KIN5S6 AND MIN5S7 (M NA, K) - 3 NEW MIXED-VALENT THIOINDATES OF ALKALI-METALS AND THEIR CRYSTAL-CHEMICAL RELATIONS

Crystalline samples of the deeply colored title compounds can be prepa red by starting from stoichiometric mixtures of the elements. In contr ast to hitherto known thioindates of the alkali metals the title compo unds are the first examples of mixed valence thioindates with respect to indium. Assuming an ionic description indium appears as In+, In3+ a nd covalent pairs of [In2](4+) in combination with S2- and M+. NaIn5Si 7 ((Na0.88In0.12)In5S7) and KIn5S7 ((K0.88In0.12)In5S7) crystallize is otypically to In6S7, and KIn5S6 ((K0.8In0.2)In5S6) isotypically to T1I n(5)S(6) in space group P2(1)/m (no 11) with lattice constants a = 903 .0(4), b = 384.9(2), c = 1706.9(3) pm, beta = 102.35(2)degrees (NaIn5S 7), a = 907.2(3), b = 388.1(2), c = 1734.4(7) pm, beta = 102.59(2)degr ees (KIn5S7), a = 879.4(2), b = 390.5(1), c = 1748.8(3)pm, beta = 95.1 4(3)degrees (KIn5S6). It is shown, that the substitution of In+ and Tl +, respectively, by alkali metal cations of nearly equal size leads to surprising complications in the structural characterization of these solids. On the one hand it is not possible to replace In+ in In6S7 com pletely by M+ under formation of MIn5S7, on the other hand one can alw ays detect significant amounts of In+ on the position of K+ in KIn5S6. Structural implications referring also to principal questions of the purity of these phases judged on X-ray powder diffraction data are dis cussed.