Thermoelectric properties and electronic structure calculations of low thermal conductivity Zintl phase series M16X11 (MÂ =Â Ca and Yb; XÂ =Â Sb and Bi)

Zintl phases of type M16X11 (M = Ca and Yb; X = Sb and Bi) have been synthesized by both the flux or solid state reaction methods. In order to evaluate the potential of these phases for thermoelectrics, the electrical resistivity (ρ), thermopower (α) and thermal conductivity (κ) have been measured on hot pressed samples over a wide temperature range of 10-1000 K. The ρ and α trends are typical of a degenerate semiconductor. However, α remains low (∼30 μV/K or less). Hall measurements indicate a rather low charge carrier mobility ∼3-7 cm2/V s, except for Ca16Sb11. It is notable that low thermal conductivity values ranging from ∼2.5 to 0.8 W/m K are measured for these phases at higher temperatures up to 1273 K. Particularly for Yb16Sb11, the lattice part of thermal conductivity is found to be between 0.5 and 0.7 W/m K. However, the challenge in this M16X11 system is to achieve high thermopower. In order to understand more fully the measured transport properties, band structure calculations have been performed. Consistent with experimental data, these calculations suggest that all the systems are indeed metallic. The electronic structures for Ca-based phases show one dimensional metallic behavior with small and almost symmetric density of states around the Fermi level.