Die Kristallstruktur von Tl 3 Bi 2 I 9 : Eine komplexe Ausdünnungs‐ und Verzerrungsvariante des Perowskit‐Typs

The Crystal Structure of Tl 3 Bi 2 I 9 : A Complex Defect and Deformation Variant of the Perovskite Structure Type Black crystals of trithallium(I) enneaiododibismuthate(III) Tl 3 Bi 2 I 9 were obtained by slow cooling of a melt with composition Tl : Bi : I = 3 : 2 : 9 from 800 K to ambient temperature. The compound is diamagnetic and decomposes peritectically at 619(2) K. X‐ray diffraction of single‐crystals revealed that Tl 3 Bi 2 I 9 crystallizes in the monoclinic space group P 2 1 / n with lattice parameters a = 1438.9(1), b = 771.54(7), c = 2091.0(1) pm and β = 93.309(6)°, ( Z = 4). The crystal structure can be described as a defect variant of the perovskite type ( ABX 3 ), where according to the formula Tl(Bi 2/3 □ 1/3 )I 3 the Bi atoms occupy only two thirds of the octahedral sites B . The vacancies are located in every third plane resulting in double layers of corner sharing [BiI 6 ] polyhedra. By distortion and rotation of the [BiI 6 ] polyhedra the coordination of the Tl atoms (cuboctahedral sites in the perovskite type, c.n. = 12) is reduced to bicapped trigonal prisms (c.n. = 8). The group‐subgroup relation between the Tl 3 Bi 2 I 9 structure and the perovskite type results in a subgroup index of 144.