THE PENTATELLURIDES M(2)TE(5) (M=AL, GA, IN) - POLYMORPHISM, STRUCTURAL RELATIONS, AND HOMOGENEITY RANGES

The hitherto unknown crystal structure of the black solid Al2Te5 is so lved by Rietveld refinement of X-Ray powder data: a = 1359.29(3) pm, b = 415.27(1) pm, c = 983.92(2) pm, beta = 126.97(1)degrees, space grou p: C2/m (no. 12), Z = 2. In contrast to Ga2Te5 and In2Te5Al2Te5 is ver y sensitive to hydrolysis. It can formally be described as Te[AlTe3/3T e1/1](2), containing layers made up of chains of cis-edge-sharing AlTe 4 tetrahedra [AlTe3/3Te1/1] and additional Te atoms. In2Te5-I and In2T e5-II are characterized by layers with a similar topology, Ga2Te5 howe ver is different. It has no layer structure, but contains chains of tr ans-edge-sharing GaTe4-tetrahedra and additional Te-atoms according to the formulation Te[GaTe4/2](2). It can be regarded as a variant of th e TISe type structure. From heterogeneous samples with the nominal com position In0.5Ga1.5Te5 single crystals of a new stacking variant (In2T e5-III) of the In,Te, structure type can be isolated. The composition of the crystals, determined by single crystal structure analysis, is I n0.77Ga1.23Te5, with a = 1613.2(3) pm, b = 424.6(1) pm, c = 1330.5(2) pm, beta = 97.39(1)degrees, space group C2/c (Nr. 15), Z = 4. This str ucture type is not yet known for unsubstituted In2Te5. The range of ho mogeneity for Ga2Te5 with respect to the substitution of Gallium by In dium is given by Ga2-xInxTe5 (x < 0.4). Within the limits of experimen tal error however a substitution of Te in Ga2Te5 by Se cannot be detec ted.