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)Q4 (t=V,Nb,Ta; Q=S,Se) led to Tl2AgTaS4 as the only intermediate compound in these systems. Tl2AgTaS4 is monoclinic, mP32, s.g. P21/n with a=7.932(3)Ä, b=10.395(9)Ä, c=10.025(4)Ä, ß=90.60(4)?; Z=4. The crystal structure was determined from diffractometer data and refined to a conventional R of 0.052 for 1283 Fo's and 74 variables. The characteristic feature of this new crystal structure is the formation of complex anionic layers, 2-oo-[AgTaS4](2-) which are seperated 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 2.562Ä and 2.266Ä, 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.