On polychalcogenides of thallium with M(2)Q(11) groups as a structural building block. I - Preparation, properties, X-ray diffractometry, and spectroscopic investigations of Tl4Nb2S11 and Tl4Ta2S11

The new ternary compounds Tl4Nb2S11 and Tl4Ta2S11 were prepared using Thall ium polysulfide melts. Tl4M2S11 crystallises isotypically to K4Nb2S8.9Se2.1 in the triclinic space group P (1) over bar with a = 7.806(2) Angstrom, b = 8.866(2) Angstrom, c = 13.121(3) Angstrom, alpha = 72.72(2)degrees, beta = 88.80(3)degrees, and gamma = 85.86(2)degrees for M = Nb and a = 7.837(1) Angstrom, b = 8.902(1) Angstrom, c = 13.176(1) Angstrom, alpha = 72.69(1)de grees, beta = 88.74(1)degrees, and gamma = 85.67(1)degrees for M = Ta. The interatomic distances as well as angles within the [M2S11](4-) anions are s imilar to those of the previously reported data for analogous alkali metal polysulfides. Significant differences between Tl4M2S11 and A(4)M(2)S(11) (A = K, Rb, Cs) are obvious for the shape of the polyhedra around the electro positive elements. The two title compounds melt congruently at 732 K (M = N b) and 729 K (M = Ta). The optical band gaps were estimated as 1.26 eV for Tl4Nb2S11 and as 1.80 eV for the Tantalum compound.