Ba4FeCuS6: a new mixed metal sulfide with a pseudo-zero-dimensional structure containing rare CuS3 units

Abstract

Herein, we describe the synthesis of a new mixed transition metal sulfide, Ba₄FeCuS₆, using two methods: (a) the high-temperature reaction of elements under vacuum and (b) the reaction of metal oxides and carbonates under flowing CS₂ at high temperatures. Both methods successfully produced polycrystalline phases of the quaternary sulfide. Single crystals of the title phase were prepared by heating the polycrystalline sample with a Pb flux followed by slow cooling of the reaction mixture. The Ba₄FeCuS₆ structure adopts the monoclinic crystal system (space group: P2₁/n). The partitioning of the atomic charges of the sulfide can be easily done considering the Fe(III) species in the structure, i.e., [Ba(II)]₄[Fe(III)][Cu(I)][S(−II)]₆. The Cu atoms, Cu1 and Cu2, of the structure are distributed at two split positions. Four sulfur atoms are bonded to each Fe atom, forming a distorted tetrahedral geometry. In contrast, only three S atoms are part of the coordination polyhedron of each Cu atom to stabilize the trigonal planar geometry of the S atoms. The FeS₄ and Cu1(S₃)/Cu2(S₃) polyhedral units are interconnected by corner sharing to create anionic pseudo-zero-dimensional units. The Ba(II) cations neutralize the negative charges on these [FeCu_1–xS₆]^(x+8)− units. Each Ba atom is surrounded by seven S atoms that form a distorted mono-capped trigonal prismatic geometry. The polycrystalline Ba₄FeCuS₆ is semiconducting with a direct bandgap of 1.06(2) eV. The magnetic studies of the sample show an antiferromagnetic transition below 30 K. It will be interesting to explore the photovoltaic properties of Ba₄FeCuS₆ in the future as it is a direct bandgap semiconductor with E_g ∼ 1 eV.