BaFeAgS: a new antiferromagnetic and semiconducting quaternary sulfide.

The single crystals of a quaternary sulfide, BaFeAgS, have been synthesized by reacting elements at 873 K inside a sealed fused silica tube. The title phase is the first ordered quaternary compound of the Ba-Ag-Fe-S system. The crystal structure of BaFeAgS is characterized by a single-crystal X-ray diffraction study at 298(2) K. It crystallizes in the space group 52h - 2/ of the monoclinic crystal system with unit cell dimensions of = 8.6367(5) Å, = 12.0291(7) Å, = 13.2510(7) Å, and = 109.015(2)°. This compound is stoichiometric, and its structure contains twelve unique crystallographic sites: four Ba, one Fe, one Ag, and six S sites. All atoms of the structure occupy the general positions. The BaFeAgS structure consists of one-dimensional chains of 1∞[FeAgS] that are extended in the [100] direction. The negative charges on these chains are counterbalanced by the filling of Ba cations in between the 1∞[FeAgS] chains. The Fe atoms are bonded to four S atoms that form a distorted tetrahedral geometry around the central Fe atom. Each Ag atom in this structure is coordinated with four S atoms in a distorted tetrahedral fashion. These FeS and AgS motifs are the main building blocks of the BaFeAgS structure. The corner-sharing of FeS and AgS tetrahedra creates one-dimensional chains of 1∞[FeAgS]. This structure does not contain any homoatomic or metallic bonds and can be charge-balanced as (Ba)(Fe)(Ag)(S). The optical absorption study performed on a polycrystalline BaFeAgS sample reveals a direct bandgap of 1.2(1) eV. The magnetic studies reveal the antiferromagnetic behavior of BaFeAgS below 50 K. The thermal conductivity and theoretical electronic structure of BaFeAgS are also studied in detail.