Die Pentatelluride M2Te5 (M = Al,Ga, In): Polymorphie,Strukturbeziehungen und Homogenitätsbereiche

The Pentatellurides M₂Te₅ (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, β = 126.97(1)°, 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[AlTe_3/3Te_1/1]₂, containing layers made up of chains of cis-edge-sharing AlTe₄ tetrahedra [AlTe_3/3Te_1/1] 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[GaTe_4/2]₂. It can be regarded as a variant of the TlSe type structure. From heterogeneous samples with the nominal composition In_0.5Ga_1.5Te₅ single crystals of a new stacking variant (In₂Te₅-III) of the In₂Te₅ structure type can be isolated. The composition of the crystals, determined by single crystal structure analysis, is In_0.77Ga_1.23Te₅, with a = 1613.2(3) pm, b = 424.6(1) pm, c = 1330.5(2) pm, β = 97.39(1)°, 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 Ga_2-xIn_xTe₅ (x < 0.4). Within the limits of experimental error however a substitution of Te in Ga₂Te₅ by Se cannot be detected.