Complex chalcogenides of the Va metals:: Preparation and crystal structure of Tl2AgTaS4

Roentgenographic investigations in the pseudobinary sections Tl(3-x)Ag(x)T(V)Q(4) (T = V, Nb, Ta; Q = S, Se) led to Tl2AgTaS4 as the only intermediate compound in these systems. Tl2AgTaS4 is monoclinic, mP32, s.g. P2(1)/n with a = 7.932(3) Angstrom, b = 10.395(9) Angstrom, c = 10.025(4) Angstrom, beta = 90.60(4)degrees; Z = 4. The crystal structure was determined from diffractometer data and refined to a conventional R of 0.052 for 1283 F-0's and 74 variables. The characteristic feature of this new crystal structure is the formation of complex anionic layers, (infinity)2 -[AgTaS4](2-) which are separated by Tl+-ions. The layers are built up by an essentially planar network of edge-and corner sharing MS4-tetrahedra. The average Ag-S and Ta-S bond-lengths are 2.562 Angstrom and 2.266 Angstrom resp. Edge sharing leads to severe distortions of the AgS4-tetrahedron but leaves the TaS4-tetrahedron essentially unaffected. The two crystallographically independent Tl+-cations are characterized by irregular sulfur coordinations.