Ultralow thermal conductivity of TlAgTe

Tl 4 Ag 18 Te 11 was reported to crystallize in a cubic structure with disordered thallium atoms filling cuboctahedral voids. Our electronic structure calculations of Tl 4 Ag 18 Te 11 revealed a zero-band gap, based on an I 4 mm 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 Tl 4 Ag 18 Te 11 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 "Tl 4.05 Ag 18 Te 11 " 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. This thallium telluride is bestowed with an ultralow thermal conductivity in comparison to state-of-the-art thermoelectrics.