Ta3SBr7—A New Structure Type in theM3QX7Family (M=Nb, Ta;Q=S, Se, Te;X=Cl, Br, I)

The layered compound Ta₃SBr₇has been prepared by direct reaction of the elements at 550°C for 2 weeks in evacuated Pyrex ampules, and its structure determined by single-crystal X-ray diffraction. Ta₃SBr₇crystallizes with monoclinic symmetry in space groupCm;a=12.249(2) Å,b=7.071(2) Å,c=8.829(2) Å,β=134.421(8)°,V=546.16(23) ų,Z=2,R=0.027,R=0.032. The structure, related to the Cd(OH)₂type, consists of triangular tantalum clusters located between every other layer of a closed-packed mixed S/SBr ordered anion framework. The slightly distorted, quasi-infinite²_∞[Ta₃SBr₇] slabs stack parallel to the crystallographicabplane in a new variant, with each successive²_∞[Ta₃SBr₇] slab related by a 1/2b+c stacking vector. This stacking mode is the sixth layered structure type observed in theM₃QX₇system. Ta₃SBr₇represents the first Ta₃QX₇compound discovered that is not isostructural with its niobium counterpart and is the second of the two one-slab per unit cell stacking types possible in theM₃QX₇system Nb₃SBr₇being the other. Synthesis, structural characterization, and lattice energy calculations of Ta₃SBr₇and related compounds are reported.