Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V₂VI₃ or Pn₂Ch₃ (Pn = Pnictogen, Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui-chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase-change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Bi₂ S₃ , Sb₂ Te₃ , Sb₂ Se₃ , Sb₂ S₃ , and β-As₂ Te₃ ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Sb₂ Te₃ , and β-As₂ Te₃ ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.