Thermoelectric Properties and Electronic Structure of the Cage Compounds A₂BaCu₈Te₁₀ (A = K, Rb, Cs):  Systems with Low Thermal Conductivity

The anisotropic, two-dimensional structure of the compounds A2BaCu8Te10 (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 thermoelectric material. It contains Cu8Te12 pentagonal dodecahedral cages filled with Ba2+ atoms. These cages are fused together to make [BaCu8Te10]2- slabs which are separated by alkali metal atoms. Electronic band structure calculations on Cs2BaCu8Te10 at the density functional theory (DFT) level show a complex 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 measurements, however, suggest that these materials are narrow gap semiconductors with a band gap of approximately 0.28 eV. Electrical conductivity, thermopower, and thermal conductivity measured on polycrystalline ingots are reported, and the results are discussed in the context of the calculated electronic structure and the PGEC model. It is suggested that while the PGEC aspects of the compounds may have an effect on the thermoelectric properties, they are not the dominant factor responsible for the very low thermal conductivity of these materials.