The Crystal Structure of Cesium Enneachlordithalliate, Cs3Tl2Cl9

Abstract

Cs₃Tl₂Cl₉ is shown to possess a rhombohedral unit of structure with a₀=9.58A, α=83° 48′, containing two molecules. The crystals exhibit extensive and quite perfect 60° twinning on the basal plane, so that most specimens simulate complete sixfold symmetry, but when this is taken into consideration, a structure based upon D_3d⁶ — R $\text{3̄}$ c accounts well for observed intensity data. The structure may be described conveniently as an aggregate of Cs⁺ and Tl₂Cl₉^—3 groups packed together in such manner that all large atoms fit into an approximately closest‐packed arrangement. The Tl₂Cl₉^—3 group is considered to be a covalent complex. The derived structure is compared with the known structures of Cs₃As₂Cl₉ and KMgF₃.