Phases in the Ba3Fe1+xS5 series: The structure of β-Ba9Fe4S15 and its low-temperature α polymorph

The crystal structure of β-Ba₉Fe₄S₁₅ shows that it is a phase in the infinitely adaptive series of compounds Ba₃Fe_1+xS₅, 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. BaS₆ 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, Ba₃FeS₅. The other end member is Ba₃Fe₂S₅. 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, $\text{I4}{}_1\text{a}\text{, z = 4}$. 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.