Crystalline and Glassy Phases in the Ternary System Tl/Bi/Cl: Synthesis and Crystal Structures of the Thallium(I) Chloridobismutates(III) Tl3BiCl6 and TlBi2Cl7
Résumé
Slow cooling of melts composed of TlCl and BiCl3 allows for the isolation of the compounds Tl3BiCl6 (1) and TlBi2Cl7 (2). 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. 2 is obtained from a dark brown melt of composition TlCl + 2 BiCl3. On rapid cooling, this melt solidifies to a metastable dark red glass which at ambient temperature crystallizes to a light amber crystalline powder within some weeks. The structure of 2 was determined by powder diffraction (triclinic, P ). A distinct lone pair effect is present causing an irregular coordination on the two independent Bi atoms. Taking Bi–Cl bonds up to 3.5 Å into account, both Bi atoms gain coordination number seven. 203Tl and 205Tl solid state NMR and XANES spectra on the Bi and Tl-LIII 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.
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