Phases in the Ba 3Fe 1+ xS 5 series: The structure of β-Ba 9Fe 4S 15 and its low-temperature α polymorph

The crystal structure of β-Ba ₉Fe ₄S ₁₅ shows that it is a phase in the infinitely adaptive series of compounds Ba ₃Fe _{1+ x}S ₅, 0 ⩽ x ⩽ 1. The material is synthesized by reacting a slightly sulfur-rich mixture at 900°C in a sealed quartz ampoule. Lattice constants are a = 25.212(3), Å, b = 9.594(1), Å, c = 12.575(1), Å, Pnma, z = 4. Three thousand thirty-three structure amplitudes were refined to R = 0.049. BaS ₆ trigonal prisms share triangular faces to form infinite columns; the columns in turn share edges and create nearly hexagonal enclosures. Within these rings are additional Ba and S and tetrahedral interstices are created which can be occupied by Fe. The variation of the Fe occupancy from ring to ring gives rise to phases in which one dimension is an integral multiple of the 8.5-Å repeat observed in one end member of the series, Ba ₃FeS ₅. The other end member is Ba ₃Fe ₂S ₅. At temperatures below 900°C a polymorphic phase is formed. Its lattice constants are a = b = 9.634(1), Å, c = 34.311(3)Å, {I4 ₁}/{a}, z = 4 . One thousand five hundred eighty-three structure amplitudes were refined to R = 0.0483. Trigonal prisms and bisdisphenoids articulate to form a complex three-dimensional structure. Two of the S atoms in the structure have statistical site occupancies.