Syntheses and Crystal Structures of the Thallium(I) Iodobismuthates(III) Tl7Bi3I16 and Tl3BiI6

Dark grey (dark red with transmitting light) crystals of heptathallium(I) hexadecaiodo‐tribismuthate(III), Tl₇Bi₃I₁₆, were obtained by slow cooling of a melt from 800 K to ambient temperature and, with higher crystal quality via solvothermal synthesis in aqueous HI by slowly cooling from 428 to 363 K. The compound is diamagnetic and melts congruently at 630(5) K. X‐ray diffraction on single‐crystals revealed that Tl₇Bi₃I₁₆ crystallizes in the orthorhombic space group Cmcm with lattice parameters a = 2473.4(5), b = 1441.9(2), c = 3616.9(7) pm. The crystal structure can be interpreted as a layered intergrowth of fragments from the CsNiCl₃ and K₅Dy₃I₁₂ structure types with isolated BiI₆]^3? octahedra and Bi₂I₁₀]^4? double octahedra. Rotation and distortion of the complex anions establish coordination numbers (c.n.) between 7 and 9 for the Tl⁺ cations. Dark red crystals of trithallium(I) hexaiodo‐bismuthate(III), Tl₃BiI₆, are only accessible via hydrothermal synthesis in aqueous HI and slowly cooling from 428 to 363 K. Thermal analysis reveals a peritectoid decomposition at 540(5) K into the neighboring phases Tl₇Bi₃I₁₆ and TlI. Tl₃BiI₆ crystallizes in the monoclinic space group P2₁/c with lattice parameters a = 1352.6(3), b = 899.6(2), c = 1353.8(3) pm, and β = 104.18(3)°. In the crystal structure isolated BiI₆]^3? octahedra are arranged according to the motif of a face‐centered pseudo‐cubic packing. Due to the tilted orientation of the BiI₆]^3? groups the Tl⁺ cations have c.n. of 8 and 9. Although the crystal structure of Tl₃BiI₆ looks like a distorted variant of the elpasolith type, there is no symmetry relation according to a group subgroup formalism.