Syntheses, Crystal and Electronic Structures, and Characterizations of Quaternary Antiferromagnetic Sulfides: Ba₂MFeS₅ (M = Sb, Bi)

Two new quaternary sulfides, Ba₂SbFeS₅ and Ba₂BiFeS₅, were synthesized by using a conventional high-temperature solid-state reaction method in closed silica tubes at 1123 K. The two compounds both crystallize in the orthorhombic space group Pnma with a = 12.128(6) Å, b = 8.852(4) Å, c = 8.917(4) Å, and Z = 4 for Ba₂SbFeS₅ and a = 12.121(5) Å, b = 8.913(4) Å, c = 8.837(4) Å, and Z = 4 for Ba₂BiFeS₅. The crystal structure unit can be viewed as an infinite one-dimensional edge-shared MS₅ (M = Sb, Bi) tetragonal-pyramid chain with FeS₄ tetrahedra alternately arranged on two sides of the MS₅ polyhedral chain via edge-sharing (so the chain can also be written as ₁^∞[MFeS₅]⁴⁻). Interestingly, the compounds have the structural type of a Ba₃FeS₅ high-pressure phase considering one Ba²⁺ is replaced by one Sb³⁺/Bi³⁺, with Fe⁴⁺ reduced to Fe³⁺ for in order to maintain the electroneutrality of the system. As a result, the isolated iron ions in Ba₃FeS₅ are bridged by intermediate MS polyhedra in Ba₂MFeS₅ (M = Sb, Bi) compounds and form the ₁^∞[MFeS₅]⁴⁻ chain structure. This atom substitution of Ba²⁺ by one Sb³⁺/Bi³⁺ leads to a magnetic transition from paramagnetic Ba₃FeS₅ to antiferromagnetic Ba₂MFeS₅, resulting from an electron-exchange interaction of the iron ions between inter- or intrachains. Magnetic property measurements indicate that the two compounds are both antiferromagnetic materials with Néel temperatures of 13 and 35 K for Ba₂SbFeS₅ and Ba₂BiFeS₅, respectively. First-principles electronic structure calculations based on density functional theory show that the two compounds are both indirect-band semiconductors with band gaps of 0.93 and 1.22 eV for Ba₂SbFeS₅ and Ba₂BiFeS₅, respectively.