Highly dynamic chalcogen chains in silver(I) (Poly)chalcogenide halides: A new concept for thermoelectrics?

Coinage-metal(I) polychalcogenide halides represent a new class of materials featuring high ion dynamics of different substructures capable of an effective phonon scattering process in the solid state. The interplay of mobile coinage-metal ions such as Ag ⁺ or Cu ⁺ on the one hand and the temperature-driven redox reaction of a linear, partially covalent-bonded Te chain on the other hand is responsible for tremendous variations in the electronic structure of such materials. A huge drop of the Seebeck coefficient within a very small temperature range and an extremely low thermal conductivity are the key properties of Ag ₁₀Te ₄Br ₃, the first representative of this substance class. Two different sets of compounds with the general formula (CM) ₁₀Q ₄X ₃ and (CM) ₂₃Q ₁₂X (CM = coinage metal Cu or Ag, Q = chalcogen, X = halogen) have been found, and recent experiments point toward the existence of formerly unknown copper(I) (poly)chalcogenide halides and two more silver(I) (poly)chalcogenide halides in this field. A comparable linear Te chain is present in a number of different compounds such as some alkaline-earth polychalcogenides M ₅Te ₃, the mineral stuetzite Ag _4.5Te ₃, and the closely related compounds Ag ₁₁AsTe ₇ and Ag ₁₂Te ₆S. In most cases the thermoelectric potential of these materials has not been verified. A topological approach for the description of the structural features has been developed to understand the electronic properties of these complex materials in detail.