Synthesis and Properties of Ba 6 Fe 2 Te 3 S 7 , with an Fe Dimer in a Magnetic Singlet State.

A new quaternary sulfide telluride, Ba 6 Fe 2 Te 3 S 7 , was synthesized by a solid-state reaction, and its crystal structure is novel. X-ray diffraction data on powder and single crystals reveal an orthorhombic lattice with a = 9.7543(3) Å, b = 18.2766(6) Å, and c = 12.0549(4) Å, and the noncentrosymmetric space group Cmc 2 1 (No. 36). The properties of the compound were studied by magnetic susceptibility investigations, specific heat measurements, Mössbauer spectroscopy, and density functional theory calculations. Assuming Ba 2+ and, as verified by the Mössbauer spectra, Fe 3+ , the charge balance requires the presence of a polytelluride, suggested to be a straight-chain [Te 3 4- ] polyanion. Further, the crystal structure contains [Fe 2 S 7 ] 8- dimers of two vertex-sharing tetrahedra, with a nearly linear Fe-S-Fe atom arrangement. The dimer exhibits antiferromagnetic coupling, with a coupling constant J = -10.5 meV ( H = -2 J S 1 S 2 ) and S = 5/2, resulting in a spin singlet ground state. The interdimer magnetic interaction is so weak that the magnetic dimers can be treated as individuals.