PREPARATION AND CRYSTAL-STRUCTURE OF TL3AL13S21, A NOVEL TERNARY SULFIDE STRUCTURE TYPE

Light-yellow single crystals of Tl3Al13S21 were grown from a stoichiometric mixture of the binary compounds Tl2S and Al2S3 at 1000-1050 K in evacuated silica tubes. The title compound crystallizes in the monoclinic space group Cm with a = 25.242(5), b = 14.900(3), c = 9.259(1) angstrom, beta = 103.81(1)-degrees and Z = 4. The structure was determined from single-crystal X-ray diffraction data and was refined to final R and R(w) indices of 0.056 and 0.053, respectively. In this new tetrahedral structure type slightly distorted AlS4 tetrahedra (Al-S 2.177-2.329 angstrom) are connected via common corners, forming a polymeric, largely covalent, three-dimensional framework. There is a layer-like ...AABAAB ... ordering perpendicular to c of this tetrahedral arrangement. The Tl atoms are located in tunnels along [001], with their coordination by 12, 11 or 9 sulfur atoms being rather irregular owing to the stereochemical activity of the lone electron pair (Tl-S 3.109-3.974 angstrom). The structure can be described as containing a distorted close packing of 21 S + 3 Tl per formula unit, with an ordered occupation of 13 of the tetrahedral voids by Al.