Highly Dynamic Chalcogen Chains in Silver(I) (Poly)Chalcogenide Halides: a New Concept for Thermoelectrics?

Abstract

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 coinagemetal 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 10 Te 4 Br 3 , the first representative of this substance class. Two different sets of compounds with the general formula (CM) 10 Q 4 X 3 and (CM) 23 Q 12 X (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 5 Te 3 , the mineral stuetzite Ag 4.5 Te 3 , and the closely related compounds Ag 11 AsTe 7 and Ag 12 Te 6 S. 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.

Silver(I)-(poly)chalcogenide Halides – Ion and Electron High Potentials

“…10. Surprisingly, the lower content of halide in this systems compared More recently, the newest example of a silver(I) polychalcogenide halide, Ag 20 Te 10 BrI [92], was reported also featuring Peierls-distorted Te strands and a highly mobile silver substructure. While the covalently bonded anion substructure also contains Peierls-like distorted Te chains the remaining noncovalently bonded anion substructure is characterized by another variation of corrugated and interpenetrated honeycomb nets.…”

“…The mentioned topologic approach can be generalized to other classes of compounds (e.g. sodium polytellurides) [92], to minerals like stuztite (Ag 4.53 Te 3 ) [94] and stutzite analogues [95]. 3 -nets which interpenetrate each other.…”

Structure Topology in Silver(I)‐(poly)chalcogenide Halides: A Helpful Tool to Understand Structure Relations and Properties

“…Syntheses, structure analyses and physical properties of the title compounds were published elsewhere [2][3][4][5][6][7]. The compounds were either synthesized by high temperature melting and annealing procedures or solvothermal reactions.…”

“…Dependent on the overall X content of a compound, the halide ions are forming strands (X content of 1 to 4 in the modified sum formula of Scheme 1) or Kagomé-like 3.6.3.6 nets (X content of 6 and 7). The polyanion substructure is characterized by a partially covalently bonded and linear arrangement of (Te-[Te 2 ]-Te) n units capable to perform a dynamic Peierls distortion in the solid state [5][6][7]. Such a scheme or formalism has several advantages.…”

“…In almost every case it is fundamental to determine the structure of a given material to understand and optimize its physical properties. Recently, the discovery of new compounds in the classes of coinage metal chalcogenide halides and polychalcogenide halides like Ag 5 Q 2 X with Q = S-Te and X = Cl, Br [2], Ag 3 SX [3], Ag 19 Q 6 X 7 [4], Ag 10 Q 4 X 3 with Q = S, Se or Te and X = Cl, Br or I [5], Ag 23 Te 12 X with X = Cl, Br [6] or Ag 20 Te 10 X 2 with X = Br, I [7] demanded for a new way of structure description because of the complexity and the high grade of dynamic disorder within the different substructures. Twinning caused by polymorphism, huge translation periods with several hundred atoms per unit cell, and low scattering power at higher diffraction angles because of the ion mobility are common features of these classes of com- simple but effective building principle and a topological structure approach, which are both based on the separate description of anion substructures and the introduction of discrete building blocks.…”