Crystalline and Glassy Phases in the Ternary System Tl/Bi/Cl: Synthesis and Crystal Structures of the Thallium(I) Chloridobismutates(III) Tl3BiCl6 and TlBi2Cl7

Slow cooling of melts composed of TlCl and BiCl₃ allows for the isolation of the compounds Tl₃BiCl₆ ( 1 ) and TlBi₂Cl₇ ( 2 ). Compound 1 is formed by sublimation at 480 °C from the black melt of 3 TlCl + 1 BiCl₃ as colourless crystals. The crystal structure determination (tetragonal, P4₂/m) consists of nearly regular octahedral BiCl₆]^3– anions and two independent Tl⁺ cations, which have coordination number 8 in form of a slightly distorted cube and 10 in form of an Edshamar polyhedron, respectively. The structure is not isotypic with the recently reported naturally occurring form of Tl₃BiCl₆, the mineral steropesite. Compound 2 is obtained from a dark red melt of composition TlCl + 2 BiCl₃. On rapid cooling, this melt solidifies to a metastable dark red glass which at ambient temperature crystallises to a light amber crystalline powder within some weeks. The structure of 2 was determined by powder diffraction (triclinic, P\bar{1} ). A distinct lone pair effect is present causing an irregular coordination on the two independent bismuth atoms. Taking Bi–Cl bonds up to 3.5 Å into account, both bismuth atoms gain coordination number seven. ²⁰³Tl and ²⁰⁵Tl solid state NMR and XANES spectra on the Bi and Tl‐L_III edges of both glassy and crystalline TlBi₂Cl₇ show that a close structural similarity exists between both forms. In contrast, the Raman spectra show distinct differences in the bands of the Bi–Cl vibrations region.