band dispersion are in excellent agreement with experiment.