Low-Temperature Ordering in the Cluster Compound (Bi8)Tl[AlCl4]3

 
With the synthesis of (Bi₈)Tl[AlCl₄]₃  a new polycationic bismuth species has been discovered by Maximilian Knies. Black, shimmering needle-like crystals could be obtained by the reaction of Bi, BiCl₃ and TlCl in the ionic liquid [BMIm]Cl·4AlCl₃.

Apart from the homonuclear Bi₈²⁺ polycation ,forming the characteristic square antiprism, the structure hosts linear, one dimensional strands of _∞¹[Tl(AlCl⁴)³]^2- extending along the crystallographic [001]-direction. The slightly distorted Bi₈²⁺ polycations form arachno-clusters according to modified wade’s rules. They display a disordered orientation that allows to describe the crystal structure in the space group P6₃/m. The room temperature structure may thus be understood as a hexagonal perovskite structure type which was first observed in BaNiO₃. Here, Ni⁴⁺ is replaced by Tl⁺ and oxygen and Ba²⁺ positions are occupied by [AlCl₄]⁻ and Bi₈²⁺, respectively. The compound therefore represents the first known perovskite structure with two complex ions.

Low temperature investigations revealed an increased ordering of the bismuth polycations reducing the symmetry. Using group-subgroup relations, the low temperature structure has been described in the space group P6₅, a sub group of order 24 (symmetry reduction follows a t2, k4 and k3 transition) of the room temperature space group.

The work was honored with a front cover. The article can be found in in the special issue “Structure, Properties, and Bonding in Solid State Compounds“ in Inorganics:

Knies, M., Kaiser, M., Lê Anh, M., Efimova, A., Doert, T., Ruck, M., Inorganics 2019, 7(4), 45.