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 solved 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 group: C2/m (no. 12), Z = 2. In contrast to Ga2Te5 and In2Te5Al2Te5 is very sensitive to hydrolysis. It can formally be described as Te[AlTe3/3Te1/1](2), containing layers made up of chains of cis-edge-sharing AlTe4 tetrahedra [AlTe3/3Te1/1] and additional Te atoms. In2Te5-I and In2Te5-II are characterized by layers with a similar topology, Ga2Te5 however is different. It has no layer structure, but contains chains of trans-edge-sharing GaTe4-tetrahedra and additional Te-atoms according to the formulation Te[GaTe4/2](2). It can be regarded as a variant of the TISe type structure. From heterogeneous samples with the nominal composition In0.5Ga1.5Te5 single crystals of a new stacking variant (In2Te5-III) of the In,Te, structure type can be isolated. The composition of the crystals, determined by single crystal structure analysis, is In0.77Ga1.23Te5, 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 structure type is not yet known for unsubstituted In2Te5. The range of homogeneity for Ga2Te5 with respect to the substitution of Gallium by Indium is given by Ga2-xInxTe5 (x < 0.4). Within the limits of experimental error however a substitution of Te in Ga2Te5 by Se cannot be detected.