Compounds of the type A₂^IB^IIC^IVX₄ constitute a well-known family of materials with a large number of different members (A = Cu,Ag,Li,Na; B = Mn,Fe,Co,Ni,Zn,Cd,Hg,Mg,Ca,Sr,Ba,Pb,Eu; C = Si,Ge,Sn; X = S,Se,Te). These compounds have applications as tunable semiconductors, e.g., in solar cell absorbers or as photocatalysts for water splitting. In compounds with A = Cu, copper usually has the oxidation state +I. Replacing the C^IV component in such copper compounds with a trivalent cation, such as Sb³⁺, might result in mixed-valent Cu⁺/Cu²⁺ species.

Martin Lerch, Technical University of Berlin, Germany, and colleagues have investigated this type of replacement and substituted the tin in Cu₂ZnSnS₄ with antimony. They synthesized a new member of the A₂^IB^IIC^IVX₄ family, Cu₂ZnSbS₄. The team prepared this compound by ball milling the corresponding binary sulfides, i.e., CuS, ZnS, Sb₂S₃, as well as Cu together. After milling, the samples were annealed at 250 °C in an H₂S atmosphere.

The product was characterized by X-ray powder diffraction. The team found that Cu₂ZnSbS₄ crystallizes in a stannite-type structure in space group I42m. The team measured the magnetic properties of the compound to check for mixed-valent copper, but found no evidence for the presence of Cu²⁺ or Sb³⁺. Instead, theoretical calculations and the results of ¹²¹Sb Mössbauer spectroscopy indicate remarkably strong covalent Sb^V–S and Sb–Sb bonding.

• Cu₂ZnSbS₄: A Thioantimonate(V) with Remarkably Strong Covalent Sb–S Bonding,
  Eva M. Heppke, Steffen Klenner, Oliver Janka, Rainer Pöttgen, Thomas Bredow, Martin Lerch,
  Inorg. Chem. 2021.
  https://doi.org/10.1021/acs.inorgchem.0c03601