Synthesis, Structure, and Thermoelectric Properties of Barium Copper Polychalcogenides with Chalcogen‐Centered Cu Clusters and Te₂^2– Dumbbells

Abstract

BaCu6–xSTe6 and BaCu6–xSe1–yTe6+y were synthesized from the elements at 663 K. These chalcogenides adopt a new structure type, cubic space group Pm$\bar {3}$, with a = 6.9680(2) Å in the case of BaCu5.93SeTe6. Therein, the Cu atoms form cubic clusters, centered by Se atoms, where statistically 2.07 corners are unoccupied. All Te atoms are part of Te22– dumbbells, leading to a charge‐balanced formula when x = 0: Ba2+(Cu+)6Se2–(Te22–)3. While Te atoms can be incorporated on the Se site, no evidence was found for the ability of Se atoms to replace Te in the Te22– pairs. Band structure calculations on different BaCu6SeTe6 models revealed a very small band gap at the Fermi level; all these chalcogenides with x > 0 should thus be p‐doped semiconductors, which we experimentally confirmed for BaCu5.7Se0.6Te6.4.

Thermoelectric Properties of the Quaternary Chalcogenides BaCu5.9STe6 and BaCu5.9SeTe6

“…13 We have not been able to synthesize such a compound; the highest amount of Cu per cell experimentally obtained was 5.93 (2). The material should be an extrinsic ptype semiconductor with 5.9 Cu atoms per cell, nominally with 3 × 10 20 carriers cm −3 if no other defects were present.…”

Chalcogenides as thermoelectric materials

“…We have been investigating Ba-containing copper chalcogenides since over a decade, anticipating that the large Ba cations would limit the Cu ion migration to a unit cell or a smaller Cu cluster therein [55]. To that extent, we demonstrated the stability of BaCu 6-x QTe 6 with Q = S, Se [71]. of an increased electrical conductivity and a basically unchanged Seebeck coefficient [84].…”

Thermoelectric properties and stability of Ba3Cu16 − xSe11 − yTey

“…In this structure, the formally d 10 -d 10 closed-shell interactions between the Cu atoms occur due to the hybridization of the filled d orbitals with the empty s and p orbitals. 27,28 The occurrence of such bonds in coinage chalcogenides is not rare; other examples include BaCu 6 − x Se 6 − y Te y , 17 Ba 3 Cu 14 − x Te 12 , 13 Tl 2 Ag 12 Se 7 , 29 and Tl 2 Ag 12 Te 7 . 30 These Cu-Cu bonds, all shorter than 3 Å, not only result in the formation of Cu clusters with 26 Cu atoms but also bond these clusters in three dimensions.…”

“…Accordingly, the structures of several quaternary barium copper chalcogenides were uncovered. [13][14][15][16][17][18][19][20] Some of these compounds including isostructural Ba 3 Cu 16 − x S 11 − y Te y 19 and Ba 3 Cu 16 − x Se 11 − y Te y 20 only form when mixed occupied chalcogen sites are present. Recently, we proved the high stability of Ba 3 Cu 16 − x S 11 − y Te y under the measurement conditions applied.…”