Ultralow thermal conductivity of Tl4Ag18Te11

Abstract

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 “Tl_4.05Ag₁₈Te₁₁” 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.