New polymolecular bismuth monohalides. Synthesis and crystal structures of Bi₄Br_xI_4-x (x = 1, 2, or 3)

New mixed bismuth monohalides Bi₄Br_xI_4-x (x = 1, 2, or 3) were prepared for the first time by the reactions of bismuth metal with bismuth trihalides taken in stoichiometric amounts. Their crystal structures were established by single-crystal X-ray diffraction analysis. The Bi₄Br₃I and Bi₄BrI₃ compounds are isostructural and crystallize in the orthorhombic system, and Bi₄Br₂I₂ crystallizes in the monoclinic system. The crystal structures of all three phases contain one-dimensionally infinite molecular chains consisting of the [Bi₄X₄] fragments whose structures are identical with those of the individual Bi₄I₄ and Bi₄Br₄ molecules. The molecules are packed in layers. Different packing modes of the layers were found for different bismuth monohalides. The Bi₄ClI₃ compound, which is apparently structurally similar to Bi₄Br₃I and Bi₄BrI₃, was also prepared.