Ta3SBr7 - A new structure type in the M(3)QX(7) family (M = Nb, Ta; Q = S, Se, Te; X = Cl, Br, I)

摘要

The layered compound Ta3SBr7 has been prepared by direct reaction of the elements at 550 degrees C for 2 weeks in evacuated Pyrex ampules, and its structure determined by single-crystal X-ray diffraction. Ta3SBr7 crystallizes with monoclinic symmetry in space group Cm; a = 12.249(2) Angstrom, b = 7.071(2) Angstrom, c = 8.829(2) Angstrom, beta = 134.421(8)degrees, V = 546.16(23) Angstrom(3), Z = 2, R = 0.027, R = 0.032. The structure, related to the Cd(OH)(2) type, consists of triangular tantalum clusters located between every other layer of a closed-packed mixed S/SBr ordered anion framework. The slightly distorted, quasi-infinite (2)(infinity)[Ta3SBr7] slabs stack parallel to the crystallographic ab plane in a new variant, with each successive (2)(infinity)[Ta3SBr7] slab related by a 1/2b+c stacking vector. This stacking mode is the sixth layered structure type observed in the M(3)QX(7) system. Ta3SBr7 represents the first Ta(3)QX(7) compound 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 the M(3)QX(7) system Nb3SBr7 being the other. Synthesis, structural characterization, and lattice energy calculations of Ta3SBr7 and related compounds are reported. (C) 1998 Academic Press. [References: 16]