Crystal chemistry of the BaNbS system: A layer structure of approximate composition Ba 2NbS 4(S 2) 0.5

Black platy crystals from the product of a reaction mixture of 6BaS : 3Nb : 7S reacted at 1000°C were hexagonal with a = 6.909(4) Å, c = 49.25(2) Å, {P6 ₃}/{mmc}, Z = 10 . A pronounced subcell with a = 6.91Å, c = 5.5 Å indicated that this was a layer structure consisting of stacking of close-packed BaS ₃ layers. Three dimensional X-ray diffraction data were collected from a single crystal using monochromatized Mo Kα radiation. From the 1535 measured reflections, 782 unique structure amplitudes were obtained of which 608 greater than 2 σ( F) were used to solve the structure. The final R = 0.1065, ωR = 0.0793; for 91 reflections with l = 9 n, R = 0.0397 and for the 517 reflections l ≠ 9 n, R = 0.138. The structure is based on the stacking of close-packed BaS ₃ layers with the sequence CBDBABDBC BCDCACDCB, where D designates a disordered layer. The disordered layers contain two crystallographically independent Ba with partial site occupancies and disordered S ₂ and S ions. Nb occupy octahedral interstices and form two different arrangements; a unit consisting of 3 face-sharing octahedra and a unit of 2 face-sharing octahedra. These octahedral units are separated by the disordered layers. The NbNb distances in the chain of 3 are 3.29 Å and they are 3.57 Å in the double unit.