The Structure of Caesium Enneachlordiarsenite, Cs3As2Cl9

Cs₃As₂Cl₉ is shown to have a hexagonal unit of structure with a₀ = 7.37A, c₀ = 8.91A, containing one molecule. The atomic arrangement is based upon closest‐packing of caesium and chlorine, with D₃² — C 3 2 as the most probable space‐group, but the amount of distortion from the closest‐packed positions is not large enough to permit of the unambiguous elimination of D_3d³ — C $\text{3̄}$ m. A structure based upon D₃² accounts well for observed intensity data. Arsenic is shown definitely by the intensity data to have only a pseudo‐coordination number of six, and the structure may be qualitatively described as containing AsCl₃ molecules embedded in an environment of caesium and chloride ions in such a way that all large atoms fit into an approximately closest‐packed arrangement. This is the first structure of this type to be discovered. The ratio of 3Cl per Cs is a particularly happy one for structures of this kind, and a study of this structure gives much insight into the existence and stability of the related group of compounds discussed in the Introduction.