Thermoelectric properties and electronic structure of the cage compounds A(2)BaCu(8)Te(10) (A = K, Rb, Cs): Systems with low thermal conductivity

The anisotropic, two-dimensional structure of the compounds A(2)BaCu(8)Te(1 0) (A = K, Rb, Cs) fits the description of a "phonon glass electron crystal " (PGEC) which is proposed to be a desirable feature of a good thermoelectr ic material. It contains Cu8Te12 pentagonal dodecahedral cages filled with Ba2+ atoms. These cages are fused together to make [BaCu8Te10](2-) slabs wh ich are separated by alkali metal atoms. Electronic band structure calculat ions on Cs2BaCu8Te10 at the density functional theory (DFT) level show a co mplex electronic structure near the Fermi level. The Fermi level falls in a deep valley in the density of states, a pseudogap in which very few states exist. Heat capacity, magnetic susceptibility, and infrared absorption mea surements, however, suggest that these materials are narrow gap semiconduct ors with a band gap of approximately 0.28 eV. Electrical conductivity, ther mopower, and thermal conductivity measured on polycrystalline ingots are re ported, and the results are discussed in the context of the calculated elec tronic structure and the PGEC model. It is suggested that while the PGEC as pects of the compounds may have an effect on the thermoelectric properties, they are not the dominant factor responsible for the very low thermal cond uctivity of these materials.