Ultralow thermal conductivity of Tl4Ag18Te11

Tl4Ag18Te11 was reported to crystallize in a cubic structure with disordered thallium atoms filling cuboctahedral voids. Our electronic structure calculations of Tl4Ag18Te11 revealed a zero-band gap, based on an I4mm space group model with ordered Tl atom sites. Utilizing differential scanning calorimetry, we demonstrated that the material melted incongruently at 723 K. Our studies of the thermoelectric properties of hot-pressed Tl4Ag18Te11 and its nonstoichiometric homologs, revealed an unusual n-type extrinsic semiconducting behavior, combined with an ultralow thermal conductivity. The thermal conductivity of a sample of the nominal composition "Tl4.05Ag18Te11" reached 0.19 W m(-1) K-1 at 500 K, with this value being one of the lowest achieved by crystalline materials to date. This can be attributed to the highly disordered Tl atoms and the large unit cell with its highly complex structure, in accord with the low Debye temperature of 98 K.